Discriminant analysis reveals limited association between forest habitat types and the environment in western United States land classification

AIMS: Critical assessment of the connection between units of the habitat type system and physiographic, climatic and soil factors in interior western United States land classifications. LOCATION: Interior western United States including Utah, SE Idaho, W Wyoming and Colorado, N Arizona, NW New Mexico and E Nevada, representing 11 Bailey's ecoregion sections and covering 389 519 km². METHODS: We analysed 2754 plots from the extensive FIA database, representing 185 different habitat types. We used two techniques: discriminant analysis represented by Random Forests and ordination represented by principal components analysis to discriminate among habitat type classification units, and assessed their relationships with the physical environment. RESULTS: Neither habitat types nor series correspond well to environmental (climatic, physiographic, edaphic) differences. Plant indicator species (sensu Daubenmire) representing the habitat types generally failed in differentiating between important factors of the physical environment. CONCLUSIONS: The failure of the habitat type classifications in discriminating between key environmental factors calls into question the basic premise that habitat types, as used in much of the western United States, are representative of basic ecological units of land. Given its broad acceptance and importance in land management, a fundamental reexamination of the habitat type concept is warranted.