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Plant Canopy Analysis Gaylon S. Campbell, Ph.D. Decagon Devices and Washington State University

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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

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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

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Intercepted PAR and Biomass Production Cumulative intercepted photosynthetically active radiation (PAR) linearly related to total biomass production

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Tools for detailed analysis Radiation budget & view factors Fisheye analysis Plant canopy light environment Fisheye analysis Ceptometer – measures light interCEPTed by canopy

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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

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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: m 2 m -2 How do we measure Destructive sampling Light attenuation Ceptometer Fisheye photograph

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LAI – destructive sampling Harvest leaves from 1 m 2 canopy area Physically measure surface area with optical meter Advantages: direct measurement Disadvantages: destructive, time consuming, wilting

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LAI-light attenuation theory A a a LAI is transmission and n is the number of leaves This is only true if all of the leaves are horizontal Unit ground Area a Leaf with area = a

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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)

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Extinction coefficient 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 τ to LAI for any canopy, given

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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

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LAI from Ceptometers Measure above-canopy radiation Measure below-canopy radiation Meter calculates τ

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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

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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

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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

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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

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Determining view factors Simplest analysis A object is the area of the picture taken up by the object of interest A total is the total area of the photograph

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View Factors

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Determining transmission coefficients 1.Project the fisheye picture onto a grid

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30 60 90 E 120 150 180 S 210 240 270 W 300 330

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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 = τ ψ

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30 60 90 E 120 150 180 S 210 240 270 W 300 330 = 1.00 ~ 0.05 ~ 0.7

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Calculating average understory radiant fluxes τ d is the transmission coefficient for diffuse radiation But, if we average over a day or longer, it approximates the total radiation transmission coefficient so: Φ bc = average radiant flux density below canopy S T avg = average total radiation above canopy

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Calculating times and duration of direct beam Plot sun path on grid Segment into time steps (hours) E S W N

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LAI from fisheye photo First method G 58 = 0.5 for all leaf angle distributions! Determine τ at ψ = 58° from fisheye photo

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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 τ fisheye and τ arbitrary 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)

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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

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