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 (F/sub PAR/) 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/sub PAR/ maps derived from IKONOS and Landsat ETM+ imagery. The spatial variability of F/sub PAR/ within the MODIS products was examined by incorporating the field measurements and aggregating the high-resolution F/sub PAR/ maps to the MODIS scale. Characterization of the temporal accuracy of the MODIS F/sub PAR/ products was conducted through comparisons with continuously operating canopy light interception measurements. The MODIS product tended to overestimate F/sub PAR/ relative to both ground-based measurements and Landsat-derived estimates of F/sub 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/sub PAR/ values. Neither the field measurements nor the high resolution image F/sub 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/sub 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/sub PAR/ maps, and consider possible ways to address these issues in future work.