Validation of MODIS F/sub PAR/ products in boreal forests of Alaska

Moderate Resolution Imaging Spectroradiometer (MODIS) fraction of photosynthetically active radiation absorbed by vegetation (FPAR ) products covering the boreal forest of interior Alaska were analyzed and compared with field measurements of canopy light harvesting over a multiyear period (2001 to 2004), as well as to high-resolution F PAR maps derived from IKONOS and Landsat ETM+ imagery. The spatial variability of FPAR within the MODIS products was examined by incorporating the field measurements and aggregating the high-resolution FPAR maps to the MODIS scale. Characterization of the temporal accuracy of the MODIS FPAR products was conducted through comparisons with continuously operating canopy light interception measurements. The MODIS product tended to overestimate FPAR relative to both ground-based measurements and Landsat-derived estimates of FPAR, particularly in the more sparsely vegetated burn scars on which we focus related research, but adequately captured seasonal variability associated with vegetation phenology. A combination of canopy closure and ground cover vegetation was identified as the source of most of the discrepancies between the MODIS estimated and field measured FPAR values. Neither the field measurements nor the high resolution image FPAR maps based on the field measurements characterized the light environment of the ground cover vegetation (i.e., <10 cm height); thus, absolute validation of the MODIS products was incomplete - despite the extensive spatial and temporal characterization of FPAR dynamics in the study region. We discuss these results, explore some other possible sources for observed differences between the MODIS, field, and high-resolution FPAR maps, and consider possible ways to address these issues in future work

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