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Plant Canopy Analysis

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Plant Canopy Analysis

Gaylon S. Campbell, Ph.D.

Decagon Devices and Washington State University

- Why do we care about the radiation environment of plant canopies?
- Calculate Leaf Area Index (LAI)
- Crop growth stage
- Ecosystem health
- Radiation use efficiency

- Calculate Leaf Area Index (LAI)

- 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

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

- Radiation budget & view factors
- Fisheye analysis

- Plant canopy light environment
- Fisheye analysis
- Ceptometer – measures light interCEPTed by canopy

- 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

- 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

- Harvest leaves from 1 m2 canopyarea
- Physically measure surface area with optical meter
- Advantages: direct measurement
- Disadvantages: destructive, time consuming, wilting

Leaf with area = a

Unit ground Area

a

a

A

a

- t is transmission and n is the number of leaves
- This is only true if all of the leaves are horizontal

LAI

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

spherical canopy

χ= 1

(most canopies)

vertical canopy

χ= 0

(onions < 1)

horizontal canopy

χ= ∞

(strawberries χ= 3)

- We can use the leaf angle distribution to determine an extinction coefficient (G) at a particular zenith angle

What does G do for us?

- allows us to relate τto LAI for any canopy, given c

- Zenith angle (ψ)
- Time and location
- latitude and longitude

- Time and location
- 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

- Measure above-canopy radiation
- Measure below-canopy radiation
- Meter calculates τ

- 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

- 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

- 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

- 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

- Aobject is the area of the picture taken up by the object of interest
- Atotalis the total area of the photograph

- Project the fisheye picture onto a grid

30

330

60

300

90 E

270 W

240

120

150

210

180 S

- Visually estimate the fraction of sky visible in each grid sector (1 = full sky)
- Average the value for each zenith angle band = τψ

t ~ 0.05

30

t ~ 0.7

330

60

300

90 E

270 W

240

120

t = 1.00

150

210

180 S

- 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:

Φbc = average radiant flux density below canopy

ST avg = average total radiation above canopy

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

N

E

W

S

- First method
- 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)

- Determine τ at ψ=15, 45, and 75° from fisheye photo
- Calculate G at each zenith angle with an arbitrary value of χ
- Calculate new values of τ for each zenith angle using G from step 2 and an arbitrary LAI value
- Calculate sum of squared errors between τfisheye and τarbitrary
- Use solver to minimize SSE by adjusting arbitrary LAI and χ values
- Results in decent estimate for both LAI and χ!

- HemiView software (ΔT Devices)
- Import digitized fisheye photo
- Software does all of the functions that we talked about doing manually
- τ, LAI, χ