Effects of complex geomorphic history on soil and vegetation patterns on arid alluvial fans

The influence of geomorphic history on soil development and vegetation patterns was examined on an alluvial fan in the northern Sonoran Desert. The fan comprised three depositional surfaces ranging in age from early-mid Pleistocene to Holocene. Many plant species showed downfan increases or decreases in cover, as others have reported previously. Soil variables, however, failed to show the simple downfan textural gradient that has traditionally been viewed as the underlying control of downfan variation in plant species composition. Furthermore, depositional surfaces differed not only in terms of pedogenic development and associated physical and chemical characteristics, but also in terms of the plant species each supported. These results emphasize that alluvial fans constitute a three-dimensional landscape mosaic composed of fan surfaces, each with distinct edaphic conditions and vegetation. An appreciation of this geomorphic complexity, which is typical in arid environments, is critical to the understanding of vegetation patterns on alluvial fans.