Summary

Description

The U.S. Geological Survey (USGS) has generated and mapped isobioclimate classes for the contiguous United States. These isobioclimate classes were created as part of an effort to map standardized, terrestrial ecosystems for the nation using a classification developed by NatureServe (Comer and others, 2003). Ecosystem distributions were modeled using a biophysical stratification approach developed for South America (Sayre and others, 2008) and now being implemented globally (Sayre and others, 2007). Bioclimate regimes strongly influence the differentiation and distribution of terrestrial ecosystems, and are one of the key input layers in the ecosystem delineation process. The Rivas-Martínez methodology used to produce these classes was developed from a consideration of bioclimatology and its relationship to phytogeography (Rivas-Martínez, 2004; Rivas-Martínez and others, 1999, 2004). This approach develops a number of bioclimatic indices calculated from a variety of data on temperature and precipitation (e.g. average temperature of the coldest month, total precipitation of the warmest four-month period, a continentality index and a thermicity index). Daymet data, which was developed from 18-year (1980-1997) climatological records and is available at a spatial resolution of 1 kilometer, was used as the source data for these indices (Thornton, 1997). Once calculated the values of these indices are compared to established thresholds for the differentiation of thermotypic (warm/cold gradients) and ombrotypic (wet/dry gradients) regions, and the results are used in sets of decision rules to identify classes. The classification is implemented in four levels: macrobioclimates, bioclimates, thermotypes (thermoclimatic belts) and ombrotypes (ombroclimatic belts). The final isobioclimates dataset represents areas of the 127 unique thermotype-ombrotype combinations that were mapped for the contiguous United States.