Published in Remote Sensing of Environment 113: 2463-2475<\/b><\/p>\n
Estimation of photosynthetic light use efficiency (\u03b5) from satellite observations is an important component of climate change research. The photochemical reflectance index, a narrow waveband index based on the reflectance at 531 and 570 nm, allows sampling of the photosynthetic activity of leaves; upscaling of these measurements to landscape and global scales, however, remains challenging. Only a few studies have used spaceborne observations of PRI so far, and research has largely focused on the MODIS sensor. Its daily global coverage and the capacity to detect a narrow reflectance band at 531 nm make it the best available choice for sensing \u03b5 from space. Previous results however, have identified a number of key issues with MODIS-based observations of PRI. First, the differences between the footprint of eddy covariance (EC) measurements and the MODIS footprint, which is determined by the sensor’s observation geometry make a direct comparison between both data sources challenging and second, the PRI reflectance bands are affected by atmospheric scattering effects confounding the existing physiological signal. In this study we introduce a new approach for upscaling EC based \u03b5 measurements to MODIS. First, EC-measured \u03b5 values were \u201ctranslated\u201d into a tower-level optical PRI signal using AMSPEC, an automated multi-angular, tower-based spectroradiometer instrument. AMSPEC enabled us to adjust tower-measured PRI values to the individual viewing geometry of each MODIS overpass. Second, MODIS data were atmospherically corrected using a Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm, which uses a time series approach and an image-based rather than pixel-based processing for simultaneous retrievals of atmospheric aerosol and surface bidirectional reflectance (BRDF). Using this approach, we found a strong relationship between tower-based and spaceborne reflectance measurements (r2<\/sup><\/em>=0.74, p<0.01) throughout the vegetation period of 2006. Swath (non-gridded) observations yielded stronger correlations than gridded data (r2<\/sup><\/em>=0.58, p<0.01) both of which included forward and backscatter observations. Spaceborne PRI values were strongly related to canopy shadow fractions and varied with different levels of \u03b5. We conclude that MAIAC-corrected MODIS observations were able to track the site-level physiological changes from space throughout the observation period.<\/p>\n","protected":false},"excerpt":{"rendered":" Published in Remote Sensing of Environment 113: 2463-2475 Estimation of photosynthetic light use efficiency (\u03b5) from satellite observations is an important component of climate change research. The photochemical reflectance index, a narrow waveband index based on the reflectance at 531 and 570 nm, allows sampling of the photosynthetic activity of leaves; upscaling of these measurements […]<\/p>\n","protected":false},"template":"","format":"standard","cptt_secteur":[1179],"cptt_theme":[1183,1185,1180,1181],"cptt_categ_publi":[1149],"cptt_auteurs_ministeriels":[1079],"class_list":["post-81983","cpt_publication","type-cpt_publication","status-publish","format-standard","hentry","cptt_secteur-forests","cptt_theme-ecosystems-and-environment","cptt_theme-forest-ecology","cptt_theme-forestry-research","cptt_theme-forests","cptt_categ_publi-scientific-article","cptt_auteurs_ministeriels-drolet-guillaume"],"acf":[],"yoast_head":"\n