37? /iBii

9$%8

LIGHT SPECTRA DISTRIBUTIONS IN TEMPERATE CONIFER-FOREST CANOPY

GAPS, OREGON AND IN TROPICAL CLOUD-FOREST

CANOPY, VENEZUELA

DISSERTATION

Presented to the Graduate Council of the

University of North Texas in Partial

Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

By

Susan Monteleone, B.S., M.S.

Denton, TX

December, 1997

Monteleone, Susan, Light spectra distributions in temperate conifer-forest

canopy gaps. Oregon and in tropical cloud forest canopy. Venezuela. Doctor of

Philosophy (Biology), December 1997,198 pp., 5 tables, 66 illustrations,

references, 117 titles.

Light spectra distributions were measured in two different montane

forests: temperate and tropical. Spectral light measurements were made in

different sized canopy gaps in the conifer forest at H. J. Andrews Experimental

Forest in Oregon, USA. Researchers at Oregon State University created these

gaps of 20 m, 30 m, and 50 m in diameter. In the tropical cloud forest, spectral

light measurements were made in two plots that were permanently established at

La Mucuy Parque Nacional in Venezuela, in collaboration with researchers at

Universidad de Los Andes.

In both studies, spectra and distributions of physiologically active light

were analyzed: red, far-red, R/FR ratio, and blue light. Horizontal light

measurements were taken at 1.0 m above the forest floor. Also, light was

measured in vertical profiles. Oregon light measurements were regressed with

numbers of conifer seedlings and basal areas surveyed in the gaps.

Horizontal light distributions varied in both temperate and tropical

systems. Distribution patterns were predicted by the morphology of the canopy

gaps in Oregon, and the relief patterns in Venezuela. Attenuation of light in

forest systems is often assumed exponential following Beer's law, i.e., an

homogeneous path through plant canopy. In Oregon, vertical profiles showed

light in canopy gaps were often not homogeneous. Profiles in Venezuela were

heterogeneous because measurements were taken in full plant canopy.

As distribution of red, far red, and blue light wavelengths, and R/FR ratios

changed along cardinal axes in gaps of different sizes, species' seedling

associations changed. Significant relationships (p < 0.05) between conifer

seedling numbers and basal areas were found. More than 50% of the variation

in seedlings was explained by patterns in light-color distribution. In 30 m gaps,

western Hemlock seedlings were highly significantly affected by red and far red

light (R2 > 90; p < 0.001). Only R/FR ratios were associated significantly with

species distributions in 20 m gaps. Gap sizes strongly affected associations.

37? /iBii

9$%8

LIGHT SPECTRA DISTRIBUTIONS IN TEMPERATE CONIFER-FOREST CANOPY

GAPS, OREGON AND IN TROPICAL CLOUD-FOREST

CANOPY, VENEZUELA

DISSERTATION

Presented to the Graduate Council of the

University of North Texas in Partial

Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

By

Susan Monteleone, B.S., M.S.

Denton, TX

December, 1997

ACKNOWLEDGMENTS

This project is the culmination of many hours contributed by willing and

supportive assistants: Magdiel Ablan, and Diana Victoria Acevedo. I thank Dr.

Miguel Acevedo for his encouragement to pursue this research and for making

possible the opportunity to work in Venezuela.

Many thanks are extended to Oregon State University and to H. J.

Andrews Experimental Forest Research Facility. Special thanks to Drs. Andrew

Gray and Thomas Spies for their collaboration on this project and their

cooperation during our stay at H. J. Andrews; and to Art McKee for his logistical

assistance in the use of H. J. Andrews Experimental Forest and associated

facilities.

At the Universidad de Los Andes in Venezuela for their support of this

project. My sincere gratitude and appreciation goes to Dra. Michele Ataroff at

the Center for Ecological Research for her collaboration and tireless support,

and further thanks to Dr. Carlos Estrada for his invaluable assistance while

working on this project at La Mucuy.

TABLE OF CONTENTS

Page

Chapter

1. ROLE OF SHADE AND LIGHT SPECTRA IN FOREST DYNAMICS 1

Introduction Plant perception of shade

Changes in light intensity Changes in light spectra

Proposed mechanisms of light perception in plants Plant photoreceptors Phytochrome Blue absorbing pigment Regulatory function of light via phytochrome Adaptive significance of photoreceptors

Dynamics of light in forest canopies

2. LIGHT SPECTRA SURVEY IN CANOPY GAPS OF A TEMPERATE MONTANE CONIFEROUS FOREST 22

Introduction Methods

Site description Study area Data collection and management Sample plots and one-meter ground measurements Vertical light profiles and attenuation coefficients

Results One-meter ground measurements Vertical light profiles and attenuation coefficients

Discussion

3. LIGHT SPECTRA IN CANOPY GAPS AND TREE SEEDLING ESTABLISHMENT IN A TEMPERATE MONTANE CONIFER FOREST 66

Introduction Methods

Site description and study area Seedling survey Spectral data collection and management PAR in gaps Statistical analysis

Results PAR in gaps Distribution of light and seedlings

Number of seedlings Seedling basal area

Regression of seedling establishment and light quality Analyses by gap size Analyses by axes

Step-wise regression analyses Discussion

4. LIGHT SPECTRA SURVEY IN A TROPICAL MONTANE FOREST LA MUCUY PARQUE NACIONAL IN VENEZUELA 100

Introduction Methods

Site description Data collection and management Sample plots One-meter ground measurements Vertical profiles and attenuation coefficients Hemispherical photographs

Results PAR in forest canopy One-meter ground measurements

Plot A Plot B

Vertical light profiles Plot A Plot B

Extinction coefficients Plot A Plot B

Open-canopy area calculated from hemispherical photographs

Discussion

5. CONCLUSIONS 151

Objectives

6. APPENDIX: TABLES OF RESULTS FROM LINEAR REGRESSION ANALYSIS OF SEEDLING DATA AND CANOPY GAP LIGHT 1623

Table 1 Table 2

7. LITERATURE CITED 180

CHAPTER 1

ROLE OF SHADE AND LIGHT SPECTRA

IN FOREST DYNAMICS

Introduction

Light is one of the most limiting of environmental factors that affects the

establishment, growth, and development of plants. Light has two important

aspects, fluence rates or light intensity and spectral quality, both of which are

effectively altered by shade environments. The plant's perception of shade in

canopy gaps is based on a dichotomy of plant photosynthetic and

photomorphological responses to the light environment.

Plant Perception of Shade

Changes in Light Intensity. -Vegetative canopies present the greatest

adaptive challenge of terrestrial plants to changes in light environments (Holmes

1981). Plants must receive adequate light intensity in a photosynthetically-

active range of radiation (PAR; 400 nm- 700 nm) to maintain their net

photosynthetic capacity. When intensity falls frequently below light compensa-

tion points, the irradiance at which loss of assimilation due to respiration is

balanced by rates of photosynthesis, plants

must adapt to survive. Canopy shade is a reduction in light intensity that causes

2

plant adaptation to maximize net photosynthesis (Schwartz and Koller 1978;

Barrett and Fox 1994; Hirose and Werger 1995).

Plants adapt to radiation environments in vegetation canopies with a

range of shade-tolerant and shade-intolerant responses. In the coniferous

forests of the Oregon Cascades in the Pacific northwest (PNW), Douglas-fir is a

shade-intolerant species, whereas western hemlock and Pacific silver fir are

shade-tolerant species (Franklin 1963; Spies and Franklin 1989,1991; Gray

1995). Shade-tolerant species are morphologically adapted to low light

intensities (Salisbury and Ross 1985). Leaf size, thickness of palisade cells,

and the activation of phytochrome results in the increased production of

chlorophyll to compensate for low light conditions (Kasperbauer 1988).

Seedlings can be shade-tolerant as juveniles to gain early establishment in the

canopy understory. They are maintained in low light until they are released from

light-limited growth in the event of a canopy-gap creation (Oliver and Larson

1996).

Many shade-tolerant species appear to require openings in the canopy to

become established (Uhl et al. 1988; White and Pickett 1985), but some species

are capable of growing directly up into canopies that have reduced densities

(Canham 1989). In the Oregon Cascade old-growth forests, Douglas-fir and

western hemlock are co-dominant in forest stands where major disturbances

have not occurred. Western hemlock canopies are typically dense, reducing

forest-floor light intensities to less than 5% of full sunlight (Spies and Franklin

1989). Seedlings of Douglas-fir, a shade-intolerant species are unable to

3

become established in canopy gaps smaller than 1000 m2. Further, typical gap-

creation events in H. J. Andrews forest, an old-growth experimental forest area

in the PNW, are from standing snags that allow less light to the forest floor than

gaps created by fallen trees.

In the tropics, ecological groups of species have been defined by their

light tolerances (Swaine and Whitmore 1988; Smith and Huston 1989). Pioneer

species are shown to be dependent on gap-phase regeneration for germination

and growth. The trigger for germination in all tropical species (reviewed by

Swaine and Whitmore, 1988) required increased red light observed after canopy

removal, or increased temperatures of soil exposed to direct radiation. Pioneer

species generally show a tendency toward longer seed dormancy than non-

pioneer species, and germination is cued to disturbance indicators linked to

changes in light quality (Brokaw 1985a).

Within non-pioneer or climax species, there is a gradient of seedling

growth responses to exposure to greater amounts of radiation found in different

gap sizes (Swaine and Whitmore 1988). One end of the response continuum

requires great amount of light to grow rapidly. These species tend to have high

mortality in low canopy shade. At the other extreme, some species do not

require great amounts of light for release. These species have slower growth

rates and are less likely to regenerate following catastrophic loss of canopy

cover.

Changes in Light Spectra.-Perception of shade by vegetation is not limited

4

to reduction in light intensities; changes in spectral quality have been shown to

precede alterations in plant resource allocations. One microclimate factor that

has received relatively little attention is the color of light in forest canopy and

gap environments (Woodward 1989; Canham 1989; Poulson and Piatt 1989;

Smith etal. 1992; Clark etal. 1993; Cornelissen 1993; Endler 1993; Jans etal.

1993; Ackerly and Bazzaz 1995), specifically the spatial patterns of color and its

role in the successful establishment and growth of species in a forest gap

(Franco 1986; Casal etal. 1990; Endler 1993). Understanding patterns of the

distribution of colors of light in forest gaps might help to elucidate functional

mechanisms that work in tandem with molecular light receptors in the plant's

cellular membrane (Frankland and Letendre 1978; Raven 1983; Smith etal.

1990; Fosket 1994). Measurements of light across a forest gap show spatial

patterns of color might be correlated to physiological activities of seeds and

seedlings of colonizing species during the forest regeneration phase.

Proposed Mechanism of Light Perception in Plants

Photomorphogenesis is defined as the control of plant development by

ambient light conditions (Smith 1984). Plants respond to variable light

environments with relatively-sophisticated physiological adjustments, e.g.,

protein synthesis or resource allocation. Such responses could account for a

great degree of the morphological plasticity observed in higher plants, e.g.,

heterophylly. Plants use a complex array of photoreceptors to sense and

respond to light conditions as environmental cues. Plant pigments such as

chlorophyll and carotenoids mediate the photosynthetic response in plants.

5

There are photoreceptors in plants such as photochrome, a pigment protein;

cryptochrome, which responds to visible- (primary peak between 420-480 nm)

and near- (secondary peak between 340-380 nm) UV wavelengths; and the blue

light photoreceptor with which plants respond to blue photon fluence rates (Taiz

and Zeiger 1991).

Plant Photoreceptors. - Plants have photoactive pigments that function as

photoreceptors in the perception of changes in spectral quality under vegetation

canopies. Photoreceptor molecules or biological pigments absorb light at

specific wavelengths, activating the signal transduction pathway (Robinson et al.

1993; Fosket 1994). Light is perceived by the plant at the environmental level

via a molecular receptor. Empirical evidence indicates that activation of a

photoreceptor enhances movement of proton and calcium ions across cell

membranes (Raven 1983). Modulation of these ion ports could be important

action sites for photoreceptors.

Photoreceptors are classified into five categories based on structure

(Hendry 1993). Chlorophylls are tetrapyrroles in their cyclic form and

phytochromes are tetrapyrroles in their linear form. Chlorophyll a and b are

dominant in terrestrial plants and have peak absorption in blue, yellow, and red

wavelength ranges. Chlorophylls are the primary pigments in the photosynthetic

activity of plants.

Phytochrome.- Phytochrome belongs to a group of pigments called

tetrapyrroles and accompanies the chlorophylls and hemes. Phytochrome is

composed of an apoprotein covalently attached to a linear tetrapyrrole

6

chromophore (McNellis and Deng 1995). It exists in two interconvertible forms:

Pr, which responds to red (650-680 nm) photon fluence rates and Pfr, which is

converted from Pr when exposed to far red (710-740 nm) photon fluence rates

(Taiz and Zeiger 1991). Phytochrome is synthesized as Pr, the bioactive form.

When exposed to saturating red light, with an absorbance maximum at 665 nm,

about 80% is converted to Pfr in vivo (McNellis and Deng 1995). There are

fluctuations in the pools of Pr and Pfr by synthesis of Pr, by destruction of the Pfr

form by proteolysis, and by slow reversion of Pfr back to the Pr conformation that

takes place in the dark (Taiz and Zeiger 1991).

There are several types of phytochromes identified in Arabidopsis, a

model species for transgenic plants; these molecules are thought to be encoded

by five distinct genes: PHYA, PHYB, PHYC, PHYD, and PHYE. Phytochrome A

is a light-labile protein and is generally isolated in light-etiolated plant tissues

(McNellis and Deng 1995). Concentrations of phytochrome A decrease 100-fold

when plants are exposed to white light. It is held to be the principal receptor for

continuous far red light. High FR/R ratios is one means by that phytochrome A

is thought to facilitate the emergence of seedlings from soil in deep shade light

environments (McNellis and Deng 1995).

Germinability of seeds under plant canopies is dependent, in many

situations, on the red-to-far red ratios (R/FR) reaching the seeds (Grime 1981;

Mohr and Drumm-Herrel 1983). Canopy shade results in the alteration of

spectral composition of light that influences successful regeneration of seedlings

by affecting seed germination and seedling growth. Germination in certain seed

7

species are inhibited by the depletion of red wavelengths as light is filtered

through the canopy. These species normally require a canopy gap to maximize

their germination success. Thus, light conditions of red light relative to far red

light at the top microzone of soils in canopy gaps is a focal factor in the

germination of forest species' seeds (Foster and Janson 1985; Forget 1992 a, b\

Alvarez-Buylla and Garcia-Barrios 1991; Kennedy and Swaine 1992; Hammond

and Brown 1995; Rokich and Bell 1995; Loiselle etal. 1996).

Phytochrome B is a light-stable protein, as are phytochromes C, D, and E.

However, phytochrome B is proposed as the principal receptor for red light, and

hence is postulated to mediate red-light-induced phytochrome responses such

as day length perception via R/FR equilibrium (Vince-Prue 1983; McCormac et

al. 1992) and shade-avoidance responses (Aphalo etal. 1991; McNellis and

Deng 1995).

Plant photoreceptors possibly initiate early signaling events that plants

use to initiate cellular development, and consequently affect morphogenetic

patterns. The hypothesized mode of action for phytochrome regulation of plant

functions is a signal transduction sequence (Raven 1983) perhaps mediated by

calcium uptake and calmodulin activity (Taiz and Zeiger 1991).

Mediation of membrane functions is especially evident during perceived

phytochrome activity in plants. This might be the interaction necessary to

facilitate the controlled uptake of calcium through cell membranes via

membrane-interactive phytochrome. This membrane interactivity could explain

the amplified effects of low concentrations of phytochrome in plant systems

8

through a calmodulin-mediated system (Raven 1983; Taiz and Zeiger 1991).

The effect of phytochrome on plants is categorized by the intensity of light

required to elicit the response (Taiz and Zeiger 1991). Some responses are

elicited by fluence rates as low as 0.1 nmol m2 s"1 (or one-tenth the light emitted

from a single flash of a firefly!) and are called very low fluence (VLF) responses.

This low amount of red light would convert less than 0.02% of the total

phytochrome to Pfr. Exposure to far red light converts 97% of Pfr to Pr, leaving

3% as Pfr, more than enough to elicit VLF responses. Note that far red light

cannot reverse VLF responses. Low fluence (LF) responses are not initiated

until fluence rates reach 1.0 p.mol m2 s"1. These are the classic photoconversion

responses, e.g., lettuce seed germination. Another category of responses is

elicited by high fluences (HF). Continuous radiation periods are required for

hours at fluences in excess of 10 nmol m2. Action spectra for HF responses are

in the far red and blue regions and are not photoreversible.

When responses require such high intensities of light, it might be found

that more than one photoreceptor is involved. Mancinelli (1989) explained that it

is possible that cryptochrome and phytochrome interact and even very low levels

of Pfr might be enough to elicit the interaction. Because it is not possible to

remove all Pfr from plants, we cannot control for the effect of Pfr to isolate the

effect elicited by blue light acting on cryptochrome.

Blue Absorbing Pigment.- There is evidence that suggests that

phytochrome and blue/UV photoreceptors interact at the molecular level ( Mohr

and Drumm-Herrel 1983; Mancinelli 1989; Elmlinger etal. 1994). However, both

9

the mode of expression and the mechanisms of interaction have not been

elucidated. There are four modes of interaction postulated (Mancinelli 1989): 1)

direct interaction between photoreceptors; 2) interaction at the level of the signal

transduction chain; 3) interaction at the level of post-signal transduction

processes; or, 4) independent action. Note that postulate 1 is a photoreceptor

interaction, whereas postulates 2-4 are interactions between products of actions

of the photoreceptors. Both photoreceptors are involved in photoregulation of

plant growth and development (Mancinelli 1989); however, this is not taken as

evidence of interaction. The nature of cryptochrome is yet unknown and

phytochrome is responsive to UV and blue light as well as red and far red light.

Thus, responses to blue light can be mediated by either phytochrome or

cryptochrome.

It has been argued that no blue light receptor exists because only a few

photochemical responses have been observed only when plants are exposed to

blue light. Isolation of the pigment was further confounded by the observed

response of phytochrome to blue wavelengths (Tanno 1983) and the

photoreversal of blue effects by FR exposure (Briggs and lino 1983; Obrenovic

1992). Also, there is an obligate sequence in which the blue "receptor" must be

activated before phytochrome can be initiated in the synthesis of anthocyanins

(Mohrand Drumm-Herrel 1983).

More evidence for the existence of a blue light receptor lies in the

controlled response of stomatal closure by blue light. Both blue and red light are

effective in photosynthesis and also effect stomatal closure. However, blue light

10

was more than twice as effective than red light in stomatal regulation, especially

under low light conditions. Zeiger et al. (1983) have isolated a blue light

photosystem that regulates changes in stomatal opening in Paphiopedilum

harrisianum (family Orchidaceae), a species whose guard cells do not contain

chlorophyll, which regulates uptake of potassium ions. Blue light is thought to

cause potassium ion movement independent of carbon dioxide or auxin

concentrations (Salisbury and Ross 1985). Zeiger etal. (1983) also attributes

any response to red light as an indirect effect to exposure, such as intercellular

changes in carbon dioxide concentrations from photosynthesis or activity of

chlorophyll receptors, an argument that further solidifies their evidence in

support of the existence of a blue light receptor.

Levels of red and blue light vary temporally, seasonally, and with cloud

cover (Holmes and Smith 1977). Generally, blue light is associated with shorter

internodes, smaller leaf areas, reduced growth rates, and increased nitrogen to

carbon ratios (Thomas 1981). Relative amounts of red and blue light in nature

are probably more important than intensities, suggesting that the interaction

between blue light and red and far red light photoreceptors is of evolutionary

importance.

Regulatory Function of Light via Phytochrome.- Phytochrome has a

regulatory function in the expression of nuclear genes, which might play an

integral role in controlling plant functions. Light-regulated elements have been

identified as promoter regions on plant genomes (Taiz and Zeiger 1991). A

protein factor called GT-1 was isolated by Kay et al. (1989). This factor binds to

11

the light-regulated regions of the rbcS genes of different species, making a

promoter not regulated by light into one influenced by light. This is one means

by which phytochrome can act upon the regulation of the expression of genes by

environmental light cues.

In a study by Elmlinger etal. (1994), levels of glutamine synthetase (GS)

in Scots pine was investigated. GS synthesis is reportedly regulated by light in

the genus Pinus. The study of the light-regulated coaction of the synthesis of

isoforms GS2 and Fd-GOGAT showed coordination of both enzymes via

phytochrome and a blue/UV photoreceptor (Elmlinger et al. 1994). In seedlings

less than 10 days old, phytochrome was the photoreceptor regulating enzyme

synthesis. After 20 days, blue light becomes necessary for any further enzyme

synthesis. Further investigation using dichromatic light showed that

phytochrome was the "effector" of GS synthesis under all conditions, and blue

light amplifies the responsiveness of the system towards Pfr. However, if Pfr

levels were kept low, blue light was not able to elicit the synthesis of GS protein.

On the basis of evidence from earlier studies corroborating the current results, it

was observed that coarse regulation of gene expression is mediated by

phytochrome, and "fine-tuning" takes place at the translational or post-

translational level (Elmlinger et al. 1994).

Adaptive Significance of Photoreceptors.- Mohr and Drumm-Herrel (1983)

argue that the amplification of anthocyanin synthesis in response to blue/UV

light is an evolutionary adaptation to production of a protective mechanism to

damaging UV radiation. Other effects of light cues on plant development has an

12

adaptive significance, e.g., shade-avoidance responses. Allocation of resources

toward stem growth to avoid light competition is one such avoidance response to

shading. Neighbor effect due to reflected FR light has been shown to be of

adaptive value to plants on the basis that plants are able to detect slight

changes in the spectral balance via the phytochrome photoreceptor

Ballare etal. 1987, 1990,1991; Smith etal. 1990).

This process is adaptive because strong selection pressure is exerted to

allocate resources on the basis of an economic principal. The more rapidly

competitive situations are detected, the more advantageous is the adaptive

response, and the advantage goes to the most responsive individuals (Bjorkman

and Powles 1981; Franco 1986; Ballare etal. 1987,1990,1991; Begonia and

Aldrich 1990; Casal et al. 1990; Smith et a\. 1990; Baraldi et al. 1992; Davis and

Simmons 1994).

Dynamics of Light in Forest Canopies

The conceptual model of vegetation as a dynamic mosaic, called the gap-

mosaic concept (Watt 1947; Bormann and Likens 1979; Shugart and Urban

1989), has generated an interest in the spatial dynamics of canopy gaps and

understory canopies in forest systems. In the northeastern United States,

researchers have investigated the relative importance of gap geometry or

various predominant microsite factors that are known to affect seedling

establishment and growth, such as changes in soil nutrients and moisture

resulting from gap creation and vegetation succession (Runkle 1982, 1985;

Runkle and Yetter 1987; Whitmore 1989; Battles etal. 1995, 1996; Battles and

13

Fahey 1996).

In the Pacific northwest of the US, where portions of this light study was

conducted, researchers have investigated the spatial and biological dynamics of

natural and artificial gaps and have related the information to the ability of

dominant species to use these sites as regeneration niches. Franklin (1963)

studied the species and the success of regeneration in different types of clear-

cuttings in H. J. Andrews forest. He studied strips oriented north and south,

strips oriented east and west, small patches 0.25-4 acres in size, seed-tree

cutting, and staggered-setting clear cuts. Oriented clear-cutting strips were

distributed across various elevations (2,025-2,650 feet) and slopes (60-40%).

Seed-tree cuttings were designed to leave trees in the clearing to act to reduce

soil temperatures by providing shade in the gap, and to act as a proximal source

of seeds for the regeneration effort.

Stand shade was established using a method described by Silen (1960,

cited in Franklin 1963). This method relates tree height to solar elevation, and

slope percent to amount of shade cast from a stand edge. A significant

relationship (p < 0.05) was determined when stand shade was related to the

establishment of natural regeneration of Douglas-fir seedlings (measured in

number per unit area). And when data from the three major types of cuttings

were pooled (east-west strips, north-south strips, and patch clear cuts), a highly-

significant relationship (p < 0.01) between gap shading and regeneration was

found. The east-to-west-oriented strips and patches tended to have stronger

relationships than plots oriented north-to-south. Apparently, in north-to-south-

14

oriented gaps, soil surfaces reached temperatures that were damaging to the

delicate, newly-established seedlings, whereas, in east-to-west gaps,

intermittent sunlight was less damaging than the continuous exposures in other

gaps.

Shugart (1987) compared the mode of tree death to the mode of tree

regeneration to determine that had the greater effect on observed patterns of

species in the forest. Whereas most biologists group factors affecting tree

regeneration and death into broad categories, Shugart coupled these modes into

four roles that trees play in a forest ecosystem based on the dichotomy of

whether species can produce a gap upon death and whether species require a

gap for regeneration. In role 1, species both require a gap for regeneration and

produce a gap upon their death. Role 2 categorizes species that create a gap

upon death but do not require a gap for regeneration. The third category, role 3,

is a tree that needs a gap to regenerate but does not produce gaps at its own

death. And role 4, trees neither create a gap nor do they require a gap to

regenerate.

A representative species from role 1, the yellow poplar (Uriodendron

tulipifera) or tulip tree commonly found in temperate forests in the southern

Appalachians, is described as a species that attains a large size and creates

gaps upon its demise. In addition, these trees require a canopy gap to

regenerate. Seed germination success is best for this species in sites with

adequate moisture and light, i.e., canopy gaps. Seeds are wind dispersed and

survive for long periods in the seed bank, approximately seven years. Hence,

15

these seeds more than likely use a regeneration event quickly when the

opportunity presents itself. Mature trees of yellow poplar are shade tolerant,

and grow to 50-55 m in the canopy. These trees generally die from windthrow,

and standing snags from this species are rare. Yellow poplar provides an

example of a mode of persistence referred to as "gap-phase replacement"

(Shugart 1987). It is important to note that, although species can be assigned to

the roles Shugart described, the effects are not mutually exclusive; hence,

creation of a gap by a species in role 1 can be used as a regeneration niche by

species occupying role 3 in a forest ecosystem, a species that requires a gap for

regeneration but does not create gaps upon its death.

Boreal species at higher latitudes might be affected strongly by the angle

of incident sunlight. At high latitudes, sun angles are elongated relative to the

earth's surface. Thus, shadows cast by standing trees are longer and the area

shaded by trees in this habitat would be larger than trees of the same type found

at lower latitudes. At higher latitudes, the death of solitary trees and the creation

of a small opening in the canopy might not be effective in releasing subdominant

trees in the lower canopy from their competitive disadvantage. Either their death

would not create a gap of sufficient size to affect shade-intolerant species that

require gap creation (role 3), or boreal trees, if shade-tolerant, would neither

create nor require gaps in their regenerative process and could be categorized

as role 4 species.

After studying a range of spatial and temporal scales of disturbances and

the stereogeometry of gaps created by such disturbances in coniferous forests

16

of the PNW, Spies and Franklin (1989) discussed the dynamics of species

interactions during post-disturbance succession. Douglas-fir, a shade-intolerant

pioneer (Swaine and Whitmore 1988; Whitmore 1989), is known to dominate the

canopy after coarse-scale disturbances that open broad patches in the forest

canopy. Douglas-fir does not regenerate well in small, closed gaps where

shade-tolerant species such as western hemlock, western redcedar, and Pacific

silver fir can invade and eventually dominate the canopy.

Fine-scaled disturbance often affect little more than the crown structure of

overstory species (Spies and Franklin 1989), but coarser-scale disturbances

such as root pathogens, wind damage, or pest infestation tend to change

uniform patches in forest canopy created by narrow-crowned, tall species such

as Douglas-fir or broad-crowned shorter species such as hemlock or redcedar,

into a mosaic of openings used as regeneration niches by established seedlings

in the lower canopy. Even shade-tolerant species usually require canopy gaps

to reach the upper strata in old-growth coniferous forests.

Seedling densities (in number m"2) were greater in gaps of both mature

and old-growth forests than in growth under the canopy. In gaps surveyed,

western hemlock seedlings were found, but Douglas-fir seedlings were not found

to be growing in gaps because gap sizes were insufficient for this shade-

intolerant species that requires gaps 300-1000 m2 in size (Spies et al. 1990). In

addition, gaps tended to play a more important role in old-growth forest

regeneration of hemlock. In mature stands, Douglas-fir crowns transmit a

greater amount of light than in old-growth forests because of canopy morphology

17

differences. Crowns were affected by low leaf-areas in younger trees and by

canopy mortality patterns. Hence, increased light availability is rarely due to the

formation of gaps in mature forests, and hemlock, an extremely shade-tolerant

species, is not limited by this low light environment. In old-growth forests, the

canopy is dominated by western hemlock, a broad, closed-canopied species,

and light beneath is extremely limited. Hence, regeneration in old-growth conifer

forests tends to be limited to areas of gap formation.

In the tropics, investigators have studied canopy gap regimes in context

of the intermediate disturbance hypothesis (Levin and Paine 1974; Connell

1978; Terborgh 1992; Terborgh etal. 1996; Vandermeer etal. 1996) that

presents an explanation for the observation that diversity begets stability in

areas that experience low intensity disturbances, but with greater frequency

(Denslow 1980; Brokaw 1985a, 19856,1987; Alvarez-Buy I la and Garcia-Barrios

1991; Forget 1992; Lowman and Moffett 1993; Steege etal. 1994; Collins etal.

1995; Yavitt etal. 1995).

Denslow (1988) suggests that competition between rain forest species

plays an important role in resource partitioning and that limiting resources differ

among forest gaps of different shapes and sizes. Distribution of resources that

affect the dispersal and establishment of competing trees should vary in gaps of

different spatial and temporal stereogeometry. She proposed that it can be

shown empirically how diversity of habitat form in tropical forests, that creates

greater niche diversity, affects organisms with relatively similar requirements to

coexist.

18

The most conspicuous changing environmental parameter in tropical

canopies is light. Brokaw (19855) suggests that light quality is of greater

importance in gap dynamics than changes in light intensity. Experiments

showed germination of Cecropia obtusifolia is stimulated by exposure to high

R/FR ratios. Experimentally alternating red and far red light exposure of

seedlings showed the necessity for long periods of red light and the fast reversal

of red effect by exposure to far red light. Thus, C. obtusifolia seeds can

distinguish between long exposures found in large gaps versus exposure to

short durations of radiation in small gaps or sunflecks.

In Barro Colorado Island tropical forest, Brokaw (1985a) studied tree

regeneration in 30 gaps of different sizes. He determined that large gaps (> 150

m2) differed from smaller gaps in species composition, growth rate, and size-

class distribution. After six years, recruitment of pioneer species was reduced,

and density declined after an initial peak in some large gaps. This study

supported the conclusion that varied sizes of gaps are an important source of

spatial heterogeneity in the dynamics of this forest.

In a study of resource partitioning in a tropical forest in Malaysia, Brown

and Whitmore (1992) refute the importance of gap size in the contribution of

species diversity by virtue of greater heterogeneity. Rather, this research

corroborates earlier work (Pompa et al. 1988; Canham 1989) that reported on

the relative importance of the frequency of disturbance and the duration of

periods of release from suppression of canopy coverage.

Chazdon et al. (1996) summarized data on understory light regimes in

19

tropical forests presented by researchers worldwide. The light environment in

understory canopy varies resulting from attenuation through foliage, reflectance

from surfaces in forest, and penumbral effects, sunflecks, caused by small holes

in the canopy. Mean intensity of light at the forest floor ranges from 5-25 fimol

m"2 s"1 (PPFD), or 1-3% of full-sky values. Median light intensities are lower

because much of the PPFD is due to sunflecks, which provide above 50 jj.mol m"

2 s"1. R/FR ratios are dramatically reduced in canopy shade, from 1.05-1.35 in

full sunlight to 0.21 in dense shade.

Most research on the influence of light intensity and R/FR ratios has

focused on photosynthetic characteristics of plants. But adaptive responses of

plants in light-limited environments do not adequately explain seedling growth

responses in tropical forest understory. Lee et al. (1996) researched the effect

of PPFD and R/FR ratios on six native Asian tree species. In this experimental

design, researchers varied light intensity independent of changes in R/FR ratio.

Seedlings were assessed by increased seedling height, an indication of

competitive release. Also, stem diameter, stem volume, carbohydrate storage

capacity, and dry mass accumulation were correlated with carbon fixation.

Three levels of intensity were used: low (40% PPFD), medium (11 % PPFD), and

high (3% PPFD). At low and medium intensities, light was enriched in red or far

red wavelengths. At high intensities, light was enriched in red only. Lee et al.

(1996) found seedling height was strongly influenced by both intensity and R/FR

ratios, the effect was most pronounced at 11 % PPFD. Changes in stem

diameter were most affected by altered intensities. And allocation of biomass to

20

roots in all species was most affected by red-enriched, high-light intensities.

Low R/FR conditions reduced allocation to leaves, but varying intensities

generally affected biomass allocations more than changes in R/FR ratios.

Taxa in this study were chosen to represent a broad range of shade-

tolerance responses in rain forest environments, from pioneer, shade-intolerant

species to very shade-tolerant species. As expected, shade-intolerant species

responded to reduced R/FR ratios, but developmental responses between

species were varied and conclusions were tentative.

In summary, the complexity of developmental patterns discussed in the

above studies demonstrate of the subtlety of seedling shade responses to forest

light environments. Ecological differences between species indicates that

molecular mechanisms controlling light responses are inconsistent, i.e., the

"electivity" of species' responses are subjected to selective pressures,

enhancing species' plasticity, even among closely related taxa.

To broaden the understanding how altered light environments distributed

in forest understory might affect seedling growth, I undertook this study to

document and describe relative light conditions in two types of wet, montane

forests: temperate and tropical. In this light survey, a snap-shot in time was

assessed for patterns or tendencies in the distribution of color in six artificially-

created conifer canopy gaps of different sizes in Oregon, and in ~160 m2 of

tropical understory in a cloud forest in Venezuela. Comparisons between

latitudes were not addressed in this study because of the inherent difficulties in

maintaining consistency in sampling techniques between locations. Differences

21

in the angle of incident radiation would also need to be standardized for viable

comparisons. In Oregon, seedling survey data collected by Gray (1995) was

used to assess the significance of distribution patterns in color within conifer

canopy gaps.

CHAPTER 2

LIGHT SPECTRA SURVEY IN CANOPY GAPS OF A TEMPERATE

MONTANE CONIFEROUS FOREST

Introduction

Spectral quality has been shown to vary in forest stands of different

species. Freyman (1968) found that quantity and quality of light can differ

between stands of aspen, lodgepole pine, and Douglas-fir. Floyd et al. (1978)

say that spectra become enriched in green (550 nm) wavelengths relative to

either blue (450 nm) or red (625 nm) wavelengths in canopy stands of mixed oak

and poplar, and the proportion of infrared light transmission increases with stand

density. Messier and Bellefleur (1988) showed that transmission characteristics

of pioneer and climax stands of birch-beech-sugar maple stands differ in light

quantity and quality on sunny versus cloudy days. It has been well established

that changes in the ratio of red and far red light (R/FR) are to be expected in

shade conditions (Franklin and Letendre 1978; Morgan 1981; Mitchell and

Woodward 1988; Kozlowski et al. 1991).

Studies of light intensity and the effect of shade on conifer species

establishment has been discussed by several researchers (Franklin 1963; Spies

et al., 1990; Spies and Franklin 1989,1991). In a comparative study between

sugar maple foliage and red pine canopy, conifers had greater transmission

23

capabilities (Vezina and Boulter 1966). But information about the spectral

distribution of color in a conifer forest canopy gap was absent from the literature.

In this chapter, I document and discuss the spatial patterns of

physiologically-active wavelengths of light in gaps of various sizes, artificially

created in a conifer forest in the PNW. I predicted the following relationships

would be found in the surveyed gaps. Where shading was greatest in the gap, I

anticipated finding lower red and blue percent transmissions, and greater R/FR

ratios. I further conjectured that these patterns would be more evident in the

smaller gaps, because of greater canopy closure.

In Chapter 3, I related these patterns to the observed distributions of

established seedlings of dominant trees which were surveyed by Gray (1995) at

sites along the axes where light data were recorded.

Methods

Site Description

H. J. Andrews Experimental Forest is located 44° 15' N, 122° 15' W in the

Blue River Ranger District of the Willamette National Forest, about 50 miles east

of Eugene, Oregon in the Pacific northwest (PNW). The forest ranges in

elevation from 410 m (1350 feet) to 1630 m (5340 feet). The area contains

approximately 6,400 ha (15,800 acres) of natural forest ecosystems which were

first set aside in 1948 as a research area, and later became one of the 15 major

ecosystems research sites in the US funded through NSFs Long-Term

Ecological Research (LTER) Program.

24

The PNW climate has wet, mild winters and dry, cool summers. In

Andrews at lower elevations (430 m), mean monthly temperatures range from

-17 °C in January to 18 °C in July (USDA Forest Service 1988). Mean annual

precipitation varies with elevation. Snow is more common at higher elevations

than lower elevations. Rainfall ranges from 230 cm at lower elevations to 355 cm

at upper elevations. This rainfall is received mainly from November through

March.

The site is characterized as an old-growth coniferous forest (> 400 y;

USDA Forest Service 1988). The common species of conifers in the lower

elevation forest stands are Douglas-fir (Pseudotsuga menziesii), western

hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata). At upper

elevations are found the noble fir (Abies procera\ a shade-intolerant species),

the Pacific silver fir (Abies amabilis; a shade-tolerant species) and also the

western hemlock and Douglas-fir. After periodic disturbances such as fire,

Douglas-fir is the dominant climax species and develops even-aged, pure

stands. Typical old-growth stands, however, usually exhibit co-dominance

between Douglas-fir and western hemlock. Wildfire has been the natural

disturbance in these old-growth forests. Small-scale disturbances are wind

throw, landslides, pests, and erosion.

25

Study Area

Four sizes of experimental gaps were created in 1990 by a team of

researchers at Oregon State University for a study of tree seedling

establishment in forest gaps (Gray 1995). All gaps possessed the same

southwestern aspect with slopes not exceeding 20%. Gap diameters were

scaled to the average height of trees in the stand. The gap sizes designated by

the gap diameter to tree height ratios were 0.2, 0.4, 0.6, and 1.0. All gaps were

replicated once, with replicated gaps labeled 1 and 2, respectively. The larger

gaps were 50 m in diameter. The remaining gaps were created in proportion to

the size of the largest gap. Gaps of size 0.6 were 60% of the 50 m-diameter,

giving them a diameter of 30 m. Gaps of size 0.4 were 40% of the largest gap,

giving them a diameter of 20 m. Gaps of 20 m most nearly approximated

naturally-occurring gaps in this stand of trees (Gray 1995). Gaps of size 0.2

were not used in this study.

Data Collection and Management

Data on spectral characteristics of light were collected using an Ocean

Optics SD-1000 fiber optic spectrometer, a lightweight and portable field

instrument which measures the electromagnetic spectrum over a UV-visible-NIR

range of 275 nm to 775 nm. The instrument has a spectral bandwidth resolution

(FWHM; full width at half the maximum peak) of 10 nm when used with an 0.2

mm diameter fiber, with a sampling interval of 0.5 nm (specification provided by

the manufacturer). All fibers are 0.22 nominal aperture (NA), and when bare-

26

ended, had a 25 degree field-of-view.

Two data channels collect simultaneous readings from separate optical

fibers conveying light to identical spectral optical units. The master channel was

established as a "reference" fiber which was placed in an area that maximized

an unobstructed view of full sky, i.e., the center of the gap.

The reference light reading was taken in each gap using a bare-ended

optical fiber with a 25 degree field of view attached to an extendable pole to

capture full incident radiation impinging the forest gap. Reference light readings

were taken simultaneously with the sample reading to calculate a percent of light

transmission of the relative incident light received by the gap. In addition, using

a Protomatic PAR light meter, full solar radiation (in foot candles) was measured

in each gap at the same time each light reading was taken in the gap. All light

sampling was completed in a four-hour solar noon window from 1100 h to 1500

h. The light measurements taken at any point along the vegetation transects in

this time frame are best comparable because the sun is at its zenith or nearly so

(Anderson 1966; Evans 1966; Russell etal. 1989).

Sequentially, the height of the pole supporting the reference fiber at the

center of the gaps was altered to be proportional to the difference in the gap's

diameter relative to the largest gap of 50 m. This "reference" pole was set at a

3.0 m height in the 50 m-diameter gap. In the 30 m-diameter gap, the reference

pole was set at 5 m height, and in the smallest gap, at 20 m-diameter, the

reference pole was extended to 7.5 m. The length of the reference pole

27

increased in smaller gaps to compensate for decreased canopy opening.

Assuming an idealized gap geometry and an acceptance angle of a = 25°

for the bare-ended reference fiber, it can be estimated that for a canopy average

height of 60 m, a minimum gap diameter of 60 tan (a / 2) = 28 m was required for

an unobstructed view. Therefore, no obstruction occurred in the mid-sized or

largest gaps. But, in the smaller gap size, 0.04, a proportion of the fiber's field of

view was obstructed. It is possible to estimate what proportion of clear-sky

obstruction occurred in the gaps by estimating the height at which the fiber's

field of view became obstructed and assuming again that the trees bordering the

gaps were on average 60 m in height. In the 20 m gaps, approximately 49% of

the field-of-view was obstructed by the trees at the edge of the gap. It could be

possible to correct reference readings in 20 m gaps using vertical profile light

readings taken from the dirigible (described in a forthcoming section). However,

I proceeded with the analyses in all gaps without adjusting the references in the

two smallest gaps. Future research could be conducted to develop a reference

correction from vertical profiles.

Light to the slave channel was delivered by a "sample" fiber which was

carried to points along the plot axes where downward light spectra were

measured at 1 m height in the gap. Both channels were set up by connections

to two fibers linked by aperture-controlled shutters. The shutter system allows

the operator to close the spectrometer for dark references, measurements which

are subtracted from data to correct for the background electronic noise of the

28

instrument. The reference fiber was a 50 m long with a diameter of 0.2 mm. The

sample fiber was 70 m long with a diameter of 0.2 mm. Both sample and

reference fibers and their shutters were connected to the spectrometer using 1 m

long fibers of 0.2 mm diameter which defined the spectral resolution on both

channels. In every gap, light measurements were taken at a 1 m height in the

gap with a cosine-corrected, hemispherical sensor connected to the sample fiber

optic.

Data files were collected and stored in binary format for compressed file

management until they were processed from binary to ascii format for statistical

and graphical pattern analyses. Percent transmission of light through the

canopy was calculated as sample spectral values divided by reference spectral

values.

Sample Plots and One-meter Ground Measurements

At H. J. Andrews forest, an existing grid system was laid out in a north to

south and east to west direction and marked with PVC stakes at 2 m increments

using a plot-grid method (Gray 1995). I conducted my light surveys in three of

the four sizes of created gaps and used the existing grid pattern for my sampling

design. The designation for all gaps in which light was surveyed are 104, 204,

106, 206, 110, and 210. In the 20 m-diameter gaps (gaps 104 and 204), light

measurements were taken every 4 m along cardinal axes, north to south and

east to west. In the 30 m-diameter gaps (gaps 106 and 206), light

measurements were taken every 6 m along the axes. And in the 50 m-diameter

29

gaps (gaps 110 and 210), light was sampled every 10 m. Increments were

chosen to remain proportional to the size of each gap as denoted by the names

of each size gap in the series; 0.4, 0.6, and 1.0 (Figure 1).

Sunlight was present longest at the eastern end of the east to west axis in

most gaps. Last to receive afternoon sun was the southern edge of the north to

south axis. For example, in gap 104 the sun entered the gap at 90 degrees E at

1200 h and exited the gap at 204 degrees SW between 1500-1630 h (Table 1).

All compass readings were uncorrected to degrees declination and were read to

magnetic north.

Table 1. Angle of entry and exit of the sun along the perimeter of three surveyed forest gaps. No correction to true north was made.

Gap Entry Time Exit Time

radians hours radians hours

104 90 E 1200 204 SW 1630

106 101 SE 1115 200 SW 1630

210 81 NE 1030 205 SW 1630

Notations were made in the field regarding object obstructions in the path

of downward light. This scheme applies to all figures representing light

distributions in all gaps, and the presence of noted obstructions in red light

figures apply to all forthcoming discussions of color distributions and spectral

values recorded in these gaps.

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Vertical Light Profiles and Attenuation Coefficients

A vertical profile of light transmission was measured in a 20 m and 30 m

gap using a helium dirigible. A single-fiber optic with cosine-corrected sensor

attached was secured to the balloon in such a manner as to collect downward

directional light. Measurements were recorded as the balloon was raised in 5 m

increments to a total height of 50 m in gap 104 and a total height of 35 m in gap

106.

Spectral values from an unreferenced sample channel were used in the

vertical light profiles. The data were smoothed by applying a seven-point moving

average to avoid enhancing the high-frequency noise inherent in the sample in

the forthcoming derivative analyses. Each spectral value along the spectral axis

was averaged by the three values both before and after the central point (x3) for

a seven-point average.

„ , '(*o) +'(*i)+'(*2)+'(xs)+'(x4>+t(x5) +t(x6) ... A*3)= (1)

where f(xj is the filtered spectral values at x3 and t(x) is the spectral value at x,.

The next series of values centers on x4) Xg, and so on. The smoothed spectral

data were used in all further analyses of vertical profiles.

The following function gives the derivative of f(x) at a center point x3 using

a seven-point approximation

32

4F(X2) -J(XQ)+9AXX)-45/X2) +45(/X4)-9J(Xs) +F(x6)

dk 60 h (2)

where h is the wavelength (nm) interval between calculation points (Kelly 1967).

The derivative of spectral values was plotted versus wavelength for in-depth,

visual exploratory analysis of specific spectral distributions.

Spectra of light in the 300 nm - 750 nm range were used in comparisons

of light at certain heights in the canopy, and in comparisons within and between

plots. Also, comparisons were made using specific values of actinic

wavelengths of red (660 nm), far red (730 nm) and blue (430) light. Percent

transmission values were averaged ± 5 nm about the action wavelengths listed

above.

There are two approaches to relating how light changes as it passes

through plant canopy (Anderson 1966). One approach is to vertically integrate

and quantify the occurring absorption of light just above the canopy floor. The

other is to estimate the availability of light at points along the plant canopy

expanse. The latter method allows for ecological evaluation of how the light

factor affects the establishment of vegetation distributed throughout the plant

canopy, e.g., epiphytic growth or changes in leaf area indices (LAI) of tree

crowns.

Monochromatic light is believed to be absorbed exponentially, following

Beer's law, as it passes through the atmosphere and canopy (Collingborne

33

1966). Vertical attenuation coefficients (K) were calculated for red (660 nm), far

red (730 nm), and blue (430 nm) wavelengths ( X ) at each canopy height by

applying Beer's Law,

InL-lnl (3)

where K is the attenuation coefficient, I0 is the incident light at the top of the slice

of thickness Z, and I2 is the transmitted light at the bottom of the slice (Figure 2).

Attenuation coefficients were plotted versus canopy height to visually assess

where in the canopy light was most greatly attenuated.

Graphical Presentations

Maximum values were recorded in Table 2. The distribution of light across

primary axes in gaps was assessed across gap sizes. Certain sites in the gaps

received high levels of percent transmission relative to other sites in the gap.

Large ranges in percent transmission presented a problem in standardizing axes

for cross-gap comparisons. Data were normalized by the division of every value

in the gap by the maximal value, returning values ranging uniformly between 0.0

and 1.0.

34

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Table 2. Maximum transmissii R/FR ratio values in all ga

on and ps.

Diameter (m) Gap1 % Red % Far Red % Blue R/FR ratio

20 104 0.03 0.03 0.12 8.38

30 106 94.52 191.21 57.60 0.92

50 110 390.22 701.60 69.91 1.61

20 204 0.11 0.18 0.52 1.28

30 206 21.1 28.59 10.66 0.79

50 210 45.69 64.48 27.97 1.09 t Caveat: Gap 110 data are suspect because a different computer program was used to collect values in this gap versus remaining gaps.

Data were presented graphically by gap and also as averages between

two replicate gaps to simplify the comparison of the distribution of light across

the primary axes in the gaps. Because axis ordinates differed between gap

sizes, axes used with averaged data were standardized to facilitate direct

comparisons between sites on each axis. Spectral maps were composed to

assess patterns or trends in the distribution of light in forest gaps of different

sizes. Normalized data were used in these graphical presentations. Also,

maximal values were presented graphically for comparison across all gap sizes.

Results

One-meter Ground Measurements

Figure 3 shows the distribution of the maximal values reported in Table 2

for comparison across gap sizes. R/FR ratio values are read on the secondary

36

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37

axis. There was a clearly increasing trend in maximum values with increasing

gap size. Differences between replicate gaps were also evident. Percent

transmission for red light (1R and 2R) showed large differences for gaps of the

same size.

The following figures show comparisons of light in different gap sizes

across two primary gap axes, north to south and east to west. Along the north to

south axis, all gap sizes received greater average percent transmissions in red

light toward the northern axis (Figure 4). Along the east to west axis, red light

transmission was more irregular but showed increased values toward the center

for all gap sizes.

The distribution of average far red transmissions was similar to red

transmissions along both the north to south and east to west axes in all gaps

(Figure 5). Patterns were complicated for R/FR ratios. Along the north to south

axis, the trend of R/FR ratios was similar between the 30 m and 50 m gaps, but

R/FR ratios switched and were greatest in the north axis of the 30 m-diameter

gaps (Figure 6). Patterns were bimodal for the 50 m gap; peaks occurred in

both sides of the axes. The smaller gaps generally showed greater R/FR ratios

along the west axis.

The distribution of blue light transmission along gap axes showed a clear

pattern in 20 m-diameter gaps, an increasing trend from one side of the gap to

the other was evident along both cardinal axes (Figure 7).

Distribution maps for all wavelengths in one 20 m-diameter gap, gap 104,

38

Figure 4. Distribution of red light averaged between gap replicates (n=2) by size. Data are standardized to the maximum percent transmission value in each individual gap before averaging.

Distribution of red light All gaps avg by size

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 Fraction of diameter N(+) to S(-)

Distribution of red light All gaps avg by size

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39

Figure 5. Distribution of far red light averaged between gap replicates (n=2) by size. Data are standardized to the maximum percent transmission value in each individual gap before averaging.

Distribution of far red light All gaps avg by size

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Figure 6. Distribution of RFR ratios averaged between gap replicates (n=2) by size. Data are standardized to the maximum percent transmission value in each individual gap before averaging.

Distribution of RFR ratios All gaps avg by size

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Figure 7. Distribution of blue light averaged between gap replicates (n=2) by size. Data are standardized to the maximum percent transmission value in each individual gap before averaging.

Distribution of blue light All gaps avg by size

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42

were compared in Figure 8. Similar patterns were observed for red and blue

light along cardinal axes. Comparing far red transmissions and R/FR ratios, an

inverse pattern was observed. Where far red transmissions were low, along the

west and south plot axes, higher R/FR ratios were seen. Transmission

differences were apparently greater in these areas of the gap than where both

red and far red transmissions were high, along the north and east plot axes.

In the second 20 m gap, gap 204, patterns were more complex (Figure 9).

Red and far red transmissions were low along the west axis, but R/FR ratios

were no higher along this axis than others in the gap. Blue transmissions were

uniform, peaking near the center of the gap and declining toward the axes

boundaries, as was also shown in gap 104 (20 m).

In 30 m and 50 m gaps, percent transmission values were distinctly lower

than was found in gaps 20 m in diameter. In the 30 m gaps, few points along the

cardinal axes showed peaks in transmission, and were generally near the center

of the gaps, except for peaks in all wavelengths along the west axis in gap 206

(Figures 10-11). No obstruction or nearby debris in the gap was noted at this

site. And in both gaps, although red and far red transmissions were low,

patterns in R/FR ratios were pronounced, peaking near the center of the gaps

and declining toward the gap edges.

In gap 110, red, far red, and blue transmissions were more pronounced

along the west axis (Figure 12). In gap 210, peaks were observed in red, far

red, and blue wavelengths near the center of the gap, and extended further

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48

along the north axis (Figure 13). R/FR ratios were more pronounced along the

north to south axes, but were also evident along the east to west axis.

Vertical Light Profiles and Attenuation Coefficients

Figures 14 a and b show the patterns of digital numbers or called

hereafter spectral values (SV) for the full spectrum of light at each height in a

vertical profile in a 20 m-diameter forest opening, gap 104. From 20-50 m

heights, the shape of the spectrum changed only slightly, with a decrease below

8 SV at the peak of the spectrum. From 5-15 m heights, light transmission was

severely curtailed, and values measured were less than 1 SV overall.

At the 50 m height, eight or nine peaks were (see numbered peaks in

Figure 15). These peaks were more greatly attenuated as light moved through

the canopy gap opening. Also, it appears that peaks shifted toward the blue

wavelengths by approximately 25 nm. Peak 1 at 50 m heights was at

approximately 390-400 nm. At the 5 m height, peak 1 was at 375 nm. This shift

is confirmed by the position of peaks 2 and 3 at 50 m versus at 5 m heights.

Extinction coefficients of red, far red, and blue spectral values in gap 104

are shown in Figure 16. The pattern of attenuation at 5 m increments changed

sharply between the 10 and 20 m heights, but remained consistent throughout

the rest of the profile. Hence, from the 45 m to 20 m heights in the canopy, light

was attenuated exponentially, and agrees with the assumptions of a

homogeneous light path which validates the application of Beer's law as it

applies to light transmission through forest canopy gaps.

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54

Figure 17 shows how light changed vertically in a profile of a 20 m-diameter gap.

Data are spectral values; light was not referenced in this profile. Blue light was

the most prominent color throughout the canopy gap profile. Next was red, then

far red light. Spectral values were greater with increased height in the canopy.

At 15 m, all light values fell below 1 SV.

In a 30 m gap, the trend in distribution of color throughout the profile was

similar to trends observed in the 20 m gap, but the magnitude of the spectral

values differed (Figure 18). Whereas blue light (430 nm) reached a magnitude

greater than 6.5 SV in gap 104, blue light only attained 4.5 SV or less in gap

106. The magnitudes of red and far red light were comparable between 20 m

and 30 m gaps.

Comparison of peaks in the derivatives of spectra from 30 m and 5 m

heights in gap 106 showed peaks and trends were very similar, though the

magnitude declined with depth in the canopy gap (Figure 19). Derivatives of

spectra accentuate changes in the peaks and troughs of the distribution of light.

Peaks numbered 1 and 2 were clearly identifiable at both heights in the gap.

Even a diminutive peak numbered 3 on the down-shoulder of peak 2 was

evident. There was no indication of a spectral shift toward the blue or red end of

the spectrum when spectra were compared. Peaks and troughs coincide nicely,

especially the troughs between peaks numbered 10, 11 and 12.

Extinction coefficients in gap 106 were relatively uniform throughout the

plant canopy light profile (Figure 20). Attenuation differences between heights

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were small (< 0.2 m"1), and Beer's law of exponential extinction of light holds true

for this conifer light gap.

Figure 21 nicely confirmed the pattern seen in the extinction profile in

gap 106. Light attenuation was uniform at all heights in the canopy, except

between 5 m and 10 m. Light was attenuated for both red and far red light and

less so for blue light at the floor of the canopy gap.

Discussion

There were distinct differences in patterns of both horizontal light

distribution and vertical profiles of light in canopy gaps of different sizes. Trends

were not repeatable between replicated gaps, making predictions about changes

in light distribution patterns difficult based only on gap size. The heterogeneous

nature of resources in any single gap make comparisons difficult between

different sites in a forest, and between gaps of similar size and shape. Gray

(1995), in a study conducted at the same sites as this light survey, attributed the

successful establishment of conifer species to the interaction of several

environmental parameters, such as temperature and moisture, on seedling

establishment. Predictions of seedling establishment based on the relation of

species' shade-tolerances, and gap sizes were not supported in his study.

I hypothesized that where shading was greatest in the gap, lower red and

blue percent transmissions, and greater R/FR ratios would be found. Percent

transmission values estimated where wavelengths were distributed in relation to

full-sky values. In most cases, percent transmission of colors was greatest near

60

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61

the center of gaps, dependent on percent of canopy closure. However, there

were trends that showed lower R/FR ratios in shaded portions of smaller, 20 m

gaps, which supports my second conjecture, that trends associated with gap

shading would be more evident in smaller diameter gaps.

Many previous studies measured light using photon fluence rates. In this

study, changes in light distribution were described using the transmission of light

referenced to full-sky above the canopy. Anderson (1966) states that percent

transmission can underestimate the amount of light actually reaching the canopy

floor. However, simultaneous readings of open sky and "under" canopy should

be more efficient in determining patterns of light distribution in an instantaneous

measurement by minimizing the effect of rapidly changing sky conditions while

taking light surveys.

There are few references in the literature of R/FR ratios in shade under

coniferous evergreen canopies, and none were found for species specific to this

study. Examples of R/FR ratios (using photon fluence ratios) for shade of other

conifer species under clear skies are as follows: spruce 0.15, 0.33; red pine

0.47-0.76, 0.33; white pine 0.25-0.26; and, jack pine 0.32 (Kozlowski et al.

1991). The present study in canopy gaps of primarily Douglas-fir and western

hemlock, R/FR ratios agreed well with those listed above, ranging below a ratio

of 2 in all cases except gap 104 (keeping in mind these readings were taken in a

canopy gap and those listed above were taken in the canopy shade of the

species noted).

62

It is generally assumed that light in a forest canopy is attenuated

exponentially according to Beer's law (Kozlowski etal. 1991) and that a forest

canopy has a photohomeostatic effect, distributing light uniformly throughout the

canopy by transmission and reflectance of light (Larcher 1995). The amount of

incident radiation absorbed is relative to the number of absorbing molecules

along the light path. Beer's law holds true for dilute concentrations of pigments

but fails at increased concentrations of molecules (Salisbury and Ross 1985).

According to the definition discussed above, each layer in the forest

canopy of equal thickness, 5 m, should absorb an equal fraction of light. These

data indicated that the coefficients of extinction were not uniform throughout the

entire canopy-gap light profile. Light did not attenuate uniformly as Beer's law

assumes it must. The increased attenuation of light at distinct strata in the

canopy gap could be due to the stratification in the canopy around gaps.

However, the light continuum in canopy gaps is greatly influenced by the

surrounding border of trees.

The sun is at a lower angle in the northern latitudes and light can

penetrate farther beneath the trees along the southern edge of the gaps (Oliver

and Larson 1996). Light from the edge of a forest stand has been observed to

influence vegetation structure as far as 120 m into the forest stand. Light

photons are spread across a greater area in northern latitudes than areas near

the equator. At northern latitudes, the intensity of light is greatly reduced due to

the increased angle of incidence of light approaching the earth. This increased

63

angle may be related to the increased attenuation of light further into the canopy

gap. Light near the gap floor would be most rapidly attenuated not because of

increased absorbance of light in the gap column but rather because of the

absorbance of light as it cuts a path through a longer portion of the adjacent

stand of trees before reaching some slice in the gap column.

Percent transmissions of these active wavelengths were often below 1 %.

However, it is not currently well understood at what percent of full incident light

actinic wavelengths can have their physiological effect on plants in their natural

environments. Photosynthetic photon flux density (PPFD) is an important

component of the light response of plants in gaps. Intensity of light (PPFD) is

most often strongly correlated to successful establishment of plants in gaps.

However, the in vivo effect of the color component of light in synchrony with the

energy component of light is a little understood facet of photobiology and is an

important focus of this study. In Franklin's (1963) study, shading was shown to

be highly significantly correlated to density of seedling establishment in gaps.

Although not quantified in his study, the effect of shade should be qualitatively

equivalent to an assumed shift in spectral quality, and responses were not

attributable solely due to a quantitative shift in solar energy (PPFD).

In his 1993 paper, Endler discussed the evolutionary effects of forest

spectral composition. He proposed that light environments can be determined

from the geometry of light paths as light intercepts or is transmitted or reflected

by objects in the forest. He reports that various sources of radiance in the

64

environment contribute to the overall effect of color in forest canopy

environments. He delimits four categories that contribute characteristic

radiances to forest environments: forest shade, woodland shade, small gaps,

and large gaps. The angle subtended by these light sources contributes very

strongly to the ambient light.

Gaps are defined as "patches of sunlight" and can range from very small

flecks of sunlight to very large patches of light due to gaps in the canopy. In

small gaps, the influence of solar radiance should dominate the spectrum and

light would be yellowish-brown. In the smaller gap 104, at the 5 m height, light

was distributed between blue (430 nm) and yellow (575 nm) wavelengths. This

would indicate that the coloration would be mostly bluish-greenish, according to

the relative responses; less than 1.5 nm"1 at this range of wavelengths. The

same pattern is evident in gap 106, but the relative responses were lower than

were found in gap 104, ranging less than 0.1 nm*1.

Spectral shifts in the vertical distribution of light were evident in the center

of gaps 104 and 106. Although full sunlight at 50 m height in the gap had strong

peaks throughout the spectral range of wavelengths (300-750 nm), even at 5 m

the spectral values were greatly diminished, especially above 450 nm. In

addition, a slight shift in wavelength peaks was detected between spectra at 50

m and 5 m heights in the 20 m forest gap. The shift was toward the blue

wavelengths. Hence, blue light might play an important role in the inhibition of

seed germination and the growth of seedlings in gaps of 20-30 m. It has been

65

speculated that cryptochrome or blue absorbing pigment might act to accentuate

the phytochrome response at the cellular level, creating an additive response in

plants to blue, red, and far red light (Mohr and Drumm-Herrel 1983; Mancinelli

1989; Elmlinger et at. 1994).

CHAPTER 3

LIGHT SPECTRA IN CANOPY GAPS AND TREE SEEDLING

ESTABLISHMENT IN A TEMPERATE MONTANE

CONIFEROUS FOREST

Introduction

It was the premise of this study to explore the permutations of changing

patterns of spectral light at important sites of seedling regeneration, e.g., forest

gaps. This research was designed with the objective of exploring the hypothesis

that seedling establishment and growth are strongly affected by changes in light

quality in canopy gaps. I hypothesized that patterns in the distribution of light

quality in a forest gap might greatly define the regeneration strategy of

colonizing species. Specifically, I anticipated finding greater seedling

germination and growth of shade-intolerant species in areas of greater percent

transmissions in red light. In areas where greater R/FR ratios would be found, I

proposed to find better germination and growth of gap specialists or shade-

tolerant species. Lastly, I proposed to investigate how blue light related to the

successful establishment of conifer species

Methods

Site Description and Study Area

This study was conducted in the same site described in Chapter 2. See

67

the description in that chapter.

Seedling Survey

Data from a seedling survey in forest gaps completed in the summer of

1995 were provided by Gray (1995) as part of a collaborative effort between

Oregon State University and University of North Texas. Natural regeneration

tree species was surveyed by OSU as part of an ongoing gap dynamics project

in northwestern boreal forest stands. Seedling data were reported along the

cardinal axes and in the quadrat areas of each gap. To correlated the

quantitative seedling data in gaps with the light survey completed in my study, I

grouped the seedling data from areas adjacent to points on the cardinal axes

where light data were taken. In gaps 104 and 204, the area around each point

on each axis was 4 m by 4 m. In gaps 106 and 206, the area around each point

was 6 m by 6 m. And in gaps 110 and 210, the area around each point on the

axes was 10 m by 10 m. The number of established seedlings and seedling

basal area (in 0.1 mm increments) were recorded in the gaps by surveying the

0.25 m2 established vegetation plots in each gap. Total basal areas were

calculated by multiplying the number of seedlings found in a basal area

category; for example, if 3 seedlings were surveyed with 0.2 mm basal

diameters, their total basal area was 0.6 mm. Total basal areas were used in all

assessments and statistical analyses.

Spectral Data Collection and Management

Spectral data collection was described in Chapter 2.

68

PAR in Gaps

Concurrent to spectral readings taken in the gaps, photometric data were

recorded in the center of each gap during the spectral light survey using a field

photometer. Photometers measure light in foot candles (fc) across the

photosynthetically-active wavelengths (PAR; 400-700 nm). Photometer readings

were converted to photosynthetic photon flux density (PPFD) by converting foot

candles to lux (fc / 0.09 = lux). Lux was then converted to PPFD (lux / 51.28205

= micro Einsteins m"2 s"1). PAR was recorded to account for differences in

spectral energy between gaps and during the period of the day surveys were

completed.

Statistical Analyses

One-way analyses of variance were employed to describe differences

between average numbers of seedlings and total seedling basal areas (cm2) in a

gap. Differences between species were assessed by axes in different gap sizes.

Also, differences between axes for each species in a gap was examined.

The effect of actinic light, 1 m above ground level, on seedling

establishment was assessed by regressing seedling species' data, number of

seedlings and seedling basal area (cm), and physiologically-active spectral light;

red (655-665 nm), far red (725-735 nm), blue (425-435 nm), and R/FR ratios.

Linear least-squares regressions (LSR) were used to determine the relationship

between seedling species and species summed as they were distributed along

axes positions in gaps (Zar 1984; SAS Institute Inc. 1990). In addition, LSRs

69

were used to show relationships between seedling data and light in different gap

sizes. Step wise multiple regressions were employed to determine the best

model for seedling-light relationships. Regressions were conducted first on both

numbers of seedlings and on seedling basal areas irrespective of gap size, then

on numbers of seedlings and on seedling basal areas using gap size classes.

Results

PAR in Gaps

Immediately evident is the trend that show the length of time spent

sampling in a gap is proportional to the size of the gap: approximately 1 h in 20

m gaps (gaps 104 and 204); 2 h in 30 m gaps (gaps 106 and 206); and about 3 h

in 50 m gaps (gaps 110 and 210; Figure 1). Gap 110 was not reported because

of technical difficulties taking readings in this gap. PAR at the forest floor in the

center of the gaps was affected by canopy closure differences resulting from the

size of the gaps. In the 20 m diameter gaps, light was below 300 nE m"2 s"1. In

the 30 m gaps, light ranged between 150-1600 \iE m"2 s. Brief reductions in light

were due to clouds passing over the sites. In gap 106, skies became overcast

toward the end of the sampling time. Gaps of 50 m diameter showed light in

these gaps was more consistent and was of greater intensity, ranging from 600-

1600 nE m"2 s. This was most likely resulted from the greater gap opening which

allowed more light to reach the center of the gap for longer periods of the day.

Distribution of Light and Seedlings

Caveat ad nauseam: The proximity of nearby vegetation or overhead

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canopy coverage greatly influenced the resulting light survey. While taking light

readings, transmission of light might have been affected by the proximity of small

shrubs, Viney maples, (Acer circinatum) or by tree limbs or downed stumps

located in the gaps. Although notes of these obstructions were made during the

field survey, supplemental information was obtained from hemispherical photos

taken at each point on gap axes in all gaps surveyed for light.

Number of Seedlings.- Distribution patterns of numbers of seedlings of

different species, Douglas-fir, western Redcedar, and western Hemlock, were

assessed with respect to patterns in spectral light in forest gaps of different

sizes: 20 m, 30 m, and 50 m diameters. Replicates of gap sizes were extremely

heterogeneous and thus, results in gaps were treated as individual samples and

not replicates of gap sizes. The discussion of results reflects this treatment of

the data.

Along north to south and east to west transects in 20 m gaps, less than 30

individuals of any species were found at any point (Figure 2). Douglas-fir did not

occur with great frequency except along the south axis in gap 204. Although, in

gap 104, seedlings were found distributed near the center, in gap 204 seedlings

were found mostly along the east axis or uniformly across the entire north to

south axis. Along every axis, blue light transmission exceeded red and far red

light transmission. Where blue light transmission peaked, and red and far red

light remained low, the occurrence of seedlings coincided in these gaps.

The distribution of R/FR ratios correspond well to the distribution patterns

72

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73

of seedlings in the smaller gaps, especially along north to south axes (Figure 3).

In gap 204, ratios were uniformly distributed along the north to south axis, and

the distribution of seedlings was similar. When R/FR ratios peaked in the south

axis in gap 104, seedling numbers were similarly high.

In 30 m gaps, light transmissions peaked near the center of the gaps,

except for peaks which occurred along the east axis in gap 106 and the west

axis in gap 206 (Figure 4). In these gaps, far red light transmissions exceeded

red, then blue transmissions. In smaller gaps, blue light showed greater

transmissions (Figure 2). Seedlings occurred generally near the center of the

gap, except in gap 106 where > 70 western Hemlock seedlings occurred along

the north axis where all light transmissions were greatly reduced.

Distributions of R/FR ratios in 30 m gaps were not clearly indicative of the

distribution of seedlings in these gaps (Figure 5). Greater numbers of seedlings

were predicted where R/FR ratio transmissions peaked. This was not evident

anywhere except with an increasing trend along the east to west transect of gap

106.

In the largest gaps, far red light transmissions were slightly greater

everywhere except along the north to south transect in gap 110 (Figure 6). In

gap 110, blue light transmission was extremely reduced relative to red and far

red light transmission. In gap 210 along the north to south transect, seedling

numbers were low where light transmissions peaked. However, along the east

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78

to west axis, seedling numbers and light transmission peaks coincided.

Transmission ratios (R/FR) were not indicative of seedling distributions in

gaps 110 and 210 (50 m gaps; Figure 7). Peaks in seedling distribution

occurred near the center of the gaps, but light distribution patterns varied and

did not indicate any patterns in seedling distribution. There were no statistically

significant differences in the distribution of numbers of individuals by species

along axes in different gap sizes (ANOVA; p > 0.05). Thus, no one axis showed

any difference in numbers of individuals along the axes in any gap. Neither was

any species found in significantly greater numbers along a specific axis

orientation in any gap size (ANOVA; p > 0.05).

Seedling Basal Area.- Distribution patterns of seedling basal.areas (cm2)

of different species, Douglas-fir, western Redcedar, and western Hemlock, were

assessed with respect to patterns in spectral light in forest gaps of different

sizes: 20 m, 30 m, and 50 m diameters. Light patterns in this series of figures

are identical to those in figures showing relationships between numbers of

seedlings and light above. Therefore, the discussion of these figures has been

restricted to the distribution of basal areas and relative light patterns in these

gaps. Seedling basal areas were indicative of the successful establishment and

growth of species where distributed in all gaps.

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gaps along all axes but the east to west axis in gap 104 (Figure 8). Along the

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81

north to south transect in gap 104, there were many small individuals (< 1 cm2;

refer to Figure 2). Distributions of R/FR ratios were variable, and patterns

associated with seedling basal areas were difficult to assess (Figure 9).

In gap 206, western Hemlock seedlings obtained greater basal areas

where light transmissions were reduced (Figure 10). This appeared to be the

case also on the north axis of gap 106 where western Hemlock basal areas were

greater than 15 cm2.

R/FR ratios peaked near the center of 30 m gaps; however, any pattern of

increase in seedling basal areas was not consistently associated with those

peaks (Figure 11). Only at the center of the east to west transect were basal

areas associated with a peak in R/FR ratios.

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distributed with percent transmissions of actinic wavelengths (red, far red, and

blue light; Figure 12). At the center of gap 210, western Hemlock basal areas

peaked at a point adjacent to a peak in light transmission. Because of the

variable nature of light and the limitations of snap-shots of light in these dynamic

systems, the proximity of these peaks to sites of the increased basal areas

might explain some of the variability at sites such as this one.

Associations between changes in R/FR ratios along the cardinal axes in

50 m gaps and changes in seedling basal areas were difficult to assess from the

variable trends such as those observed in Figure 13. Relationships along these

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87

axes were made clearer after linear regression analyses such as those

discussed in the following sections.

There were no statistically significant differences in total basal areas by

species along axes in different gap sizes (ANOVA; p > 0.05). Thus, no one axis

showed any difference in average total species basal areas along the axes in

any gap. Nor was any species found to be significantly larger (total basal areas)

when found along a specific axis orientation in any gap size (ANOVA; p > 0.05).

Regressions of Seedling Establishment and Light Quality

Analyses by Gap Size. - Significant results were reported at a = 0.05, and

highly significant results were reported at a = 0.001. All significant relationships

were positive. Seedling basal areas, relative to numbers of seedlings, appeared

to be more sensitive to changes in light quality; thus, only significant

relationships concerning seedling basal areas were reported in the body of the

report. Refer to Appendix for exhaustive results of the individual least-squares

regressions relating either seedling number or seedling basal area (cm2) to light

measurements taken in individual forest gaps (sample sizes = 8).

The following distribution maps indicate graphically the location of

significant relationships for Douglas-fir (D), western Redcedar (R), or western

Hemlock (H) by gap size (20 m, 30 m, or 50 m). Regression coefficients (R2)

were reported as percents (R2 x 100) in parentheses.

In Figure 14, no significant relationships between species basal areas

88

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and changes in spectral light along axes in 20 m gaps were found. In 30 m

gaps, Douglas-fir and western Hemlock seedling basal areas were found to be

related to changes in red light on the east axis; western Hemlock was highly

significantly related to changes in red light (R2 = 0.92). Along the south axis,

western Redcedar and western Hemlock basal areas were found to distributed

significantly relative to changes in red light in this gap size (R2 > 0.50). In 50 m

gaps, Douglas-fir basal areas were found to be highly significantly related to

changes in red light on the west axis and greater than 80% of the variation was

explained by this association. Also, basal areas of western Hemlock were

related to red light found along the north and east axes in the 50 m gaps.

Far red light in 20 m gaps showed no significant relationship with changes

in seedling basal areas (Figure 15). In 30 m gaps, Douglas-fir and western

Hemlock basal areas were associated with far red light changes; highly

significant associations (R2 = 0.92) were found for western Hemlock seedling

basal areas again on this axis (refer to Figure 14). On the west axis of the 50 m

gaps, Douglas-fir seedling basal areas were again highly significantly (R2 = 0.98)

related to changes in light, this time far red light. Western Hemlock continued to

show associations along the north and east axes in 50 m gaps (refer to Figure

14).

Only for ratios in R/FR light were seedling basal areas significantly

associated with changes in light in 20 m gaps; Douglas-fir was found along the

90

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91

north axis, and both western Hemlock and western Redcedar were found along

the west axis (Figure 16). In 30 m and 50 m gaps, R/FR light predicted the

changes in species' basal areas only along the south axes; western Hemlock in

the 30 m gaps, and both Douglas-fir and western Hemlock in the 50 m gaps.

Blue light distributions were only significantly associated to changes in

seedling basal areas in 30 m and 50 m gaps (Figure 17). Douglas-fir and

western Hemlock (R2 = 0.90) were significant along the east axis, and Douglas-

fir and western Redcedar were significant along the south axis in 30 m gaps.

Only Douglas-fir (R2 = 0.75) and western Hemlock basal areas were found to be

significantly associated with changes in light in 50 m gap west and east axes,

respectively.

Analyses by Axes.-Changes in species' basal areas relative to changes in

red, far red, ratios of red and far red, and blue light were assessed irrespective

of gap sizes (Figure 18). When all gap sizes were combined, the only significant

association with changes in R/FR light was found for Douglas-fir along the east

axis of the gaps (refer to # on far red distribution map in Figure 18). In general,

any significance found when gaps were combined explained less than 45% of

the variation that could be found between changes in light and changes in

seedling basal areas. Western Hemlock was highly significantly associated with

changes in red light on the east axis and with changes in blue light on the south

axis (R2 < 0.50).

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Step-wise Regression Analyses

Step-wise multiple regressions were conducted to explore if an aggregate

effect of different colors of light on seedling regeneration in forest gaps was

determinable. The best two-variable model for numbers of seedlings of all

species combined in the 20 m diameter gaps resulted in an R2 of 0.23460283

(p=0.0207) when R/FR ratios were entered initially, followed by blue light. In 30

m gaps, numbers of seedlings showed no significance when light variables were

added to the model (p > 0.8284). Also, no significance was found in the larger

gaps, 50 m (p > 0.1679).

No significance was found for total basal areas of seedlings (all species

combined) in gaps of size 20 m (p > 0.5837). No significant model was found for

light variables in mid-sized gaps, 30 m (p > 0.7803). And no significance was

found when light variables were added to the regression model for seedling

basal areas in 50 m gaps (p > 0.1312).

When step-wise regressions were employed on seedling data where no

gap size classes were used, results were borderline for a model using red light

but no significance was found for numbers of seedlings (p=0.0501). For

seedling total basal area (cm2), no significance was found (p > 0.0832).

Discussion

Both Douglas-fir and western Hemlock seedling distributions were

significantly related to changes in light along the east axis in 30 m gaps; western

Hemlocks were highly significantly distributed with light along these axes. In

96

Washington state, western Hemlocks dominate the stand as juveniles in both

basal areas and numbers of individuals. Western Hemlock growth is repressed

beneath larger Douglas-firs but, because they are shade-tolerant, seedlings

survive until they can form mature stratified stands (Oliver and Larson 1996).

Species should be more shade-intolerant to the north in gaps where light

availability is greatest and become more shade-tolerant moving toward the south

side of the gap (Oliver and Larson 1996). Numbers of western Hemlock, a

shade-tolerant species, were distributed significantly with blue light along the *

south axis in 30 m diameter gaps with 70% sample variance explained. Blue light

was greater than 2% transmission along the south axis in both gaps 106 and

206. Intermediate blue light transmission in shade might have created a

conducive environment for western Hemlock establishment and growth.

Western Redcedar, a hade-tolerant species, was distributed significantly

with blue and red light along the south axis in medium sized gaps. Known for its

slow growth, western Redcedar is often overtopped, and thus, is seldom found

as a monoculture (Oliver and Larson 1996).

Douglas-fir, a shade-intolerant species, was found to be significantly

related to percent transmission of blue light along the east and south axes of

medium-sized gaps. In larger gaps, Douglas-fir was highly-significantly

associated with changes in blue, red, and far red light along the west axis, an

intermediate site in south-facing gaps that received larger amounts of sunlight

during the diurnal period.

97

In several other instances, there were observable patterns that indicated

an association between changes in light distribution in gaps and the presence or

absence of species in certain areas of the gaps. There were no significant

differences in how seedlings were distributed between axes in gaps (ANOVA; p

> 0.05). Thus, means were shown to be equal amongst all axes in any gap size.

Therefore, any association found to be significant in the regressions of light

quality and seedling establishment is likely due to real differences in light

environments and associated differences in micro-environments along individual

axes in gaps.

At northern latitudes, small gaps receive little light on the forest floor. As

gap size increases, the size of a crescent-shaped area of sun increases,

appearing widest at the north side of the gap because the at noon the sun is in

the south and at its zenith (Oliver and Larson 1996). The sun follows a path

from east to south to west and the highest sun angle is at noon when the sun is

directly south. Shading in gaps was due to the shape of the crown of trees

located at the edge of the gap circle. In H. J. Andrews forest, gaps along a

southern aspect, and the southern line of trees defined shade in gaps (Gray

1995).

The pattern of distribution of R/FR ratios in a gap should indicate the

pattern of shading in a gap, because shade is known to be deficient in red light

because of preferential absorption in plant canopies. However, ratios of R/FR

were greater in small gaps (R/FR«10) relative to the ratios found in larger gaps

98

(R/FR<;2). Greater canopy closure was observed in the smaller gaps, and

canopy coverage has been observed to reduce light transmissions. However,

reference readings were obscured from a clear field of view by greater than

40%. Hence, this interference might have caused some anomalous readings in

20 m diameter gaps.

Germinability of seeds under plant canopies is dependent, in many

situations, on the light environment, especially R/FR ratios, reaching the seeds

(Grime 1981). Germination in certain seed species are inhibited by the

depletion of red as light is filtered through the canopy. Apparently, Douglas-fir

was sensitive to the ratios of red and far red light. Smaller gaps had a more

pronounced reduction in R/FR ratios along the north axis than did other gaps.

On the west axis in small gaps (20 m in diameter) numbers of seedlings

and seedling basal areas (cm2) were significantly related to changes in R/FR

ratios for all species except Douglas-fir (p < 0.05). In most cases, at least 50%

of the variation in the samples was explained by the occurrence of seedlings

along the west axes of these gaps. Numbers and basal areas of Douglas-fir

seedlings was found to be significantly associated with R/FR ratios along the

north axis of smaller gaps (p = 0.0037). In this case, 78% of sample variance

was associated with this relationship between light environment (R/FR ratio) and

seedling establishment.

Blue light has also been shown to be involved in the inhibition of seed

germination (Tanno 1983). In larger gaps, 30 m and 50 m, blue light was greatly

99

reduced along the extremities of the axes, although percent transmission of blue

light was greatest near the center points of gaps; as high as 50% of incident blue

light in gap 106. Blue light along south and east axes followed shading patterns

in the gaps; blue light was greatly reduced along these axes orientations until

much later in the day. Less blue light along these axes could have resulted in

more successful germination and establishment of Douglas-fir seedlings.

CHAPTER 4

LIGHT SPECTRA SURVEY IN A TROPICAL MONTANE FOREST

LA MUCUY PARQUE NACIONAL IN VENEZUELA

Introduction

Light is a factor of primary importance to the propagation and survival of

plants. Tropical montane cloud forests are probably among the most light-limited

forests on earth. Yet the plethora of vegetation in these habitats pay tribute to

the adaptiveness of indigenous species to the extremes of their environment.

Studies have shown tropical species to be uniquely adapted to limitations

in light environments as shade-tolerant or shade-intolerant variants (Swaine and

Whitmore 1988; Woodward 1989; Thompson etal. 1992 a, b). Other studies

discuss strategies of plant regeneration such as gap pioneering species (Brokaw

1985 a, b\ Runkle 1985a, b; Ackerly 1996; Strauss-Debenedetti and Bazzaz

1996). Although many researchers have investigated the relevance of spectral

quality on seed germination in tropical forests worldwide (Foster and Janson

1985; Forget 1992 a, b; Kennedy and Swaine 1992; Hammond and Brown 1995;

Loiselle et al. 1996), few studies in the tropics have addressed the importance of

spectral quality in terms of plant physiology (Lee 1987; Lee etal. 1996), and

fewer discuss the effect of light quality on species resource allocation strategies.

Refer to Chapter 1 for a discussion of these relevant studies.

101

Plants assess their surroundings by sensing their spectral light

environment to obtain discriminatory information that physiologically induces

behaviors that can confer an adaptive advantage to some species (Ballare et al.

1987,1990,1991). Studies have shown that proximal spectral light can greatly

influence minute-to-minute changes in plant physiology and allocation of

resources. Importantly, plants are not only affected by their light environment,

but they also greatly influence light in their environment by their presence or

absence, by their size, or by their shape or anatomy. On the basis of the

importance of the dynamics of light quality in tropical forest systems, this

baseline study was undertaken.

Understanding the interactions of light and plants in tropical systems

begins with a description of the varying light environment in the forest. In this

study, a snap-shot of the light quality environment was completed by survey to

determine if any patterns might exist that could be causal in the distribution and

allocation of biomass in a tropical montane cloud forest in Venezuela. These

data presented here also offer an opportunity to broaden the geographic

representation of existing data on light in forest environments. In this chapter,

the distributions and patterns of spectral light found in this tropical montane

forest are discussed.

Methods

Site Description

This study was done in La Mucuy, a montane cloud forest located 8° 38' N

102

by 70° 14' W, approximately 2,400 m M.S.L. in the Sierra Nevada mountain

range of Venezuela, in Parque Nacional Sierra Nevada. Temperatures remain

constant, varying little around a 12.5 °C average throughout the year (Ataroff

pers. comm.). Monthly precipitation peaks at two times during the year; at

approximately 345 mm in April, and approximately 355 mm in November (Ataroff

pers. comm.). Precipitation lows, or dry seasons, occur in February with

approximately 50 mm, and again in December at 140 mm. In this region,

humidity fluctuates between 80-100%; often by early to mid afternoon, the

atmosphere reaches saturation point and a dense fog or low clouds form and

engulf the forest (Lamprecht 1954), hence the name, cloud forest.

Species of cloud forest vegetation are diverse, with 40-60 different

categories of trees (Lamprecht 1954). Three general categories of vegetation

are 1) species of trees with the majority of their biomass in the upper canopy, 2)

species of trees whose crown occur in the lower canopy, or understory, and 3)

epiphytic species.

There are two or three vertical strata in this cloud forest tree canopy

(Lamprecht 1954). Occasionally, these three strata are not distinguishable, and

the canopy looks continuous. When stratification occurs, the upper stratum

occurs at a height of approximately 30-35 m. Representative of the overstory

vegetation are the following families: Arecaceae, Compositae, Euphorbiaceae,

Podocarpaceae, Rubiaceae, and Theaceae. The intermediate stratum occurs at

20-25 m and the lower stratum occurs at 6-15 m. These understory species are

103

represented by the following families: Begoniaceae, Ericaceae, Piperaceae,

Rubiaceae, and Solanaceae. The intermediate canopy is very dense with little

light penetrating to the lower stratus, where individuals are widely spaced and

population numbers are small. Epiphytes generally are in the Bromeliaceae and

Orchidaceae families and are distributed throughout the forest canopy, relative

to light and nutrient availability.

Data Collection and Management

Data on spectral characteristics of light were collected using an Ocean

Optics SD-1000 fiber optic spectrometer, a light-weight and portable field

instrument that measures the electromagnetic spectrum over a UV-visible-NIR

range of 275 nm to 775 nm. The instrument has a spectral bandwidth resolution

(FWHM) of 10 nm as provided by the manufacturer and when used with an 0.2

mm diameter fiber. The instrument has a sampling interval of 0.5 nm. Two data

channels collected simultaneous readings from separate glass optical fibers

conveying light to identical spectral optical units. The master channel was

established as a reference fiber that was placed in an area with an unobstructed

view of full sky. The slave channel was established as a sample fiber that was

carried to points along plot axes where downwelling light spectra were measured

at a 1.0 m heights in the understory. Both channels were set up by connections

to two fibers linked by an aperture-controlling shutter. The shutter system allows

the operator to close the spectrometer for dark references, measurements that

are subtracted from data to correct for the background noise of the instrument.

104

The reference fiber was a 50 m long with a 0.2 mm diameter. The sample fiber

was 70 m long with a 0.2 mm diameter. Both sample and reference fibers and

their shutters were connected to the spectrometer using 1.0 m-long fibers of 0.2

mm diameter, which defined the spectral resolution on both channels. All fibers

are 0.22 NA, and when bare-ended have a 25° field-of-view.

Attaching the hemispherical diffuser to the sample fiber greatly reduced

light availability to the fiber in an already heavily shaded area. Thus, no cosine-

corrected diffuser was used in this light survey. All light samples were taken

using a bare-ended fiber with a 25° field-of-view.

Data files were collected and stored in binary format for compressed file

management until they were processed from binary to ascii format for statistical

and graphical pattern analyses. Percent transmission of light through the

canopy was calculated as sample spectral values divided by reference spectral

values.

The data were smoothed by applying a seven-point moving average

reduce high-frequency noise in the sample. Each percent transmission value

along the spectral axis was averaged by the three values both before and after

the central point (x3) for a seven-point average.

^ w '(*o) +>(*i) +'(*2)+'(*3)+'(*4)+t(x5) <xe)

J{X3) (-(J

where f(x^ is the filtered transmission at x3 and t(x) is the transmission at x,. The

105

next series of values centers on x4, Xg, and so on. The smoothed spectral data

were used in all further analyses of vertical profiles. The smoothed spectral data

were used in all further analyses.

Spectra of light in the 300 nm - 750 nm range were used in comparisons

within and between plots, as well as specific actinic wavelengths of red (660

nm), far red (730 nm) and blue (430) light. Percent transmission values were

averaged ± 5 nm about the action wavelengths listed above.

All light sampling was completed within a 4 h window of solar noon,

ranging approximately from 1100 to 1400 hours. Thus, the light measurements

taken at any point along the vegetation transects in this time frame are best

comparable when the sun is at its zenith or at angles nearly so (Anderson 1966;

Evans 1966; Russell etal. 1989).

Sample Plots

At La Mucuy, two sample grids 32 m x 32 m square were arrayed along

continuous east-to-west axis. This axis extends from forest understory where

plot A was laid out, through a forest light gap where the reference fiber was

deployed, then again to a forest understory where plot B was laid out (Figure 1).

Plot A was established to the west of the forest light gap and was separated from

plot B by more than 50 m, that was established to the east of the gap. Plot A has

an overall eastern aspect with a gentle grade, whereas plot B has an overall

western aspect with a more steeply sloping grade. Axis sample sites were

delineated every 4 m with the plot center at 16 m (point 6) from the origin of both

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the north-to-south axes and the east-to-west axes for both plots. Four additional

plot sites were established in each of the NW, SW, NE, SE quadrat areas

adjacent to the intersecting axes (Figure 2). Counting all points along

intersecting transects (four each for N, S, E, and W; one for the center) and all

quadrat points in a grid (four each for NW, NE, SW, and SE) there were 33

possible sample sites for each plot. Therefore, there were a total of 66 possible

sites for both of the two plots.

One-meter Ground Measurements

All ground samples were taken in both plots A and B using 1.0 m

sampling interval. This pole was moved from point to point in the plot and

spectral data from both data channels were recorded as the sample fiber was

deployed. No measurements were taken under this tropical canopy with the

hemispherical diffusor because cosine corrected readings were greatly

attenuated under already extremely low light conditions. All measurements in

this analysis were taken using a bare-ended fiber with a 25°field of view. Data

were visually compared within and between individual plots. For these

comparisons, all samples at 4.0 m intervals were used.

Vertical Light Profiles and Attenuation Coefficients

A vertical profile of light transmission was measured at four points on the

primary axes of both plot A and plot B: N4, E4, S8, and W8. All measurements

in this analysis were taken using a bare-ended fiber with a 25°field of view.

Spectra were measured every 0.6 m at the following heights in the forest

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canopy: 1.8 m, 2.4 m, 3.0 m, 3.6 m, 4.2 m, 4.8 m, 5.4 m, 6.0 m, and 6.6 m. Total

canopy height was approximately 10-15 m.

There are two approaches to relating how light changes as it passes

through the plant canopy (Anderson 1966). One is to vertically integrate the

absorption of light occurring above the canopy floor. The other is to estimate the

availability of light at any point in the plant canopy. The latter method allows for

ecological evaluation of how the light factor affects the distribution of vegetation

throughout the plant canopy, e.g., epiphytic growth or changes in leaf area

indices (LAI).

Monochromatic light, when absorbed exponentially, follows Beer's law, as

it passes through the atmosphere and canopy (Collingborne 1966). Vertical

attenuation coefficients (K) were calculated for red (660 nm), far red (730 nm),

and blue (430 nm) wavelengths {X) between each change in canopy height by

applying a derivation of Beer's Law,

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thickness z, and Iz is the incident light 0.6 m below. Z is the distance the light

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Hemispherical Photographs

Hemispherical photographs were used to estimate direct and diffuse

radiation distribution in the canopy understory. A record of percent cover of the

overhead canopy was made using 35 mm photographs taken using a 15 mm

fisheye lense EF (1:2.8). Photographs were taken in both plot A and B at each

4 m transect point on the main axes and the quadrate adjacent points. The lens

was placed 1.0 m above the ground at permanently marked sites along these

axes. Compass orientation of the camera varied; thus, analysis of photographs

relative to the sun path was not possible. However, lens location was

standardized to allow future comparative analyses of these photos.

A total of 96 photographs were taken. The photographs were then

digitally scanned and the images were analyzed in an automated process using

a GIS raster-based analysis package (Environmental Systems Research

Institute, Inc. 1994). The GIS analysis calculated percent open sky above each

point in the sample plots where spectral light measurements were taken earlier.

Pixel values representing open-sky were grouped into one class. All other

pixel values were made to equal zero (no-data) and the number of pixels in

open-sky were summed. The sum of open-sky pixels were multiplied by the area

of each pixel (0.001) to compute area open-sky for each photo. Area was an

estimate of relative proportions; thus, it is a unitless measure.

These values of percent open sky were then regressed with the spectral

value of percent transmission for different actinic wavelengths; red (660 nm), far

112

red (730 nm), and blue (430 nm). This regression was done to establish a

relationship between changes in spectral quality along the forest floor relative to

differences in percent sky in the overhead canopy.

Results

PAR in Forest Canopy

Two days of full-sky light was plotted in Figures 4 and 5. Pyranometer

readings were recorded from the time sampling began in a gap to the time

sampling finished. These two days typified full-sky light PAR (photosynthetically

active radiation) readings taken from the forest gap in the montane forest. PAR

was plotted with corresponding visual estimations of percent cloud cover. In

general, mornings were typified by low PAR (< 50 |xE m"2 s*1). Percent cloud

cover was < 40% both mornings and PAR reached 250-350 (|iE m'2 s"1) by noon.

Percent cloud cover increased from 40% to 80% by late afternoon, and reached

100% by 14:00 hours on 25 February. Fog usually occurred by late afternoons.

One-meter Ground Measurements

Plot A.- Spectral measurements ranged from very small percent

transmissions (< 0.001%) to large percent transmissions (> 90%) in the same

plot. Sunflecks occurred frequently from small gaps in the forest canopy. In

general, however, most light was less than 5%. Because of scaling difficulties

with ranges of this nature, readings were normalized by dividing all percent

transmission values in each plot by the maximum value in a plot. Normalized

values ranged from 0-1. Maximum transmission values in both plots for red, far

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red, blue light and RFR ratios were reported in Table 1.

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% RED % FAR RED % BLUE RFR RATIO

PLOT A 0.077 0.907 0.058 1.746

PLOT B 0.025 0.053 0.035 1.525

All values were relative to unique maximum values that differed for each

light color. Figure 6 showed a sunfleck in the northwest quadrat. Light from this

small canopy gap was greatest near the opening but light spilled over to an

adjacent site north of the sunfleck.

Also, to observe patterns in lower light transmissions, maximum values

were removed. Several methods were explored to reduce the overshadowing

effect of these maximum values. First, data were subjected to a logarithmic

transformation. The preponderance of percent transmission values < 1.0

precluded the usefulness of this procedure. Data were then subjected to a

histogram equalization process using frequency of percent transmission values

in lieu of brightness values (Jensen 1996). Ultimately, the removal of maximum

values (often more than one data point was deleted to achieve optimal

comparisons of lower percent transmissions) from each gap gave comparable

results to the more laborious histogram equalization process.

Figure 7 showed how red light was distributed throughout the rest of the

forest plot relative to the maximum value in the sunfleck. Percent transmission of

red light was mostly below 0.05% of the maximum value in the plot. No

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discemable patterns were observed in the distribution of red light from site to

site in the plot; the distribution of red light under forest canopy was variable.

The sunfleck was evident again in plots of far red and blue light and RFR

ratios (Figures 8-10). In Figure 8, far red light was greatest at point 4N, 2W only.

Thus, ratios of red and far red light were greatest at point 2N, 2W, where little far

red light was measured (refer to Figure 6), relative to the sunfleck adjacent to

that site. Overall, RFR ratios were less than one-fifth of 1% of the maximum

value showing RFR ratios, and in general, are greatly reduced under broad-

leaved forest canopy.

The two maximum values for percent transmission of blue light in the

sunfleck (Figure 10) were distributed inversely from the trend observed in red

light distribution (refer to Figure 6). An increased transmission in blue light was

observed in proximity to the north of the sunfleck.

Figures 11-13 show the distributions of the lower percent transmission

values, without the referenced maximum value in the plot. Most far red light

measurements were less than 0.012% of the maximum transmission, at

extremely low values (Figure 11). Many sites in the plot approximated near-zero

readings. Patterns in far red light were very similar to those seen for red light in

the sample plot (refer to Figure 7). Ratios of R/FR showed red light were

discernibly higher in the eastern portion of the plot; values ranged between 0.10-

0.30 (Figure 12).

Blue light showed a spotty distribution in the lower transmission range

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(Figure 13). All values were less than 0.1% of the maximum value, but peaks

were observed scattered throughout the plot.

Plot B.- Light conditions were discernibly different in the vegetation

understory on the other side of the light gap in La Mucuy. Vegetation was more

densely crowded, with shrubs and bushes more commonly encountered. It was

considerably more difficult to traverse the transect lines in plot B versus plot A.

In Figure 14, percent transmission in red values was greatest at three sites in

the plot. These sites could not be predictable given the proximity of the forest

light gap near the far west edge of the plot. Also, percent transmission in far red

light ranges peaked in the southeast quadrat of plot B, in an area densely

crowded with bushes and bramble, but also adjacent to the forest light gap

where reference readings were recorded (Figure 15).

Ratios of R/FR light showed greater differences between red and far red

transmission occurred at sites northeast of the center of the plot (Figure 16).

This area was at the top of a west-facing slope, where vegetation was less

dense and more easily navigated during sampling.

Patterns of red and blue light in plot B were similar. Blue light

transmission (Figure 17) was greatest in the same proximity of greatest red light

transmission readings (Figure 14). In the minimum transmission ranges, red light

was 0.3% of the maximum reading in plot B (Figure 18). Compared to plot A,

where transmissions in red were < 0.05%, red light was more prevalent in plot B.

This is unexpected, because vegetation density, while not quantified, was

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distinctly greater in plot B, especially at the sample fiber height where low

shrubs and bushes occurred.

In plot B, far red light transmission was greatest (the higher of the

minimum transmissions were between 0.3-0.4%) in the southeast quadrat, again

where a dense stand of low, bushy vegetation was encountered (Figure 19). In

Figure 20, ratios of R/FR light were distinctly depressed in this same quadrat.

This site lacked a tall vegetative overstory because few trees were located in

this area. Perhaps the lack of a vertically distributed overstory, as found

throughout most of plot A, resulted in greater values in transmission reaching the

dense, low canopy found in this site in plot B hence, greater ranges of

transmission were recorded at these sites.

Most transmissions in blue light were below 0.1% (Figure 21). Again, as

seen in plot A, sites of peaks in red and blue transmission were similar (refer to

Figure 18).

Vertical Light Profiles

Plot A - The vertical distribution of light in the overstory was measured in

0.6 m increments because segment lengths were 0.6 m. Four sites were

sampled with sites arranged around the center of the plot: N4, S8, E4, and W8.

Figures 22 a and b illustrate how, when comparing all heights sampled, light

varied from 1.8 m near the bottom of the forest understory, to 6.6 m up into the

vegetation canopy. The distribution of light over the entire range of wavelengths

was below 0.35% transmission at all heights in the canopy. In general, light was

132

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Figure 22a. Vertical spectrum for each height at sites in plot A.

SITE N4 SITE S8

PERCENT TRANSMISSION

0.030

0.020

0.010

0.000*

0.030

0.020

0.010

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0.030

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0.010 •

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4.8 m

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PERCENT TRANSMISSION

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0.020

0.010

0.000

0.030

0.020

0.010

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aoDfc 350 400 450 500 550 000 650 700 750

3.6 m

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3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 700 750

WAVELENGTH (nm)

136

Figure 226. Vertical spectrum for each height at sites in plot A.

SITE E4

PERCENT TRANSMISSION

SITE W8

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0.020

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0.020

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V

6.6 m

6.0 m

5.4 m

4.8 m

4.2 m

3.6 m

J 3.0 m

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1.8 m

PERCENT TRANSMISSION

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0.020

0.010

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6.6 m

6.0 m

5.4 m

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4.8 m

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4.2 m

/ 0.000

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0.010

0.020

0.010

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350 400 450 500 550 600 650 700 750

WAVELENGTH (nm)

137

greatly reduced in the PAR range (400-700 nm) at heights below 4.2 m. Peaks at

the infra-red end of the spectrum (> 700 nm) remained in evidence throughout

the profile, although greatly reduced in some sites. Readings in the UV portion

of the spectrum were greatly increased near the forest floor as opposed to in the

upper canopy, Only at sites S8 and W8, above 4.2 m heights, were greater

percent transmissions measured in PAR in the upper canopy. At site W8, far red

wavelengths (~ 730 nm) peaked at 4.8 and 5.4 m heights in the canopy.

There was a preponderance of far red light relative to other wavelengths

measured throughout the entire vertical profile at all four sites in plot A (Figure

23). Percent transmissions were highest at site W8 and lowest at site N4. At

sites S8 and E4, transmission of far red light was highest in the lower strata of

the profile. At site W8, far red transmissions were highest above 4.2 m. There

was no trend in far red light distribution at site N4. Blue light was distributed

mostly in the upper strata of the canopy, except at site E4. Red light

transmission was uniform at sites N4 and E4. At S8 and W8, red light

transmissions were greatest in the upper strata, but percent transmissions were

much less at site S8 higher in the canopy.

Plot B.- In Figures 24 a and b, vertical profiles of selected spectra in 1.2

m increments on the north-to-south axis in plot B at sites N4 and S8 are shown,.

The tree canopy did not go beyond the 4.2 m height at site S8. There were

distinct differences in the spectral patterns between sites. Percent transmissions

for all spectral curves were below 0.12%. At 6.6 m in the canopy at site N4, the

138

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139

Figure 24 a. Vertical spectrum for selected heights at sites N4 and S8 in plot B, at La Mucuy, Venezuela. Light sampling done to 4.2 m only at site S8.

N4 S8

WAVELENGTH (nm)

WAVELENGTH (nm)

WAVELENGTH (nm) WAVELENGTH (nm)

WAVELENGTH (nm)

WAVELENGTH (nm)

WAVELENGTH (nm)

140

Figure 24b. Vertical spectrum for selected heights at sites E4 and W8 in plot B, at La Mucuy, Venezuela.

E4 S8

WAVELENOTH <nm)

WAVELENOTH <nm)

WAVE LENGTH (nm)

WAVELENOTH <nm)

WAVELENOTH (nm)

WAVELENOTH (nm)

FET

WAVELENOTH (nm)

WAVELENOTH (nm)

WAVEUENOTH | n m ) WAVELENOTH (nm)

141

spectral distribution was greatest in the PAR wavelengths, 400-700 nm.

Compared to site S8 where UV wavelengths, ~ 300-330 nm, peaked

above other wavelengths at 3.0 m and 4.2 m, percent transmission in UV

wavelengths were attenuated at site N4. Peaks in far red wavelengths were

acute at 6.6 m at site N4, near the canopy top, and again at 4.2 m at site S8 near

the canopy top.

In Figure 24 b, most percent transmission values were below 0.035% at

both sites E4 and W8 on the east-to-west axis in plot B. Peaks in wavelengths

near 300 nm were pronounced at all heights in the canopy for both sites, except

height 6.6 m at site E4, where most light was extremely attenuated. It is possible

the sample fiber was immediately under an object, such as a tree branch, which

obstructed the light path, although care was taken to avoid this circumstance.

Percent transmission in the PAR range was reduced at all heights at both sites;

no values exceeded 0.0025%. Far red wavelengths (730 nm) were uniform at all

heights at site E4, except at 6.6 m where light was greatly attenuated. And at site

W8, far red wavelengths peaked near the top of the canopy, at 6.6 m, and

showed a decreasing trend with increased depth in the canopy, as did UV

wavelengths (300 nm).

In plot B, sites N4 and S8, transmission in red, far red, and blue light

decreased from greater transmissions higher in the canopy (6.6 m) to lower

transmissions deeper in the canopy (1.8 m; Figure 25). Figure 25 shows percent

transmissions were extremely reduced at both sites E4 and W8, < 0.02%. This is

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143

not unexpected given the reduction in percent transmission in PAR shown at

sites E4 and W8 in Figure 24 b.

Extinction Coefficients

Plot A.- As was found in the smaller gaps in H. J. Andrews Experimental

Forest in Oregon, the vertical attenuation coefficients at La Mucuy, Venezuela

were not uniform throughout the profile because of variability of absorption at

different heights in the canopy (refer to Chapter 2). It appears that the canopy at

La Mucuy was not homogeneous, and that Beer's law might not apply in broad-

leaved tropical montane forests. In plot A, several strata in the canopy appeared

to absorb light at different rates (Figure 26). Greater extinction values were

interpreted as greater light extinction. Strata in which the coefficient values

increased abruptly might indicate areas in the canopy where greater leaf area

ratios might be found.

Changes in distribution of percent transmission values resulted from

abrupt changes in extinction coefficients in the canopy profile. For example, at

site W8 in Figure 23, percent transmission values decrease greatly for red and

blue light transmission between 4.2 m and 4.8 m heights, and more gradually for

far red light transmission. In Figure 26, extinction coefficients decreased at the

4.2 m height in the canopy; attenuation patterns were blue > red > far red, as the

transmission profile indicated.

Plot B.- In Figure 27, extinction coefficients of light in plot B are shown for

the four sites sampled. At site N4, increased attenuation of light occurred at 5.4

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146

m and 2.4 m in the canopy. At other sites, attenuation of light varied at different

magnitudes, even in areas where transmission values were < 0.02%.

Open-Canopy Area Calculated from Hemispherical Photos

The percent open area in canopy over each site in plot A was calculated

from an analysis procedure using hemispherical photos taken at each site in the

plot. Areas of open-sky were regressed with percent transmission of red, far red,

and blue light, and with R/FR ratios measured at each site in the plot. No

significant relationships were found between areas of open canopy and spectral

percent transmission (R2 = 0; p > 0.9832).

Discussion

Light becomes the most limiting environmental factors in tropical montane

forests, assuming that moisture is abundant in cloud forests, even during dry

seasons. At La Mucuy, apart from a few areas in the plots that were exposed to

sunflecks or areas of brief increased solar intensity, most percent transmission

values in these plots were extremely low, generally less than 5%. PAR

measurements in the reference gap indicated that incident light was less than

400 nE m'2 s"1 throughout most of the sampling period. Additionally, cloud cover

becomes a salient factor in the reduction of incident radiation by late afternoons.

Removal of the saturated and near-saturated values from the light

distribution pattern provided a means to determine if low percent transmission

ranges were variable or uniform. The distribution of percent transmissions

across the forest floor showed light under this canopy was extremely variable,

147

even at low percent transmissions. Measurements of fluency rates in each

wavelength were not recorded in this study. Thus, it is difficult to discuss the

physiological relevance of spectral values at such low percent transmissions,

other than to say that if these data typify the light environment in this habitat,

plants that thrive there must be able to function efficiently under such low light

conditions.

The distribution of R/FR ratios in plot A was not uniform. This plot was on

an east-facing hillside with a gentle-to-steep slope in different parts of the plot.

The east edge of the plot was facing the reference light gap and the slope to the

east of the primary north to south cross axis was much steeper, > 30%. The

western half of the plot was on the apex of this slope where the gradient leveled

off. Light transmissions might have been more pronounced on the eastern side

of the plot because of the aspect of the plot or its proximity to the light gap. This

might have resulted in greater R/FR ratios because less red light was

attenuated. However, no patterns were observed in the distribution of red light in

this portion of the gap. Likewise, no pattern was discerned for far red light.

Distributions appeared random and variable. Only when ratioed did the pattern

become evident. The significance of in a difference between 0.2% and 0.4%

transmissions is a matter for further research.

Lee (1987) described R/FR ratios, based on quantum ratios between 658-

662 nm and 728-732 nm, in two tropical forests, La Selva in Costa Rica and

Barro Colorado Island (BCI) in Panama. In full sunlight, R/FR ratios were

148

typically 1.28. In gaps of undefined size, R/FR ratios ranged from 0.59-1.25 and

0.97-1.17, respectively. In sunflecks with diameters < 0.5 m, R/FR ratios ranged

from 0.37-1.17 and 0.58-1.3, respectively. In a typical understory shade

environment, R/FR ratios ranged from 0.17-0.7 and 0.13-0.67, respectively.

Light environments in the understory of a tropical montane forest appear

potentially to be comparable to those found in these other tropical forests. In plot

A, R/FR ratios ranged from 0.085-1.746. In plot B, R/FR ratios ranged from

0.025-1.525. However, the larger ratios were found in sites where sunflecks

occurred at the time of sampling and were atypical of the light environment in

these plots. In general, R/FR ratios were much less than the maximum values

observed, making this type of forest extremely light limited.

The distribution of blue light (430 nm) was random; no trends were

observed in either plot A or plot B. Even with the maximal values removed,

percent transmissions in blue light were very low. Peaks in blue transmission at

sites scattered throughout the plots, although < 0.12% in plot A and < 0.3% in

plot B, might be due to increased scatter from plants adjacent to the sampling

site. Values at these sites do not correspond to peaks in red or far red light, that

would suggest they result from sunflecks occurring through the forest canopy.

Not only was light under a forest canopy heterogeneous, but the vertical

light environment was variable. In plot A, transmission of far red slightly

increased with depth in the canopy. These data were supported by the

distribution of extinction coefficients in plot A. Less far red light was attenuated

149

in the lower strata. Changes in extinction coefficients throughout the canopy

showed that apparent physical differences in the light path were affecting the

light.

In plot B, extinction coefficients were not uniformly distributed throughout

the entire canopy, as in plot A. Distinct patterns in the profiles indicated where

greater attenuation of color occurred. These patterns were supported by the

pattern of color distribution found in this plot.

The distribution of light in a canopy is assumed to follow Beer's law of

exponential absorption if the light path transverses a homogeneous matrix. This

study has shown that light, for the most part, was different in each unique 0.6 m

slice of the forest canopy. Extinction coefficients in both plots showed that the

canopy environment is heterogeneous, and light passing through this canopy is

altered at different rates at different heights in the canopy.

Interpretation of fluctuating coefficient values by the magnitude of change

was a subjective procedure, at best. In future work at this site, extinction values

might be used to predict which strata in the canopy are supporting epiphytic

plant growth. Studies of epiphytes in rainforests is likely to be highly correlated

to the distribution patterns of light in the upper strata of tropical systems. Also,

information about vertical light profiles could elucidate how mature species affect

the nursery of seedlings gaining establishment in their understory. Lastly,

studies of the relationships between species' leaf area ratios and leaf

physiognomy could benefit from a better understanding of how light behaves in

150

different stands of forest trees.

CHAPTER 5

CONCLUSIONS

Objectives

The objectives of this study were to describe the distribution of light

wavelengths in plant canopies. In Oregon, permanently-marked canopy gaps of

different sizes were surveyed to assess patterns of light distribution in conifer

canopy gaps. In Venezuela, two 32 m x 32 m permanent plots were surveyed to

assess patterns of light distribution in tropical montane cloud forest canopy.

Vertical profiles were measured at the center of conifer canopy gaps and

at four points in each square plot in tropical canopy. Extinction coefficients were

calculated and the vertical distribution of light spectra and actinic wavelengths

were assessed.

In Oregon, the relationships between species' seedlings, surveyed in H.

J. Andrews Experimental Forest by Gray (1995), to light spectra in conifer

canopy gaps were determined using linear regression analyses. Seedling

numbers and seedling basal areas (cm2) were used in the analyses.

One of the most important limiting resources in a forest is the light

environment, a heterogeneous resource that varies spatially and temporally in a

forest environment. The complexity of forests, both vertically and horizontally,

directly affects the distribution of light intensity and color. A few guiding

4CL4

152

principles have emerged from decades of research on how light behaves in plant

canopies. Observations of changes in the RFR ratios with plant shading has

been well documented by many researchers. Also, it is usually assumed that

light varies vertically in a forest canopy according to Beer's law of exponential

light attenuation.

Conflicting methodologies in the study of light in different forest systems

have not facilitated our understanding of the relationship between seedling

establishment and patterns of shade and light on forest floors. Although limited

spectral surveys have been conducted in temperate and tropical systems,

essentially no data are available on the distribution of different wavelengths in

temperate canopy gaps or in tropical understory.

The goal of this study was to determine if there are distinguishable

patterns in the distribution of spectral light in contrasting forest environments.

First, spectral surveys were conducted in conifer forest gaps of different sizes.

In this survey, the distribution of light was measured along the cardinal axes that

dissected the canopy openings. Information about seedling establishment and

growth, from a survey conducted by Gray (1995), was examined to determine if

there was a correlation between the patterns of color in forest gaps and numbers

of seedlings or seedling basal areas.

The effect of intensity of light on seedlings is well established; however,

the effects of light color on seedling establishment and growth is less-well

understood. However, no previous study was found which applied the logic that

153

patterns in seedling distribution might correlate to the distribution patterns of

color found in forest gaps. Red, far red, and blue wavelengths affect the

developmental processes in seedlings, and R/FR ratios, and blue light are

known to affect seedling germination. Also, alterations in R/FR ratios have been

shown to affect stem elongation, leaf area ratios, and resource allocation

patterns in plants.

Second, spectral surveys were conducted in a tropical montane cloud

forest. In this survey, the distribution of light was measured in two, 32 m x 32 m

plots under the forest canopy. These data provided a baseline on the spectra of

light found in this light-limited environment.

Light was determined to be distributed in predictable patterns in gaps,

especially contrasting near-center sites with near-edge sites in the temperate

forest. Patterns of shading were controlled in these plots by the southern stand

of trees in a gap (Gray 1995). Early morning light first illuminated the west edge

of the gaps. By mid-day in the 50 m gaps, light was in all but the southern edge

of the gap that did not see much sunlight until later in the afternoon. In addition

to the effect of the path of the sun in northern latitudes, shading in the gap was

affected by standing snags (dead, standing debris), shrubs and brush (such as

Viney maples), and downed logs and stumps left in the gaps (Gray 1995).

In general, PAR was shown to increase according to gap size (Gray

1995). In this light survey, percent transmissions of red light, far red light, blue

light, and R/FR ratios were shown to increase as gap diameters increased.

154

Patterns in the horizontal distribution of color in forest gaps are complex. In

general, axes expected to receive greater percent PAR also received greater

transmissions in red, far red, and blue light. Because these measurements were

taken in large canopy openings with little to no vegetation left standing, this is

not altogether unexpected. Indeed, any variations in distribution patterns could

be attributed to few sources of interference in the downward path of light in

these gaps: snags, stumps, shrubs, and bushes. However, the distribution of

R/FR ratios in these gaps showed some unique patterns. In 50 m gaps, ratios

were lower at the center and edges of the openings. The smallest gaps (20 m

in diameter), showed higher ratios along the south and west axes, where shade

endured longest. Along the east to west axis, the 30 m gaps behaved similarly.

Along the north to south axis, 30 m gaps showed an opposite pattern; ratios

were higher in the north-south transect. In general, these patterns agreed with

the shading patterns observed in gaps, but variations in the spatial environment

might have generated some variation from the expected results.

Blue light has been shown to play a role in the inhibition of seedling

germination (Tanno 1983) and inhibition of seedling elongation (Morgan 1981;

Obrenovic 1992) in some species. Regulation of synthesis of glutamine

synthetase (GS), a regulatory enzyme in the assimilation of ammonia, in species

of Pinus have been shown to be affected by exposure to blue light (Elmlinger et

al. 1994). The vertical profile in canopy gaps showed that blue light, relative to

other wavelengths, was found to be most prevalent near gap floors in 20 m and

155

30 m gaps measured. Based on this observation, the relationship of seedlings

and blue light in canopy gaps were examined more closely.

Vertical attenuation in a forest gap has been assumed to follow Beer's law

that assumes a homogeneous light path (Koslowski etal. 1991). Researchers

have referred to canopy light environments as photohomeostatic (Larcher 1995).

The results of this study showed that light in a forest gap passed along a

heterogeneous path, resulting in greater attenuation rates as light nears the

forest floor. Gap stereogeography and solar radiation patterns interact to define

light patterns reaching openings in forest canopy. Some heterogeny of plant

canopies result from the complex distribution of plant biomass throughout the

forest stand. In forest gaps at northern latitudes, contributions to heterogeny

might be greatly affected by the structure of the stand of trees surrounding the

forest gap.

Successful establishment and growth of seedlings of dominant species in

H. J. Andrews Experimental Forest light gaps were strongly related to changes

in spectral light patterns along axes in canopy gaps. In many cases, greater

seedling numbers were located in the center of gaps, regardless of size. This

would suggest that areas of greater light intensity played some role in seedling

establishment. However, this study found strong associations between the

distribution of colors along gap axes and seedling establishment. In smaller

gaps, all species were affected by ratios of R/FR light, but some unique

relationships were found on different axes. For example, in 30 m diameter gaps,

156

Douglas-fir seedlings were significantly associated with percent transmissions in

blue and red wavelengths on the south axis, where greater shading occurred.

Douglas-fir seedling establishment was affected by transmissions in blue, red,

and far red light on the east axes, also a shaded region of the gap. Likewise,

the east axis had greater associations between red, far red, and blue

transmissions and the successful establishment of western hemlock. Gray

(1995) discussed temperature effects on the establishment of western hemlock;

seedling establishment is prohibited by greater soil temperatures because of

high direct solar radiation in gaps. In addition to temperatures, distribution of

color along axes might strongly affect patterns in western hemlock establishment

(R2 > 0.50). Interestingly, western redcedar showed a preference for blue and

red light transmissions along the south axis only.

In 50 m gaps, Douglas-fir was predominantly found along the west axis,

highly significantly associated with changes in light transmissions along this

axis. No associations were detected for western redcedar in larger gaps.

However, western hemlock was associated with light changes along the cardinal

axes in larger gaps.

Denslow (1980) discussed how distribution of resources can differ

between forest gaps of different shapes and sizes. This study has corroborated

to some extent the suggestion that partitioning of light could occur in conifer

forest gaps. Species distributions along axes in different gaps as well as unique

distributions within individual gaps was demonstrated in H. J. Andrews

157

Experimental Forest.

Germination of seedlings can be dependent on blue and R/FR ratios in

forest gaps, and likely these ratios played a role in the observed patterns of

seedling distribution. Also, species' reported predilection for shade habitat was

supported by the observed distributions of dominant species seedlings surveyed

in H. J. Andrews gaps.

It is important to acknowledge that patterns in seedling regeneration

result from years of exposure to heterogeneous environmental variables, and a

single snapshot in time of light distribution cannot approach a comprehensive

examination of the importance of color in seedling establishment and growth.

The data from varied light environments found in the tropical cloud forest

have indicated that color might be an important physical factor controlling

seedling development and forest regeneration processes. Transmissions

peaked at sites where sunflecks occurred at the time of measurement, but in

general, most percent transmission values were extremely low. Hence, those

species found in this forest must have unique adaptations to survive in this light-

limited forest.

Two plots were surveyed in a montane cloud forest. The light gap was at

the bottom of two steeply sloping hillsides; hence, both tropical forest canopies

sampled were adjacent to a forest light gap. Plot A was permanently established

on the west side of this light gap, and plot B was established on the east side of

the gap, and was greater in elevation by a few meters. Plot A showed greater

158

transmissions in red, far red, and blue light, and greater R/FR ratios than plot B.

Density of plants was not quantified, but physical movements during the light

sampling was more restricted by the vegetation in plot B. The species

composition differed between plots, as well. Trees in excess of 20 m were

common in plot A. In plot B, brambles and shrubs were common in the lower

canopy and might have influenced horizontal light measurements at the 1 m

level.

Time restrictions did not allow for a light intensity survey of the plots, but

shading appeared greater in plot B than plot A. Transmission values in plot B

support this subjective observation.

Color distribution was very heterogeneous, even at very low

transmissions (< 0.05% in plot A; < 0.04% in plot B). One would assume higher

transmissions would radiate to adjacent areas from where maximum

transmissions occurred. However, this generally was not the case. Areas of

greater transmission were dispersed randomly throughout the sampling plots,

with one exception. R/FR ratios in plot A were uniformly greater toward the east

end of the east to west axis. This pattern was attributed to the proximity of the

light gap to this side of plot A and the steepness of the slope along which these

measurements were made, relative to the rest of plot A. And, in general,

transmissions were greater in this area of plot A for red light but not for far red or

blue light transmissions.

It has become apparent that the assumption of an exponential attenuation

159

of vertical light in forest systems has been too simplistic, light environments in

forest systems are more complex, and are more stratified than previously

understood. Attenuation of light became greater with increased depth in the

canopy, resulting in an extremely reduced light habitat near the forest floor.

Seed germination and seedling physiology of species in this forest is dependent

on these changing spatial and temporal patterns of light (Foster and Janson

1985; Kennedy and Swaine 1992; Forget 1992a, £>; Hammond and Brown 1995).

Sunflecks have been shown to often provide the majority of solar energy to light-

limited tropical forests (Chazdon et al. 1996).

In La Mucuy, light transmissions were distributed uniformly throughout the

6.6 m vertical profile of plant canopy in plot A, except for light at sites along the

cardinal axes that showed some transmissions were greater higher up in the

plant canopy. At two sites in plot A, far red light transmissions were higher near

the forest floor than at 6.6 m in the overstory. In plot B, light transmissions were

typically higher up in the canopy and lower near the forest floor for all

wavelengths assessed.

In future work, an assessment of how this variability in the vertical

distribution of color in tropical systems is related to the distribution of prolific

epiphytic species in the plant overstory would be most interesting. Also, this

information can be used to show how species that distribute biomass up in the

plant canopy use light by correlating leaf orientation patterns and leaf area ratios

by species functional characteristics, such as shade-tolerance or shade-

160

intolerance. Close attention to taxonomic relatedness is necessary to optimize

the comparative value of studies conducted by different researchers, and in

different locals.

In order for plants to adapt to light environments, i.e., via selection

pressure, light regimes would have to be predictable. In tropical montane

forests, low light intensities are a predictable variable. However, species have

been shown to be able to acclimate rapidly to brief and unpredictable

occurrences of increased light by sunflecks through small openings in the forest

canopy (Chazdon et ai. 1996). Also, shade-tolerant species, when released

from light suppression, quickly respond by increasing growth rates to optimize

light gap opportunities. This study documents the distribution of color in a

tropical montane cloud forest, but is only a snap-shot of the spatial light

environment that varies temporally as well as spatially. To make improved

assessments in future studies, it would be better to measure light readings

averaged over a full photoperiod and from simultaneous readings from sensors

arranged in a plot distribution both beneath tree canopy and in a canopy gap.

The following conclusions are based upon the findings of the light

surveys. No attempt was made to analytically compare the results of the two

surveys; however, several generalizations can be made regarding the

distribution and effect of color in these two different light environments.

1. The distribution of spectral light in the temperate montane forest gaps

differed relative to the size of the gaps.

161

2. Areas in the gaps receiving more shade tended to show patterns in the

distribution of spectra in a gap, e.g., gap edges versus gap

centers.

3. In the tropical montane forest, horizontally-distributed light

transmissions under a forest canopy are very low, generally < 1 %.

This suggests that cloud forests are extremely light limited.

4. The horizontal distribution of light in the tropical cloud forest site was

generally uniform.

5. The vertical distribution of light spectra in the conifer-forest gaps site

showed greater blue light near the forest floor, relative to other

wavelengths examined.

6. Vertical extinction coefficients in a conifer forest differed for each

unique 5 m slice of the forest gap; attenuation of light in forest

gaps might not follow Beer's law.

7. Vertical extinction of light in a tropical forest differed for each unique

0.6 m slice of the forest overstory; also, Beer's law of exponential

attenuation of light in tropical forest canopies might not apply.

8. Vertical distribution of light spectra are most likely affected by the

geometry of adjacent stands of trees surrounding the gaps; in H. J.

Andrews Experimental Forest, south-standing trees affected

shading in gaps surveyed (Gray 1995).

9. Vertical distribution of light spectra in tropical canopies showed

162

variations in patterns; percent transmission of some wavelengths

were greater higher in the forest canopy.

10. Future work should take into account temporal variations in light a

well as spatial patterns.

APPENDIX

TABLES OF RESULTS FROM LINEAR REGRESSION

ANALYSIS OF SEEDLING DATA AND

CANOPY GAP LIGHT

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