Plant Canopy Analysis

Colin S. Campbell, Ph.D.Decagon Devices and Washington State

University

Radiant energy and plant canopy analysis

Why do we care about the radiation environment of plant canopies?

Calculate Leaf Area Index (LAI) Crop growth stage Ecosystem health Radiation use efficiency

Radiant energy and plant canopy analysis Must have detailed knowledge of light

environment to use photosynthesis models

Partitioning ET into E and T Need to know fraction of energy intercepted

by canopy and fraction transmitted to soil

Intercepted PAR and Biomass Production Cumulative intercepted

photosynthetically active radiation (PAR) linearly related to total biomass production

Tools for detailed analysis

Radiation budget & view factors Fisheye analysis

Plant canopy light environment Fisheye analysis Ceptometer – measures light interCEPTed

by canopy

Definitions Zenith angle (Ψ) – angle between sun and the

zenith (vertical)

Transmission coefficient (τ) – fraction of sunlight transmitted through canopy to ground

Direct beam radiation – sunlight coming directly from sun (leaves a shadow)

Diffuse radiation – sunlight that has been scattered

Ψ

Plant canopy light environment -leaf area index (LAI)

Leaf Area Index (LAI) – One-sided surface area of leaves/surface area of soil

Unit area basis: m2 m-2

How do we measure Destructive sampling Light attenuation

Ceptometer Fisheye photograph

LAI – destructive sampling Harvest leaves from 1

m2 canopy area Physically measure

surface area with optical meter

Advantages: direct measurement

Disadvantages: destructive, time consuming, wilting

LAI-light attenuation theory

A

anexp

A

a

a

LAI is transmission and n is the number of leavesThis is only true if all of the leaves are horizontal

Unit ground Area

a

Leaf with area = a

Real canopies (leaves aren’t all horizontal)

Leaf angle distribution parameter (χ) Describes the orientation of the leaves

vertical canopyχ = 0

(onions < 1)

spherical canopyχ = 1

(most canopies)

horizontal canopyχ = ∞

(strawberries χ = 3)

Extinction coefficient We can use the leaf angle distribution to

determine an extinction coefficient (G) at a particular zenith angle

773.0

222

182.1774.1

sincos

G

cos

*exp

LAIGWhat does G do for us?

- allows us to relate τ to LAI for any canopy, given

What parameters do we need to calculate LAI?

Zenith angle (ψ) Time and location

latitude and longitude

Canopy extinction coefficient (G) Calculate from leaf angle distribution (χ) = 1

for most canopies

Canopy transmission coefficient (τ) Estimate from fisheye image Use ceptometer to measure directly

LAI from Ceptometers

Measure above-canopy radiation

Measure below-canopy radiation

Meter calculates τ

LAI from Ceptometer

What else does a meter need to calculate LAI? Time and location (zenith angle) Estimate of leaf angle distribution

parameter (χ)

Note: LAI measurements are best without beam radiation (only diffuse radiation) broken clouds are worst (changing

radiation conditions)

Decagon Accupar LP-80

LI-COR LAI-2000

What is a fisheye analysis? Simply a projection of a hemisphere onto a

plane In our case, a picture is taken through a special lens

that projects a full 180 degree hemisphere onto the film

What does this image allow us to do?

Determine view factors of surrounding objects

View factor determines radiative influence of one object on another

Determine light transmission coefficients through a canopy – diffuse and direct

What does this image allow us to do?

Determine when a particular location will be in direct sunlight

Determine what percentage of time a location will be sunlit

Determine τ, LAI, and χ values

Determining view factors

Simplest analysis

total

objectobject A

AF

Aobject is the area of the picture taken up by the

object of interest

Atotal is the total area of the photograph

View Factors

Determining transmission coefficients

1. Project the fisheye picture onto a grid

30

60

90 E

120

150

180

S

210

240

270 W

300

330

Determining transmission coefficients

2. Visually estimate the fraction of sky visible in each grid sector (1 = full sky)

3. Average the value for each zenith angle band = τψ (we will do this for our lab picture for 15, 45, and 75°)

754515 25.05.025.005.1 d

30

60

90 E

120

150

180

S

210

240

270 W

300

330

= 1.00

~ 0.05 ~ 0.3

N

Calculating average understory radiant fluxes

τd is the transmission coefficient for diffuse radiation But, if we average over a day or longer, it

approximates the total radiation transmission coefficient so:

avgTdbc S

Φbc = average radiant flux density below canopyST avg = average total radiation above canopy

Calculating times and duration of direct beam

Plot sun path on grid Segment into time steps (hours)

E

S

W

N

LAI from fisheye photo First method

G58 = 0.5 for all leaf angle distributions! Determine τ at ψ = 58° from fisheye photo

5.0

58cos*)ln( 58LAI

1. Determine τ at ψ=15, 45, and 75° from fisheye photo

2. Calculate G at each zenith angle with an arbitrary value of χ

3. Calculate new values of τ for each zenith angle using G from step 2 and an arbitrary LAI value

4. Calculate sum of squared errors between τfisheye and τarbitrary

5. Use solver to minimize SSE by adjusting arbitrary LAI and χ values

6. Results in decent estimate for both LAI and χ!

LAI from fisheye photo(method 2: use solver in excel)

LAI from fisheye photo(method 3: use software package)

HemiView software (ΔT Devices)

Import digitized fisheye photo

Software does all of the functions that we talked about doing manually

τ, LAI, χ