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)
Ψ
 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
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
Leaf with area = a
Unit ground Area
a
a
A
a
a

  exp  n 
A

  is transmission and n is the number of leaves
 This is only true if all of the leaves are horizontal
LAI
Real canopies
(leaves aren’t all horizontal)
 Leaf angle distribution parameter (χ)
 Describes the orientation of the leaves
vertical canopy
χ=0
(onions < 1)
spherical canopy
χ=1
(most canopies)
horizontal canopy
χ=∞
(strawberries χ = 3)
Extinction coefficient
 We can use the leaf angle distribution to determine
an extinction coefficient (G) at a particular zenith
angle
 2 cos2   sin2 
G
  1.774  1.182 0.773
What does G do for us?
- allows us to relate τ to LAI
for any canopy, given 
  G * LAI
   exp
 cos



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

Fobject 
Aobject
Atotal

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
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°)
 d  1.050.25 15  0.5 45  0.25 75 
 ~ 0.3
 ~ 0.05
 = 1.00
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:
bc   d ST avg
Φbc = average radiant flux density below canopy
ST avg = average total radiation above canopy
Calculating times and duration of direct
beam
 Plot sun path on grid
 Segment into time steps (hours)
LAI from fisheye photo
 First method
cos 58
LAI  ln(  58 ) *
0.5
 G58 = 0.5 for all leaf angle distributions!
 Determine τ at ψ = 58° from fisheye photo
LAI from fisheye photo
(method 2: use solver in excel)
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 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