National Wetland Inventory Digital Data

http://www.fws.gov/wetlands/

The U.S. Fish and Wildlife Service produces a series of digital data files on a U.S. Geological Survey 7.5 minute topographic quadrangle basis, with metadata conforming to the FGDC metadata format. The data can be access/downloaded from the link above.

NatureServe Terrestrial Vegetation Communities of the United States

http://www.natureserve.org/library/vol1.pdf

An authoritative standardized classification of the terrestrial ecological communities of the United States, naming and describing more than 4,000 community types.