Robert G. Bailey
North America contains a great variety of ecosystems, including mountains, deserts, tropical savannas, areas of permanently frozen subsoil, forests, and steppes. Ecosystems are primarily products of climate. As a source of energy and moisture, climate acts as the primary control for ecosystem distribution. As the climate changes, the other components of the system change in response, although such responses often lag behind climate change and may, through feedback mechanisms, influence climate (see chapters 3 and 11). Climate influences soil formation, helps shape surface topography, and affects plant growth and animal communities, which in turn influence the suitability of a given system for human habitation. As a result, ecosystems of different climates differ significantly.
This chapter provides a comprehensive summation of much that has been presented in previous chapters. It offers a scheme that embraces climatic factors, vegetation ecology, geomorphology and soils to delineate the ecoregions of North America. These regions may not concide precisely with regions based on other criteria, because ecosystems respond to a complex mix of factors. Nevertheless, the scheme presented herein offers a viable summation of North America's natural landscapes, relevant not only to the subsequent discussion of selected regions, but also to the broader understanding of such issues as environmental management.
Based on climatic conditions and on the prevailing plant associations determined primarily by those conditions, we can subdivide the continents into various ecosystems. The climatic classification developed by Köppen (1931) as modified by Trewartha (1968) is adaptable for this purpose. The Köppen-Trewartha classification identifies six main groups of climate, and all but one—the dry group—are thermally defined. These groups are subdivided into 15 types based on seasonality of precipitation or on degree of dryness or cold. They range from the ice caps at high latitudes to the tropical wet climates at low latitudes.
Trewartha's (1968) climate groups are briefly described as follows. The low latitudes contain a winterless, frostless belt with adequate rainfall. This is the tropical-humid climate or A group. It is subdivided into two types, tropical wet (Ar) and tropical wet and dry (Aw). The subtropical belt, or C group, occurs on the low-latitude margins of the middle latitudes, where winters are mild and killing frosts are only occasional. Two subdivisions are recognized: subtropical dry summer (Cs) and subtropical humid (Cf). Poleward from the subtropics is the temperate belt, or D group, which contains two types, temperate continental (Dc) and temperate oceanic (Do). Two subtypes of temperate conti-