Regional mapping of gross light-use efficiency using MODIS spectral indices |
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Authors: | G.G. Drolet K.F. Huemmrich F.G. Hall B.D. Amiro T.A. Black H.A. Margolis |
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Affiliation: | a Centre d'Étude de la Forêt, Faculté de Foresterie et de Géomatique, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4 b Biospheric Sciences Branch, Code 614.4, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA c Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Baltimore, MD 21228, USA d Department of Soil Science, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 e Atmospheric Sciences and Technology Directorate, Meteorological Service of Canada, Saskatoon, Saskatchewan, Canada S7N 3H5 f Faculty of Agricultural Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 g Department of Geography, Queen's University, Kingston, Ontario, Canada K7L 3N6 |
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Abstract: | Direct estimation of photosynthetic light-use efficiency (LUE) from space would be of significant benefit to LUE-based models which use inputs from remote sensing to estimate terrestrial productivity. The Photochemical Reflectance Index (PRI) has shown promise in tracking LUE at the leaf- to small canopy levels, but its use at regional to global scales still remains a challenge. In this study, we used different formulations of PRI calculated from the MODIS ocean band centered at 531 nm and a set of alternative reference bands at 488, 551, and 678 nm to explore the relationship between PRI and LUE where LUE was measured at eight eddy covariance flux towers located in the boreal forest of Saskatchewan, Canada. The magnitude and variability of LUE was significantly lower at the times when useful MODIS ocean band images were available (i.e. around midday under clear-sky conditions) relative to the rest of the growing season. PRI678 (reference band at 678 nm) showed the strongest relationship (r2 = 0.70) with LUE90a (i.e. 90-minute mean LUE calculated using Absorbed Photosynthetically Active Radiation, APAR), but only when all sites were combined. Overall, the relationships between the MODIS PRIs and LUE90a were always stronger for observations closer to the backscatter direction and there were no significant differences in the strength of the correlations whether LUE was calculated based on incident PAR or on APAR. Predictions of ecosystem photosynthesis at the time of the MODIS overpasses were significantly improved by multiplying either PAR or APAR by MODIS PRI (r2 improved from 0.09 to 0.44 and 0.54 depending on the PRI formulation).We used our PRI-LUE model to create a regional LUE90a map for the three cover types covering 47,500 km2 around the flux sites. The MODIS PRI-derived LUE90a map appeared to capture more realistic spatial heterogeneity of LUE across the landscape compared to a daily LUE map derived using the look-up table in the MODIS GPP (MOD17) algorithm. While our LUE map is only a snapshot of minimum regional LUE90a values, with appropriate gap-filling methods it could be used to improve regional-scale monitoring of GPP. Moreover, the strong relationship between midday and daily LUE on clear days (r2 = 0.93) indicates that instantaneous MODIS observations of LUE90a could be used to estimate daily LUE. Finally, pixel shadow fraction from the 5-Scale geometric-optical model was closely related to both MODIS PRI and tower-derived LUE suggesting that differences in stand leaf area and in diffuse illumination among flux sites play a role in the relationship we observed between LUE and MODIS PRI. |
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Keywords: | Photochemical Reflectance Index Gross light-use efficiency MODIS Flux towers Gross ecosystem productivity Boreal forest Regional mapping Shadow fraction |
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