The Role of Climate in the Distribution of Vegetation

ABSTRACT Vegetation develops in response to many different stimuli including climatic, edaphic, and cultural conditions. The degree to which the distribution of vegetation can be explained on the basis of climatic conditions depends in part on the proper selection of active climatic factors. Temperature and precipitation by themselves are poor descriptors of climate. Potential evapotranspiration, water surplus, and water deficit (combined in a moisture index) are found to be more useful. An analysis of the vegetation distribution in conterminous United States shows that the Thornthwaite moisture index and annual potential evapotranspiration are not only able to differentiate among the mid-latitude grassland, forest, and desert biochores but also to provide climatic limits for formation classes within each of the biochores. For example, it is possible to identify on the basis of numerical data an oak-hickory climate as distinct from the birch-maple, and a bunch grass climate distinct from the sagebrush. The problem of non-climatically controlled vegetation associations such as the low latitude pines and savanna are also discussed.