Relating a Spectral Index from MODIS and Tower-Based Measurements to Ecosystem Light Use Efficiency for a Fluxnet-Canada Coniferous Forest
Elizabeth
M
Middleton, NASA, elizabeth.m.middleton@nasa.gov
(Presenting)
Yen-Ben
Cheng, NASA, ybcheng@ucdavis.edu
Thomas
Hilker, Univ. British Columbia, thilker@interchange.ubc.ca
Karl
F
Huemmrich, Univ. Maryland Baltimore Campus, karl.f.huemmrich@nasa.go
T
Andrew
Black, Univ. British Columbia, andrew.black@ubc.ca
Praveena
Krishnan, NOAA, praveena.krishnan@noaa.gov
Nicholas
C
Coops, Univ. British Columbia, nicholas.coops@ubc.ca
As part of the NACP, we examined the retrieval of ecosystem photosynthetic light use efficiency (LUE) directly from satellite observations. Our novel approach was to compare LUE derived from tower fluxes with LUE estimated using spectral indices computed from MODIS satellite data over forests in the Canadian Carbon Program Network, using the MODIS narrow ocean bands acquired over land. We matched carbon fluxes collected at the time of the MODIS mid-day overpass for over 100 relatively clear days in five years (2001-2006) from a mature Douglas-fir forest (DF49) in British Columbia. We also examined hyperspectral reflectance data collected diurnally from the DF49 tower in conjunction with the eddy correlation fluxes and other measurements made throughout the 2006 growing season. We evaluated LUE in conjunction with the Photochemical Reflectance Index (PRI), a normalized difference spectral index that uses 531 nm and a reference band to capture high light induced stress responses afforded by the xanthophyll cycle. When tower-based canopy-level LUE was recalculated to estimate foliage-based values (LUEfoliage) for three foliage groups (sunlit, shaded, and intermediate), a strong linear relationship for PRI:LUEfoliage was demonstrated (0.6 r2 0.8, n=822, P<0.0001). A similar relationship between MODIS PRI and tower-based LUE was obtained from satellite observations (r2 = 0.76, n=105, P= 0.026) when the azimuth offsets from the solar plane were considered. MODIS PRI values were most similar to those observed in situ in the BRDF cross-plane, and provide a means to monitor forest under stress using narrow spectral band indices.
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