STRONGLY DUST-INFLUENCED SOILS AND LANDSCAPE DYNAMICS IN VEGETATED ARIDLANDS OF THE SOUTHWESTERN UNITED STATES

Since the mid 1950s, Earth scientists have become increasingly aware of the critical role dust plays in soil formation, especially in aridlands. Research in the Cima volcanic field in the Mojave Desert showed that dust-influenced soil profile development is genetically coupled to formation of most desert pavements and markedly differs from the “canonical” A/B/C profile model of V.V. Dokuchaev (1883). Vesicular horizons, a key surface horizon of desert pavement soils, and subjacent, thickening clay and calcium carbonate–enriched B horizons form mainly through accretionary and inflationary profile (AIP) development, a type of cumulative soil development. The “upward growth” model of desert pavement formation profoundly contrasts with the more well known and long established “deflation” model. In semiarid regions that lack desert pavements, increased plant density and surface stabilization also favor dust entrapment and AIP. The nature of and time-dependent changes in soils forming by AIP strongly influence patterns of recruitment and plant community composition in aridlands, partly reflecting the ecologically limiting role of water in this climate. The hillslopes of aridland hills and mountains trap dust and, in favorable circumstances, weathering and sustained dust entrapment promote formation of vegetated and soil-mantled hillslopes. Toposequence studies indicate, however, that the presence of dust-entrapping colluvium is probably necessary to maintain AIP on hillslopes over long periods of time. Dust-trapping vegetation is highly sensitive to episodic drought and/or fires that greatly reduce vegetation cover. Certain types of bedrock in aridland hillslopes weather rapidly and favor the development of soil-mantled hillslopes, but the typically thin, weakly developed soils are prone to erosion, a factor that likely favors transformation of transport-limited to detachment-limited hillslopes associated with certain types of bedrock. The conditions that favor sustained “soil production” on hillslopes (transport solely by diffusive processes, maintaining steady-state soil thickness and a spatially and temporally constant diffusion coefficient) for thousands to tens of thousands of years is unlikely in aridlands.