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Understanding the dynamics of the global carbon cycle requires an accurate determination of the spatial and temporal distribution of photosynthetic CO2 uptake by terrestrial vegetation. Stress factors may cause sub-optimal photosynthetic function resulting in down-regulation (i.e., reduced rate of photosynthesis). Photosynthetic down-regulation is related to changes in the apparent spectral reflectance of leaves. Present approaches to determine ecosystem carbon exchange rely on meteorological data as inputs to models that predict the relative photosynthetic function in response to environmental conditions inducing stress (e.g., drought, high/low temperatures). This study examines the determination of ecosystem photosynthetic light use efficiency (LUE) from satellite observations, through measurement of vegetation spectral reflectance changes associated with physiologic stress responses. This approach is possible using the Moderate-Resolution Spectroradiometer (MODIS) on Terra to provide frequent, narrow-band measurements of high radiometric accuracy. Data from reflective MODIS ocean bands were used over land to calculate the Photochemical Reflectance Index (PRI), an index that is sensitive to reflectance changes near 531nm associated with vegetation stress responses exhibited by photosynthetic pigments. MODIS PRI values were compared with LUE calculated from values of CO2 flux measured at the overpass time at a flux tower located in a Douglas fir forest on Vancouver Island in Canada. Preliminary results show a relationship between MODIS PRI and LUE when using MODIS observations in the backscattering direction. These results compare well to previous work at a boreal aspen forest suggesting this approach may be generally useful.
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