Climatic-ecological aspects of the arid American southwest, with special emphasis on the White Mountains, California

The main purpose of the present study is the development of concepts and methods suitable for deriving climatological information on the basis of phytoindication in semiarid-semihumid regions where no climatological data are available. The macroclimate of the southwestern United States can be clearly defined using regression analysis. The humid oceanic and temperate climate in the western part of this region is distinguished from the dry continental climate of the Great Basin east of the Sierra Nevada. The very important role of summer precipitation for the distribution of vegetation is explained from a climatic-ecological point of view. Although microclimatic conditions point to unfavorable conditions for plant growth-extreme amounts of radiation lead to increasing thermal stress with altitude-the gradients of soil moisture during the warm season explain high vegetation densities. Phytogeographical aspects show a clear separation between the Sierra Nevada and the White Mountains and between the White Mountains and Wheeler Peak. Lowest vegetation density is found in the Owens Valley and not, as might be expected, in the eastern part of the Great Basin. Thus, although the White Mountains are situated adjacent to the Sierra, their vegetation shows weak relations to the Sierran and Californian floristic province. In fact, Great Basin plants constitute the majority. An overall floristic comparison establishes a continuous change from the White Mountains to Wheeler Peak and an abrupt transition between the vegetation of the Sierra Nevada and the White Mountains.