SONORAN DESERT BAJADA

The perennial vegetation of a Sonoran Desert bajada was de- scribed in terms of gradients of (i) environmental variables, (ii) species popula- tions, and (iii) community characteristics. Changes in plant species composition along the bajada were related to differences in soil particle size distribution and salinity. Yang and Lowe (1956) suggested that different soil attributes characterize, and are associated with, two different vegetation types in the Sonoran Desert. Differences in physical and physiological characteristics, such as moisture equivalent and wilting coefficient, were attributed to the significant difference in soil texture. Coarse gravelly soils, with a relatively high amount of water available for plant use, were associated with the complex Paloverde-Saguaro (Cercidium-Cereus) vegetation type. Much finer soils with less avail- able water were associated with the simple creosote bush-bur sage (Larrea-Franseria) vegetation type. The vegetation of southwestern United States bajadas often shows considerable variation in species composition along the length of the bajada. One might suspect that these vegetational changes are associated with changes in soil characteristics, such as soil texture. Direct gradient analysis is a method widely used to study the spatial patterns of vegetation (Whittaker 1967). Its purpose is to clarify vegetational structure and variation in terms of gradients of (i) environmental variables, (ii) populations of species, and (iii) community characteristics.