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Vegetation Stress Detection through Chlorophyll a + b Estimation and Fluorescence Effects on Hyperspectral Imagery

^a Centre for Research in Earth and Space Science (CRESS), Toronto, ON, Canada M3J 1P3
^b Dep. of Physics and Astronomy, York University, 4700 Keele Street, Toronto, ON, Canada M3J 1P3
^c P&M Technologies, 66 Millwood St., Sault Ste. Marie, ON, Canada P6A 6S7
^d Ontario Forest Research Institute (OFRI), Ontario Ministry of Natural Resources (OMNR), Sault Ste. Marie, ON, Canada P6A 2E5
^e Provincial Geomatics Service Centre, Ontario Ministry of Natural Resources, 300 Water St, Peterborough, ON, Canada K9J 8M5

View larger version (18K): [in a new window]   Fig. 1. Schematic diagram showing the coupling between the fluorescence–reflectance–transmittance (FRT) and PROSPECT models to calculate the change in reflectance (r*) and transmittance (t*) caused by accounting for chlorophyll fluorescence.

View larger version (22K): [in a new window]   Fig. 2. Fluorescence–reflectance–transmittance (FRT) model simulation of leaf reflectance with the effects of fluorescence flux. Parameters used for the simulation: = 0.085, Ca+b content = 50 µg/cm2, DL = 0.075 mm, FL = 688 nm, BL = 30 nm, FH = 746 nm, BH = 52 nm, f ratio FH/FL = 0.94, labeled as r* (with fluorescence), r (without fluorescence).

View larger version (32K): [in a new window]   Fig. 3. Simulated leaf reflectance including the effects of fluorescence using the fluorescence–reflectance–transmittance (FRT) model (top and middle left plots) and compact airborne spectrographic imager (CASI) reflectance in the laboratory (middle right plot) and from an airborne campaign (bottom two plots). Top left plot shows simulated fluorescence emission for different values of (ranging from 0 to 0.2), and superimposed on leaf reflectance (top right plot) with peaks at 695 nm (BL = 30 nm) and 750 nm (BH = 40 nm), N = 1.5, and Ca+b = 40 µg/cm2. Derivative reflectance (middle left plot) for different fluorescence simulations shows a double peak as function of fluorescence emission. Laboratory CASI reflectance from sugar maple seedlings with and without fluorescence emission shows that such effects are captured on the derivative reflectance (middle right plot). Airborne CASI data collected over 30- x 30-m sugar maple study sites (bottom plots) from the two sites with highest field-measured Fv/Fm and Ca+b (GY41, Fv/Fm = 0.81; Ca+b = 38.8 µg/cm2) and lowest (MD35, Fv/Fm = 0.75; Ca+b = 19.08 µg/cm2) show comparable double peak features.

View larger version (19K): [in a new window]   Fig. 4. Simulated canopy reflectance (top) and derivative reflectance (bottom) using the PROSPECT and SAILH radiative transfer models for leaf Ca+b = 40 and 80 µg/cm2, N = 1.5, and canopy leaf area index (LAI) = 6, plagiophile leaf angle distribution function, and nadir view.

View larger version (24K): [in a new window]   Fig. 5. Band location of the derivative chlorophyll index (DCI) calculated as D705/D722, developed to track changes due to the double peak generated by the suggested effects of pigment and chlorophyll fluorescence at the canopy level in stressed vegetation (healthy site, GY41: Fv/Fm = 0.81; Ca+b = 38.8 µg/cm2; stressed site, MD35: Fv/Fm = 0.75; Ca+b = 19.08 µg/cm2)

View larger version (19K): [in a new window]   Fig. 6. Relationship between the reflectance curvature index [CUR; (R675·R690)/R6832] measured from leaf samples (experiment with Ca+b constant) and modeled by fluorescence–reflectance–transmittance (FRT)–PROSPECT. F'm110 was used as input for in FRT–PROSPECT model.

View larger version (20K): [in a new window]   Fig. 7. Estimation of Ca+b by inversion of the PROSPECT leaf radiative transfer (RT) model from 60 reflectance and transmittance spectra and Ca+b measured from leaves samples with variable chlorophyll content (= 35.66 µg/cm2, s = 15.87, n = 60).

View larger version (20K): [in a new window]   Fig. 8. Relationship between F'm110 measured from leaf samples (experiment with Ca+b constant) and estimated by inversion of fluorescence–reflectance–transmittance (FRT)–PROSPECT.

View larger version (78K): [in a new window]   Fig. 9. The derivative chlorophyll index (DCI) applied to hyperspectral derivative reflectance collected with the compact airborne spectrographic imager (CASI) airborne sensor. Remote sensing images were collected over the two study sites with low (top image) and high (bottom image) stress conditions to map the double peak as an indicator of stress. Images show the 500- x 500-m sugar maple areas with the 20- x 20-m study sites with highest field-measured Ca+b and Fv/Fm (GY41, Fv/Fm = 0.81, Ca+b = 38.8 µg/cm2, top image) and lowest Fv/Fm (MD35, Fv/Fm = 0.75, Ca+b = 19.08 µg/cm2, bottom image). Areas of vegetation stress are identified on the 2- x 2-m spatial resolution airborne images in red.