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.LIMATE, soil, fire, drainage, man, ani-c mals, and many other factors all influence the distribution of vegetation. The present study undertakes a detailed examination of the relationship between natural vegetation and climate over the conterminous United States in an effort to identify more clearly the nature of the relationship that exists between these two factors under different environmental situations. The phrase natural vegetation in the present context refers to the predominantly existing vcgetation under the normal pattern of climate, environmental ( including natural fires ), and edaphic conditions generally uninfluenced by the directed or willful action of man. It does not necessarily refer to a so-called climatic climax vegetation association.The present study will not utilize directly the statistics of temperature and precipitation or relate annual averages of these statistics to vegetation distribution. Rather, derived factors of climate, factors concerned 1) with the plant water need as it is related to the supply of moisture and precipitation and soil moisture storage, and 2 ) with the energy Also Professor of Geography, University of Delnware, Newark, Delaware. for plant devclopment, will be correlated with the distributions of vegetation. It is hoped that the careful selection of climatic indices that are active in vegetation growth and development can result in a new understanding of the role of climate in vegetation development, and possibly place in better perspective the roles of other environmental factors and man as influencers of vegetation distribution.
ACTIVE CLIMATIC INDICESThe 1948 climatic classification developed by Tl~ornthwaite~ made use of average monthly values of temperature and precipitation, as do most such classifications. Thornthwaite, however, emphasized the moisture factor in climate by introducing the concept of the climatic need or demand for water (whic...
