MODIS enhanced vegetation index predicts tree species richness across forested ecoregions in the contiguous USA

With the expectation of major shifts in climate, ecologists have focused attention on developing predictive relationships between current climatic conditions and species diversity. Climatic relationships appear best defined at regional rather than local levels. In reference to tree diversity, process-based models that express gross primary production (GPP) as an integrated function of climate seem most appropriate. Since 2000, NASA's MODIS satellite has provided composite data at 16-day intervals to produce estimates of GPP that compare well with direct measurements. The MODIS enhanced vegetation index (EVI), which is independent of climatic drivers, also appears a good surrogate to estimate seasonal patterns in GPP. In this paper we identified 65 out of 84 delineated ecoregions distributed across the contiguous U.S.A., within which sufficient (>= 200) Federal Inventory and Analysis survey plots were available to predict the total number of tree species, which varied from 17 to 164. Four different formulations of EVI were compared: The annual maximum, the annual integrated, the growing season defined mid-point and growing season averaged values. The growing season mid-point EVI defined the beginning and end of the active growing season. In all formulations of EVI, a polynomial function accounted for about 60% of the observed variation in tree diversity, with additional precision increasing to 80% when highly fragmented ecoregions with < 50% forest cover were excluded. Maps comparing predicted with measured tree richness values show similar patterns except in the Pacific Northwest region where a major extinction of tree genera is known to have occurred during the late Pliocene. The extent that these relationships remain stable under a changing climate can be evaluated by determining if the MODIS climate-driven estimate of GPP continues to match well with EVI patterns and systematic resurveys of forest vegetation indicate that tree species are able to adjust rapidly to climatic variation. (c) 2006 Published by Elsevier Inc.