Validation of MODIS F_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 F_PAR within the MODIS products was examined by incorporating the field measurements and aggregating the high-resolution F_PAR maps to the MODIS scale. Characterization of the temporal accuracy of the MODIS F_PAR products was conducted through comparisons with continuously operating canopy light interception measurements. The MODIS product tended to overestimate F _PAR relative to both ground-based measurements and Landsat-derived estimates of F_PAR, 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 F_PAR values. Neither the field measurements nor the high resolution image F _PAR 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 F_PAR dynamics in the study region. We discuss these results, explore some other possible sources for observed differences between the MODIS, field, and high-resolution F _PAR maps, and consider possible ways to address these issues in future work.