DIGGING DEEPER INTO THE TEMPO AND MODES OF CLIMATE CHANGE-INDUCED ENVIRONMENTAL TRANSITIONS ON HILLSLOPES, EASTERN MOJAVE DESERT (Invited Presentation)

In what are now the warm deserts of the American Southwest, marked vegetation change following the Pleistocene is implicitly viewed as a direct functional (i.e., physiologically based) response of plants to climate. This hypothesized vegetation response is a key feature of widely cited models of alluvial fan aggradation (Bull, 1991). However, at a site in the eastern Mojave Desert, the transition to “modern” desertscrub vegetation on hillslopes first required a protracted period of erosion of relatively deep, fine-grained soils during the Holocene. Those soils originated during the late Pleistocene through the entrapment and accumulation of eolian sediments in coarse colluvium. These soils absorb and retain substantial moisture and support relatively dense stands of perennial grasses. With the onset of more arid Holocene conditions, primary vegetation changes apparently involved shifts in species composition and overall reduction of perennial grass cover. Although perennial grasses persisted in these soils on south-facing hillslopes, compared to north aspects, the vegetation cover on southern aspects dropped below a threshold required to impede significant erosion. Soil loss increased the exposure of bedrock surfaces, diminishing capacity for infiltration, thereby creating self-enhancing feedback that drove further erosion. Alluvial deposits on the basin floor record the accelerated loss of hillslope soils during the middle Holocene, and the progressive loss of these soils from south aspects continues to the present. Soil loss fundamentally changed the capacity of slopes to absorb moisture and the depth and temporal distributions of plant-available soil moisture. This hydrologic transition generated a secondary vegetation change involving occupation of south-facing slopes by woody species that typify Mojave desertscrub vegetation. These shrubby species possess deeper, widely dispersed woody taproots capable of extracting deeper, but limited moisture stored within bedrock joints and fractures. Although climate shifts ultimately generate vegetation changes, the proximate mechanisms to which plants directly respond can lag behind the direct climate signal and involve complex interrelationships of vegetation, soils, and hydrologic responses.