&p.1:Abstract Significant gaps exist in the knowledge oftropical leaf spectra and the manner in which spectrachange as leaves age in their natural environment. Leafaging effects may be particularly important in tropicalvegetation growing on nutrient poor soils, such as Ama-zon caatinga, a white sand community common in theAmazon Basin. Spectral changes observed in six caatin-ga dominants include decreased reflectance and transmit-tance and increased absorptance for epiphyll-coated old-er leaves. Near-infrared (NIR) changes were most signif-icant. More detailed spectral and physical changes werestudied in one dominant, Aldina heterophylla.Over 16months, Aldina study plants produced one or two leafflushes. During leaf expansion, leaf water content andSpecific Leaf Area decreased rapidly. Over the first 6months spectral changes occurred across the spectrum,resulting in decreased transmittance and increased abs-orptance in the visible and NIR and decreased visibleand increased NIR reflectance. In contrast, significantspectral changes were restricted to the NIR over the last9 months, which showed a 10% absorptance increase as-sociated primarily with increasing epiphylls and necro-sis. At the canopy scale, increased NIR absorptance pro-vides a mechanism for producing seasonally varying for-est albedo and changing NIR to red ratios, independentof changes in other canopy attributes. In the Amazon ca-atinga studied, all canopy dominants were subject to epi-phyllic growth providing a mechanism for distinguishingthese forest types spectrally from more diverse terra-firme forest or forest types with more rapid leaf turnover,

such as second growth. These changes are observable us-ing remote sensing and could be used to map caatingaand monitor interannual or seasonal variability in phe-nology. If these results can be extended to other commu-nities with long-lived foliage, they may offer a means formapping vegetation on the basis of leaf longevity.

&kwd:Key words Leaf age · Aldina heterophylla· Amazon ·Remote sensing · Caatinga&bdy:

Introduction

Anatomical, biochemical and physiological properties ofleaves vary as a function of life strategy (evergreen vsdeciduous), physical environment, light quality, life spanand leaf age. Many changes in leaf-properties are associ-ated with changes in the spectral quality of light trans-mitted, reflected and absorbed by leaves. For this reason,spectroscopic techniques represent a potentially non-de-structive means of evaluating leaf physiological function(Gamon et al. 1995), and remote sensing of canopy spec-tral properties provides a viable approach for scaling upphysiological measurements from leaf to global scales(Field 1991). The connection between physiology andleaf optical properties has long been recognized (Knip-ling 1970; Woolley 1971; Gausman 1985) and the man-ner in which chemical and anatomical changes associat-ed with leaf aging influence leaf spectra has also beenwell established (Gausman et al. 1970, 1973; Sinclair etal. 1971). However, with the exception of Lee and Gra-ham (1986) and Lee et al. (1990), few measurementshave been made of tropical broad-leaf species and fewerhave focused on the effects of leaf aging on leaves grow-ing in situ in tropical environments. A better understand-ing of mechanisms that contribute to the spectroscopicproperties of different tropical forest communities willhelp in the classification and extraction of ecological in-formation from remote sensing. In this paper we will re-strict our discussion to the visible (400–700 nm) andNIR (700–1100 nm).

D.A. Roberts (✉)Department of Geography, EH3611, University of California,Santa Barbara, CA 93106, USATel. 805-892-2276; Fax: 805-893-3146;e-mail: dar@geog.ucsb.edu

B.W. NelsonDepartment of Botany, Instituto Nacional Pesquisas Amazonas,Manaus, Brazil

J.B. Adams · F. PalmerDepartment of Geological Sciences, University of Washington,Seattle, WA, USA&/fn-block:

Trees (1998) 12:315–325 © Springer-Verlag 1998

O R I G I N A L A RT I C L E

&roles:D.A. Roberts · B.W. Nelson · J.B. Adams · F. Palmer

Spectral changes with leaf aging in Amazon caatinga

&misc:Received: 18 November 1996/Accepted: 24 December 1997

Under nutrient poor conditions, an evergreen lifestrategy and long lived leaves can be advantageous (Cha-pin 1980; Chabot and Hicks 1982). For example, overlarge expanses of the Amazon River Basin dominated byterra firme and tall caatinga forests, where nutrient poorsoils and an evergreen life strategy are common, averageleaf life spans have been observed to vary 30 fold, withsome leaves living over 4 years (Reich et al. 1991). Overthe lifespan of a leaf, significant anatomical, chemicaland physiological changes occur (Field and Mooney1983; Reich et al. 1991; Schaffer et al. 1991; Sobrado1994; Brooks et al. 1996), many of which can have anassociated spectral response.

In addition to natural morphological and physiologi-cal changes associated with leaf aging, long lived foliageis subjected to mechanical damage that may result inspectral changes. In the humid tropics, one mechanismthat may account for significant changes in leaf opticalproperties is colonization by epiphylls. Epiphyll is a gen-eral term used to describe fungi, leafy liverworts, li-chens, algae and bacteria that form communities on leafsurfaces (Smith 1982). The impact of epiphylls on hostleaves and their ecological significance are not well doc-umented (Coley and Kursar 1996). Spectrally, epiphyllshave been shown to reduce visible light transmission andthereby limit photosynthesis (Coley et al. 1993), but theeffect of epiphylls on leaf reflectance in visible light orspectral effects in the NIR is unknown.

Spectral changes associated with leaf aging may be animportant mechanism that allows separation of differentvegetation types using remote sensing. The Amazon caat-inga, with its long-lived leaves, has distinctly low NIR re-flectance relative to adjacent forest types at a landscapescale (Fig. 1). In color-infrared photography collectedabove Amazon caatinga, shrubs and small trees with old-er foliage are readily distinguished from plants with new-ly emergent foliage by their low NIR reflectance

316

Fig. 1 Color infrared images showing the study site location, theINPA campina and a color infrared aerial photograph over thestudy site. (a) shows a Landsat Thematic Mapper (TM) image ofthe general vicinity of Manaus, with NIR, red and green reflectedlight displayed as red, green and blue, respectively (to match col-or-IR photographs). Second-growth forest is bright red, primaryforest intermediate red and caatinga dark red. (b) color-IR photo-graph of the INPA campina collected in September 1991. Plantswith new foliage are bright red, while plants with old foliage aredark red or blue tinted. Open sand is white. The locations of thefour study plants are shown&/fig.c:

(Fig. 1b). At larger scales, caatinga is readily distin-guished from adjacent terra firme forest and second-growth forest by its lower NIR reflectance, resulting in adark red forest in this color rendition of a Landsat The-matic Mapper image acquired in the vicinity of Manauson 21 August 1991 (Fig. 1a). In contrast, terra firme hasintermediate NIR reflectance, with the highest values oc-curring in crops and second-growth forest. A quantitativecomparison of randomly selected caatinga and terra firmeforest sites located in this image demonstrated statistical-ly significantly lower reflectance across the entire NIRspectrum (Table 1).

To use remote sensing for mapping vegetation, it isimportant to understand the spectral dynamics of leafaging in these dominant forest types. This study exam-ined potential mechanisms of leaf aging and epiphyllgrowth for changes in reflectance, transmittance andabsorptance at the leaf scale. We also consider howthese results may affect larger scale patterns for remote-ly sensed spectral images.

Materials and methods

To test the leaf aging hypothesis we utilized spectroscopic tech-niques to study temporal changes in visible to NIR leaf spectra. Todocument the potential role of leaf aging we measured spectrafrom six caatinga dominants that showed similar patterns (Ta-ble 2), then focused a more detailed study on a single species, Al-dina heterophylla, tracking changes in leaf spectral propertiesfrom leaf expansion to senescence. We concentrated on Aldinaheterophyllabecause it has a slow growth rate and produces infre-quent leaf flushes (once or twice in a season) offering a unique op-portunity to study leaf aging effects without the confounding ef-fects of increasing shade.

Unlike previous spectral studies of tropical leaves (Lee andGraham 1986; Lee et al. 1990) and leaf aging in trees and shrubs(e.g. Lin and Ehleringer 1982), all plants in this study were grow-ing in situ and thus were exposed to natural environmental factorsthat would be difficult to simulate in a greenhouse. Furthermore,an effort was made to track populations of leaves rather than usingleaf nodal position as a surrogate for age.

Site and species information

Known as carrasco, campina, or Amazon caatinga in Brazil, var-rilal in Peru and bana in Venezuela, most vegetation on white

sands consist of closed canopy forests, although in some placesthe canopy is low and broken by patches of bare sand. Anderson(1981) suggested standardizing the nomenclature to “Amazon ca-atinga scrub” and “Amazon caatinga forest” for the open andclosed canopy types, respectively. Amazon caatinga is floristicallydistinct from the neighboring terra firme forest growing on welldrained clay soils. Amazon caatinga formations cover a large area,totaling 282 300 km2 or 7% of the total primary forest of the Ama-zon Basin (Nascimento and Homma 1984). Some of the typicalgenera near Manaus are Aldina, Pagamea, Clusia, Ouratea, Pra-dosiaand Humiria.

The study was initiated in the “Reserva Campina”, a 900 ha re-serve located 45 km north of the city of Manaus in a poorlydrained, nutrient-limited environment that can be characterized ashaving low species diversity and low biomass (Fig. 1). The sitewas selected because it permitted easy access to plants over an ex-tended period of time. The reserve includes approximately 1 ha ofopen Amazon caatinga scrub, and 25 ha of low woodland thatgrades into tall Amazon caatinga forest. Near Manaus the Amazoncaatinga formations occur in isolated pockets and dominant spe-cies can vary from one patch to another. At the Reserva, the domi-nant woody species in low closed-canopy woodland and opensand-shrub formation are Aldina heterophylla and Pradosiaschomburgkiana.

Study design

The study extended over a 16 month period starting on 13 July 1993and ending on 26 October 1994 (Fig. 2). In the initial survey, in July1993, leaves were sampled from the six dominants in the study area(Table 2). Leaves were sampled covering a range of ages from eachspecies and placed into broad categories of young (unexpanded),mature and old. They were further categorized on the basis of epi-phyll cover as uncolonized, moderate or heavy cover. Spectra fortwo of the species are shown in Fig. 3. Aldinaspectra were collectedin more detail, sampled 12 times at monthly intervals, with a gap incoverage in December 1993, and February, May and August 1994.

317

Table 1 Statistical comparison of caatinga and terra firme forest.Data were extracted from 20 randomly selected patches of caatin-ga and 20 randomly selected patches of terra firme forest locatedon a Landsat TM scene acquired on 21 August 1991. Each patch

consisted of a 3×3 pixel average totaling 90 m on a side. Patcheswere selected to avoid small streams and canyons. Units are en-coded radiance measured by the TM sensor&/tbl.c:&tbl.b:

Target Wavelength (nm)

485 550 660 830 1650 2250

Terra firme N = 20Mean 73.27 30.79 28.99 64.78 49.4 9.3SD 0.91 0.65 0.715 2.13 2.19 9.59caatinga N = 20Mean 73.79 31.23 29.32 61.78 45.71 8.46SD 0.70 0.58 0.71 1.10 1.81 0.61T statistitc 2.024 2.280 1.452 -5.616 -5.818 -4.435P value 0.01 0.05 0.02 <0.001 <0.001 <0.001

&/tbl.b:

Table 2 Dominants sampled in the INPA Campina&/tbl.c:&tbl.b:

Genus and Species Family

Aldina heterophylla Spr. ex Benth. Fabaceaea

Clusia aff. columnaris Engl. Clusiaceaea

Humiria balamifera St. Hil. Humiriaceaa

Pagamea duckei Standl. Rubiaceaa

Pradosia schomburgkiana SapotaceaeProtium heptaphyllum (Aubl.) March. Proteaceae

a Source, Anderson et al. 1975&/tbl.b:

Physical measurements of leaves (fresh weight etc.) were restrictedto the first 4 months (July to October 1993), then from January toApril 1994 and a final set of measurements in September 1994.

Three sample trees were initially located in the Reserva in theopen sand portion on 13 July 1993 (Fig. 1b). Plants were selectedon the basis of a recent leaf flush (< 1 month) and ease of accessto illuminated crown leaves. Each of the trees were between 2 and5 m tall with new, fully expanded green leaves. Two trees (1 and3) were later determined to be extensions of the same tree, a factthat was not noticed at first since the trees have long spreadingbranches that touch the ground. Although from the same plantthese branches will be referred to as Plants 1 and 3 throughout thepaper. A fourth tree (Plant 4) with a fresh flush of unexpandedleaves was added on August 15 to provide a more exact estimateof leaf age. Six branches on each tree with fully sunlit leaveswere tagged. Leaves remained in full sun throughout the length ofthe study. Photographs of each tree were collected from a fixedlocation to ease relocation and permit a comparison of plants atthe start and end of the study. Leaf flush was nearly simultaneouson the selected trees and this species was at first thought to dem-onstrate synchronous leaf flush. However, Plants 1 and 3 pro-duced a second flush of leaves in the fall, necessitating more de-tailed tagging of individual leaves to distinguish older leavesfrom the new crop.

At roughly monthly intervals, three leaves were randomly se-lected from the six branches and destructively harvested from eachtree by snipping the petiolule. After each harvest, leaves wereplaced in sealed plastic bags with a few drops of water or watersoaked tissue. The bags were then placed in a closed styrofoamcontainer and the leaves were later transported to Manaus for mea-surements. Leaves of this species are very coriaceous and remainfresh for many days. Leaves were kept in the dark in sealed plasticbags with 100% humidity and measured within 24 h.

318

Fig. 2 Timeline for phenology and spectral sampling. The verticalposition of each collection as shown on the plot corresponds to therelative time at which measurements were made with collectionone corresponding to day zero. Precipitation measured in Manausis plotted to the left&/fig.c:

Fig. 3 Reflectance, transmit-tance and absorptance spectrafor Pradosia schomburgkianaand Protium heptaphyllum. Un-colonized, mature leaves areshown with thin, light greylines, old foliage with moderateepiphyll cover as medium greylinesand old foliage withheavy epiphyll cover as thick,dark lines. Spectral differencesdue to leaf age and epiphyllcover are far more pronouncedthan species differences&/fig.c:

Leaf spectroscopy

Leaf spectra were measured using a portable spectrometer (Per-sonal Spectrometer II, Analytical Spectral Devices, Boulder,Colo., USA) attached to an integrating sphere (Model 1800–12,Licor, Lincoln, Neb., USA). All spectra were standardized using abarium sulfate standard and the calibrated light source supplied byLicor with the integrating sphere. Integrating sphere performanceand stray light contamination were measured prior to each month’smeasurements to ensure temporal stability. No stray light correc-tion was applied to the leaf spectra after it was determined to ac-count for an error of less than 0.2% in most measurements. All in-struments were operated off batteries to ensure a stable energysource. Leaf reflectance and transmittance were measured for bothsurfaces of each leaf using leaf squares cut from the left and rightsides of the leaf taking care to avoid the mid-vein. At the start ofthe experiment a leaf map was constructed, ensuring that leafspectra were always collected from the same position on the leaf.From each leaf a pair of four spectral measurements (reflectanceand transmittance for both surfaces) were made, providing a fullspectrum and replicate for three leaves from each plant (a mini-mum of six spectral suites/plant). The spectrometer used has anominal spectral range between 329 and 1051 nm, collected in512 spectral bands at a sampling interval of 1.4 nm, although apoor signal to noise ratio at the extremes restricted the usefulrange of this instrument to 400 to 1020 nm. To ensure that a 24 hdelay was not significant, initial tests were conducted repeatingspectral measurements on a set of leaves over a 3 day period. Nosignificant changes were observed for Aldina. Each monthly sam-ple resulted in between 80 and 112 individual spectra, producing atotal of over 1300 spectra over the 16 months.

To account for the potential effects of leaf coatings or naturalleaf damage on spectra, the presence of epiphylls or necrosis wasnoted for each leaf surface measured by the spectrometer. Epiphylldevelopment was assessed by visual inspection and placed intoone of four categories: uncolonized, slight, moderate and heavy.When present, localized necrosis was recorded as absent, minor(one to two small patches) or significant.

In addition to spectral measurements, a pair of leaf disks (usinga no. 9 leaf punch, 1.26 cm diameter) was collected from each leaf.These leaf disks were patted dry and weighed on an analytical bal-ance to determine fresh weight. After weighing they were placed ina vacuum-sealed container with silica gel and dried at ambient tem-peratures for at least 4 months. After drying the leaf disks were re-weighed to determine dry weight. Percent water content was calcu-lated as the ratio of fresh weight minus dry weight divided by freshweight. SLA was determined as the ratio of the area of the freshleaf disk in cm2 divided by the dry weight (in grams). Leaf thick-ness was measured for each leaf disk at the start and end of thestudy using a set of digital calipers (Model 293-761, Mitutoyo, To-kyo, Japan).

Leaf reflectance and transmittance from both surfaces wereused to calculate leaf absorptance (1-R-T) in the upward anddownward direction. Leaf spectra, derived spectral measures andphysical measures were averaged for each tree to reduce the datavolume. Each tree was analyzed separately because of the uniquebehavior of each plant, in particular the difference between Plants2 and 4 and Plants 1 and 3.

Epiphyll identification

Leaf samples were examined to determine the nature of epiphyllsand investigate their potential role in modifying leaf spectra. Spec-imen leaves from Aldina, Ouratea, Pradosia and Protium, wereexamined in the laboratory and used to isolate epiphylls, primarilyfungi. Isolates from leaf specimens were grown on maltose yeastextract agar and identified.

Results

Phenology and physical attributes

Initial observations of several fully flushed Aldinacanopiesin July 1993, suggested synchronous leaf turnover. Thiswas found not always to be the case early in the experi-ment (Fig. 2). As mentioned, a fourth plant was added tothe study on August 15 as it produced a new flush ofleaves. Leaf retention and the number of times individualplants flushed leaves within the growth season both varied.Exact leaf ages were determined for Plant 4, which wastracked from an initial flush on 15 August 1993 throughfull expansion prior to 21 September, and bud break on 26October 1994, 1 year later (Fig. 2). For this plant, leaveswere retained at least 437 days (over 14 months). Based ona comparison of the physical properties (e.g. SLA, leafthickness) of leaves from Plant 4 to leaves from the otherplants, it is possible to estimate the timing of leaf flush forPlants 1 to 3 as occurring in mid to late June. On the basisof a late June flush, leaf retention ranged from 14 to 15months for Plants 1 and 3 to over 16 months for Plant 2.

Leaf flush in all individuals was restricted to the dryseason for both years. However, Plants 1 and 3 produceda second flush of leaves between 21 September and 16October 1993, near the onset of the rainy season. After16 October 1993, no new leaves were produced on anyof the plants until 3 September 1994, when Plants 1 and3 were observed to have almost fully expanded leaves,suggesting leaf flush in August. Leaf flush occurred ap-proximately 1 month later in Plant 2, which had fully ex-panded leaves by October 1994 and over 2 months laterin Plant 4. All individuals showed a delay of at least 2months in leaf flush in 1994, relative to 1993.

Percent water content and SLA markedly declinedover the first 3 months of the study, followed by a moregradual decrease (Fig. 4). Throughout the study, percentwater content remained lowest in Plant 2 (Fig. 4a). Aslight increase was found for all plants on 3 September1994. SLA showed similar, rapid early declines(Fig. 4b), followed by a more gradual decline up to 24April 1994. However, unlike percent water, there was nosignificant difference between the four plants in SLA.

Seasonal changes in leaf water content and SLA arenot surprising. Initial rapid changes correspond to natu-ral maturation of leaves over the first 70 days. Less rapiddeclines in SLA over the growth season may representaccumulation of organic materials such as starch or lig-nin after leaf internal structure has been set. Accumula-tion of organic materials is also suggested by a decreasein SLA with no corresponding change in leaf thicknessover the 15 month period (Table 3). Decreasing SLAcoupled with a fixed leaf thickness translated to as muchas a 1.5 fold increase in dry weight leaf density.

Leaf spectra

Results from the initial survey of leaf spectra are summa-rized using two species, Pradosiaand Protium (Fig. 3).

319

Plots are shown of reflectance, transmittance and derivedabsorptance for uncolonized leaves and leaves with mod-erate and heavy epiphyll cover. Spectra cover the visible(400–700 nm) and the NIR (700–1100 nm). Uncolonizedleaf surfaces for both species showed similar patternstypical of most broadleaf evergreen species; reflectancelows occurred between 400 and 500 nm and at 590, 620and 680 nm with a reflectance peak at 550 nm(Fig. 3a,d). Peak reflectance occurred in the NIR, at750 nm, with the strongest NIR absorption occurring at980 nm due to liquid water absorption. Visible light re-flectance of Protium was slightly higher than Pradosia,while the opposite pattern occurred in the NIR. At nowavelength was the difference greater than a few percentreflectance. Transmittance in leaves of both species waslower than reflectance in both the visible and NIR (Fig.

3b, e). Transmittance spectra had similar shapes for bothspecies but were 4 to 5% higher in Protium at 550 nmand in the NIR, potentially due to differences in leafthickness. Pradosia leaves sampled had an averagethickness of 0.328 mm compared to 0.204 mm for Proti-um. Higher transmittance in Protium resulted in lowervisible light absorptance at 550 nm (Fig. 3c,f). Ab-

320

Table 3 Leaf thickness (mm),average SLA (cm2/g = fresh ar-ea/dry weight) and average spe-cific leaf volume (cm3/g = freshvolume/dry weight) in July 1993and September 1994. N = 6&/tbl.c:&tbl.b:

July 1993 September 1994

Plant Thickness SLA SLV Thickness SLA SLV

Plant 1 0.308±0.009 112.0±1.8 3.45 0.309±0.020 77.5±3.8 2.40Plant 2 0.309±0.007 95.2±3.3 2.94 0.317±0.008 76.9±2.2 2.30Plant 3 0.317±0.008 110.3±7.0 3.50 0.312±0.007 74.0±4.0 2.35

&/tbl.b:

Fig. 4 Changes in (a) percent water content and (b) SLA forPlants 1 (■), 2 (●), 3 (s) and 4 (◆) from 13 July 1993 (0) to 3September 1994&/fig.c:

Fig. 5 Temporal changes in spectra from 400 to 1020 nm forPlant 1: (a) Reflectance, (b) transmittance and (c) absorptance forthe upper leaf surface for July, August, and November 1993 andJanuary, June and September 1994. Line thickness decreases fromJuly to September. Arrowspoint from young to old leaves&/fig.c:

sorptance in both species ranged from 95% in blue light(~450 nm) to less than 2% in the NIR.

Increasing epiphyll cover was associated with lowerreflectance and higher absorptance in both species. Dif-ferences were most pronounced in green light (~550 nm)and NIR. These results were most extreme for leaves inthe heavy epiphyll category, in which NIR reflectanceand transmittance decreased by over a factor of two, re-sulting in NIR absorptance ranging from 70% at 750 nmto 40% at 900 nm. Spectral differences between species,in comparison to differences between epiphyll cover cat-egories, were relatively minor.

The more detailed study of Aldina produced similarresults, although none of the samples reached the heavyepiphyll category. As leaves aged, green reflectance andtransmittance of the upper surface decreased while NIR

reflectance increased slightly and NIR transmittance de-creased (Fig. 5a,b). Similar observations for the visiblespectrum have been made by others (e.g. Lin and Ehle-ringer 1982), and attributed to increases in chlorophyll(Lin and Ehleringer 1982) and changes in leaf internalanatomy (Rock et al. 1994). Early changes in NIR reflec-tance and transmittance have also been noted, with ageneral shift from high NIR transmittance/low NIR re-flectance for expanding leaves followed by a decrease intransmittance and an increase in reflectance. A decreaseof this magnitude in NIR transmittance after full matura-tion has not been noted previously.

Analysis of derived absorptance spectra demonstrateda significant increase in green absorptance with no corre-sponding increases in either blue or red (~650 nm) wave-lengths (Fig. 5c). NIR absorptance also showed signifi-cant changes, increasing from near zero for young leavesto 10% for old leaves. The change in NIR absorptancewas less than changes in transmittance due to the offset-ting effect of increased reflectance.

Results for the four study plants are summarized fortemporal plots of 550, 676 and 830 nm, herein referred

321

Fig. 6 Changes in leaf spectra in green, red and NIR for Plants 1 (■),2 (●), 3 (s) and 4 (u) from 13 July 1993 (0) to 3 September 1994.Spectra include: (a) 550 nm reflectance and transmittance, (b)676 nm reflectance and transmittance, (c) absorptance at 550 and676 nm and (d) reflectance, transmittance and absorptance at 830 nm&/fig.c:

to as green, red and NIR (Fig. 6). Green reflectance de-creased in all study plants, showing the largest decreaseduring the first 70 days, followed by a more gradual de-crease to day 186 (Fig. 6a). Transmittance showed a sim-ilar pattern, but was consistently lower than reflectanceby a factor of 3 except in very young leaves. When usedto derive absorptance, decreasing reflectance and trans-mittance resulted in a significant increase in green abs-orptance over the first 70 days, followed by a more grad-ual increase until day 186 (Fig. 6c). In red wavelengths,in contrast, reflectance showed a steady increase over thesame period. Increasing reflectance was matched by aninitial rapid decrease in transmittance over the first 70days, followed by a slight decrease over the remainingperiod. The combination of increasing red reflectanceand decreasing red transmittance resulted in no netchange in absorptance except in the case of young leaves(Fig. 6c). In the NIR, reflectance showed an initial rapidincrease over the first 70 days then stabilized (Fig. 6d).Initial increases in reflectance were mirrored by a de-crease in transmittance over the same period. However,unlike reflectance, transmittance continued to declineover the study period, resulting in a net increase in abs-orptance from near zero to 10% (Fig. 6d).

Similar spectral behavior was observed in all of theplants studied. However, the magnitude of change dif-fered between each plant. In all cases, Plants 2 and 4 hadconsistently higher visible light reflectance and transmit-tance, resulting in significantly lower visible light abs-orptance. Differences were less pronounced in the NIR.The only significant difference observed was in Plant 4,which had consistently higher transmittance and lowerabsorptance than leaves from the other plants.

Epiphylls and necrosis

When examined in the laboratory the upper leaf surfacesof dried specimens were observed to bear large patchesof dark fungi. Other epiphylls, such as leafy liverwortsalgae and lichens were not identified, although they mayhave been present. When moistened, fungi had little ad-herence to leaf surfaces and little hyphal penetration, al-though a few areas did show evidence of penetration.Discoloration of the specimen leaves was primarily dueto pigmented epiphylls on the upper leaf surfaces. Themost frequently encountered specimen was identified asPestalotiasp. a member of the Melanoconiales that hasbeen reported as a plant pathogen. Additional isolates in-cluded molds and yellow and rose pigmented bacteria.

The effects of epiphylls and necrosis on NIR abs-orptance were evaluated using leaf surface informationrecorded for portions of the leaf measured by the spec-trometer. Three epiphyll density categories, uncolonized,slight and moderate were observed. An additional cate-gory, necrotic, was added to account for leaves thatshowed significant areas of necrosis. NIR absorptanceand the number of leaves in each category are reported(Fig. 7). To minimize clutter, NIR absorptance is report-

ed for a subset of months in which significant changeswere observed (Fig. 7a). Both uncolonized and slightlycoated leaves showed an initial rapid increase of 4% overthe first 70 days followed by little change over the next116 days. By the end of the study, NIR absorptance had

322

Fig. 7 (a) Changes in NIR absorptance from 13 July 1993 (0) to 3September 1994 (417) for leaves that were uncolonized, had slightand moderate epiphyll cover or were necrotic. One standard devia-tion is plotted as a thin line above each column. (b) Leaf countsfor each category as they varied over the study period&/fig.c:

increased to between 6 and 7%. In contrast, leaves with amoderate epiphyll coatings and necrosis showed signifi-cantly higher NIR absorbtance than uncolonized andslightly coated leaves of the same age. For the period be-tween day 186 and day 417 all moderately coated or ne-crotic leaves showed significantly higher NIR abs-orptance, with averages near 13%.

The relative importance of epiphylls and necrosis inmodifying Aldina spectra at the canopy level may be bet-ter revealed by the numbers of leaves that fell into eachcategory and how these numbers changed during thecourse of the study (Fig. 7b). Over two sampling periodsall of the leaves were categorized as uncolonized. On day70 the first slight coatings and the first incidence of ne-crosis were observed. Following day 70, uncolonizedleaves were always present, but slight and moderate epi-phyll coatings and necrosis become increasingly com-mon. By the final two sample periods, days 375 and 417,few leaves were uncolonized, with a majority having atleast a slight coating and between a third and half havingsignificant necrosis or moderate epiphyll coatings. As aresult, the net effect of an increase in NIR absorptance onan individual leaf basis, will be magnified by an increasein the incidence of coatings or necrosis on a canopy level.

Discussion

Physical attributes and phenology

Leaf longevity is commonly hypothesized to be adaptivein nutrient poor environments where construction costsare high and the potential for photosynthesis low. Physi-cal attributes of long-lived leaves commonly include lowSLA, high leaf thickness, and high leaf toughness. Aldi-na heterophylla, which lives in a nutrient poor, periodi-cally waterlogged environment, shows leaf thickness,leaf longevity and SLA characteristics consistent withfindings by others (e.g. Reich et al. 1991) for plants insimilar environments.

Although not synchronous between study plants, leafflush was restricted to the dry season. Dry season leafproduction and plant-synchronous leaf flush in the tro-pics have been hypothesized to be adaptive for minimiz-ing damage by insect herbivores (Aide 1988, 1992,1993). Alternatively, dry season leaf flushing and flower-ing may occur in response to rehydration of stemsthrough available water in soil or trunks following re-duced transpirational water loss due to leaf shedding(Reich and Borchert 1982; Borchert 1994). Differencesin the timing of leaf flush observed between 1993 and1994 also raise interesting questions. In 1993, leavesflushed in mid- to late June, while in 1994 leaf flush oc-curred 2 months later. These patterns match changes inthe start of the dry season, as defined as the first monthwhere potential evapotranspiration (PET) exceeds pre-cipitation. For the Manaus area, average daily PET hasbeen estimated at 3.5 mm/day or 105 mm/month (Mar-ques et al. 1980). Based on rainfall records from Mana-

us, this places the start of the 1993 dry season in Mayand the start in 1994 in July (Fig. 2). Leaf flush in bothyears occurred approximately 1 month after the start ofthe dry season, resulting in a 2 month delay in 1994 rela-tive to 1993. Changes in the start and length of the dryseason have obvious implications in terms of the age dis-tribution of leaves in this community. For example, the2-month delay in leaf flush in 1994 may have resulted inleaf ages ranging from 14 to 16 months. Had the patternbeen reversed, and the dry season started later in 1993and earlier in 1994, leaf ages may have been as short as10 months, resulting in a much more rapid turnover time.

During the 6 month dry season in 1993, differencesalso emerged in the number of times specific plantsflushed new leaves. Two of the plants (2 and 4) produceda single flush of leaves over a period of 16 months whilethe other plant (1 and 3) produced two leaf flushes sepa-rated by several months. Although genetic variabilitymay account for differences in leaf flush, spectral andphysical differences between leaves also suggest thatgrowth conditions may account for the differences.Plants that produced a single flush showed consistentlylower water content and lower visible absorptance whichare consistent with an interpretation of poorer chloro-phyll development and poorer growth. Soil augering hasshown high spatial variation in the depth to the perchedwater table within the scrub formation at the reserve.This spatial variability may account for differences ob-served between each plant.

Spectral changes and leaf aging

Spectral changes in the visible portion of the spectrumcharacterized the first 186 days of the study while de-creased NIR transmittance and increased NIR ab-sorptance characterized the remaining 9 months. Chang-es observed in the visible spectrum matched similar ob-servations by Gausman et al. (1973) for Zea maysandLin and Ehleringer (1982) for Carica. In general, ab-sorptance at 550 nm increased while reflectance andtransmittance decreased at this wavelength. Dramaticchanges in NIR reflectance for expanding and maturingleaves observed by others (e.g. Gausman et al. 1973)were restricted to the month between July and August atthe start of the experiment.

The only significant change in leaf spectra for the re-maining 9 months occurred in the NIR. In this wave-length region, leaves showed a consistent increase in NIRabsorptance and decrease in NIR transmittance. Althoughincreases in NIR absorption with increasing leaf age havebeen observed (Rock et al. 1994) the magnitude of thechange in Aldina was more than a factor of two greater.Comparison of Aldina to spectra measured in the initialsurvey demonstrate significantly greater NIR absorptanceis possible with heavy epiphyll coating. Analysis of NIRabsorption of uncolonized leaves demonstrated that asmall increase in NIR absorptance is a natural part of theaging process of these leaves. However, comparison of

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uncolonized leaves to leaves with moderate epiphylls orsignificant necrosis demonstrated much greater abs-orptance when leaves were coated or damaged. Becausenecrosis and epiphyll growth increase over time, bothmechanisms would be expected to have an increasinglysignificant affect on NIR reflectance at the leaf and cano-py scale. In this study the effects of necrosis on NIR abs-orptance could not be distinguished from the effects ofepiphylls. Furthermore, it was not possible to determinewhy necrosis of parts of leaves occurred. At times, necro-sis was observed in combination with epiphylls, while atother times, uncolonized leaves were highly necrotic. Thepoor hyphal penetration and ease of their removal fromleaf specimens suggests that epiphylls may be usingleaves primarily as a growth surface. However, the pres-ence of at least one fungal genus, Pestalotia, which isknown to be a plant pathogen, does suggest a potentiallydeleterious relationship. Increased NIR absorptance maybe a byproduct of light absorbing pigments within epi-phylls, such as melanins, necrosis caused by epiphylls, ordue to necrosis from other causes (e.g. insect damage).

At the canopy scale, canopy reflectance is a productof competing mechanisms of light absorption, primarilyby chlorophyll and liquid water and light scattering byleaves and woody materials such as bark and trunks. Inthe visible spectrum, where absorption dominates, in-creased absorption would be expected to have a minorimpact on canopy reflectance. However, in the NIR,where scattering dominates, any mechanism that resultsin an increase in absorption (such as decreased transmit-tance in Aldina, or decreased reflectance and transmit-tance in Pradosia and Protium) will have an enhancedeffect. It is in this region where we would expect the im-pact of leaf aging and epiphyll growth to have its largestimpact. In the INPA Campina, all dominant shrubs wereobserved to be impacted by epiphylls. As a result, en-hanced NIR absorptance would be expected to occur atthe canopy scale, which may account for the low NIR re-flectance observed in TM images and in color-IR pho-tography (Fig. 1a). In a similar fashion, epiphylls mightbe expected to impact terra firme forest. However, thedegree to which leaves are colonized by epiphylls variesconsiderably between different tree species (Richards1996). When combined with the higher species diversityof terra firme forest (Anderson 1981), which would beexpected to produce considerable variation in phenology,leaf turnover and susceptibility to epiphylls, it is unlikelythat epiphylls would have the same impact at the canopyscale. In contrast to terra firme forest, second-growthforest is typified by much lower species diversity (Uhland Jordan 1984). However, second-growth forest in thevicinity of Manaus is typically dominated by fastergrowing species with considerably higher leaf turnoverrates and shorter leaf life spans. In this case, short-livedleaves may not live long enough be colonized. Field ob-servations in second-growth forest by the authors supportthese conclusions. In related spectroscopic studies insecond-growth forest (D. A. Roberts, unpublished data),no significant epiphyll colonization was observed.

The ecological role of epiphylls in caatinga forestcannot be answered by this study. Based on this research,we know that they are present on all of the species westudied, and can have a significant impact on the spectralproperties of caatinga in the NIR. However, based on re-search in nutrient poor environments, it is possible tospeculate on the role they may play. For example, Jordanet al. (1980) speculate that epiphylls may play an impor-tant role in recycling nutrients in nutrient poor podzols.Richards (1996) suggests that epiphylls, through im-proved nitrogen fixation, may contribute significantly tonutrient conservation at the ecosystem level. Althoughepiphylls reduce photosynthesis, and thus may be deletr-ious to individual leaves (Coley and Kursar 1996), thisnegative impact may be offset in a very nitrogen limitedecosystem, such as Amazon caatinga (Herrera 1985).

Conclusions and implications

These results have their most direct implications for eco-logical or physiological studies that utilize remote sens-ing. The most significant finding is an increase in NIRabsorptance with increasing leaf age. To date, most ofthe spectral changes that have been documented involvechanges in visible light reflectance and absorption due tochanges in pigments, and changes in NIR reflectance andtransmittance due to changes in internal leaf structure. Asignificant increase in NIR absorptance with increasingleaf age, however, has not been documented. In thisstudy, we observed a significant increase in NIR abs-orptance as leaves aged from a variety of caatinga spe-cies growing in central Amazonia. Increased NIR abs-orptance may be attributed primarily to increased leafnecrosis and epiphyllic growth on leaf surfaces.

Increased NIR absorptance at the leaf scale may have alarger impact at the canopy scale by dampening NIR scat-tering within a canopy and thereby reducing canopy re-flectance. This may provide a mechanism for producingseasonally varying, decreasing forest albedo and increas-ing NIR canopy absorption independent of changes in leafarea, illumination or other factors. Damped NIR canopyreflectance may also be significant in terms of its effect ona variety of spectral vegetation indices (SVIs) used to esti-mate ecologically significant parameters including net pri-mary production (NPP), absorbed photosynthetically ac-tive radiation (APAR), canopy resistance, and photosyn-thetic capacity (Sellers 1985; Sellers et al. 1992; Gamon etal. 1995, Field et al. 1995). Spectral changes observed inthis study provide an alternative mechanism for producingchanges in spectral indices. For example, based on theseresults, a steady decrease in the ratio of NIR to red reflec-tance would be expected, implying a decrease in leaf areaand decrease in APAR, even if no change had occurred.

Finally, these results imply that, if leaf-aging effectsobserved in this study are extendable to other ecosys-tems, it should be possible to map caatinga over large ar-eas of the Amazon basin using remote sensing. Early inthe growth season, spectral changes should be most

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prominent in visible wavelengths while later, over themajority of the life of leaves, changes will be most pro-nounced in the NIR. In addition, it should be possible tomonitor phenological responses as they may vary withina season or from one year to the next. Once mapped itshould be possible to assign appropriate values for pho-tosynthesis and transpiration using published valuesfrom similar forests and thus improve basin-wide esti-mates. Finally, it may be possible to classify vegetationover large areas into age related classes, in addition tomore standard classification schemes.

&p.2:Acknowledgements We thank the Instituto Nacional de Pes-quisas na Amazonia for facilitating laboratory and field work dur-ing the course of the study. All field and laboratory spectroscopicmeasurements were made using equipment purchased by the Inter-national Atomic Energy Agency in support of the Amazonia Oneproject. The authors also wish to acknowledge Dr. J. ReneeBrooks, Dr. John Gamon and Marianne Wampler for helpful com-ments while preparing the manuscript. Funding for this researchwas provided in part by NASA Grant NAGW-2652, University ofWashington subcontract 470097 and the University of California,which supplied start-up funds for computing facilities.

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