Landscape Evolution, Soil Formation, and Ecological Patterns and Processes in Sonoran Desert Bajadas
Authors: McAuliffe Joseph R. 
Published: April 2006

Three alluvial piedmonts (bajadas or alluvial fans) studied in the Sonoran Desert near Tucson, Arizona are complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene—aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene—aged surfaces containing soils with strongly developed argillic (clay—rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated the importance of igneous lithology (highly weatherable intrusives vs. weathering—resistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are strongly influenced by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns. Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long—lived Larrea tridentata. Individuals of this shrub species can exhibit clone—like growth and increase considerably in size (diameter) over time spans of many centuries to millennia. The growth and persistence of these long—lived clones in some parts of the landscape apparently contribute to the exclusion of other species. However, development of large clones and dominance by L. tridentata are impossible or greatly inhibited in several landscape settings including: (1) extremely young alluvial deposits that have existed for too short a time for large clones to have developed, (2) hillslopes subject to considerable erosional disturbance, and (3) extremely thin soils underlain by impenetrable petrocalcic horizons (caliche), which magnify drought conditions and apparently contribute to episodic mortality in L. tridentata. Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant life forms. Clay—rich (argillic) horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and the temporal availability of soil water. Despite surficial stability for extremely long periods of times, sites with strongly developed argillic horizons lack L. tridentata and are instead occupied by drought—deciduous or succulent plants that are capable of highly seasonal activity in soils that exhibit high seasonal variability in water availability. Syntheses involving the study of various ecological processes (e.g., plant physiological, demographic, and interspecific interactions) with a larger landscape perspective provide a rich framework for further studies of arid land systems.