BIOGEOMORPHOLOGY OF SMALL SEMI-ARID BASINS, NORTHEASTERN ARIZONA

On the semiarid Colorado Plateau, NE Arizona, landscapes associated with weakly cemented sandstones are sensitive to climate changes on millennial to decadal scales. Besides influencing fluvial systems in this region, these climate changes may also affect processes of weathering and plant community establishment on hillslopes. To test this hypothesis, we are examining small drainage basins on the Black Mesa escarpment west of Chinle, Arizona. Here, the Jurassic sandstones cemented mainly by clay minerals weather by hydration, favoring relatively high erodibility. Two fundamental slope types are present in this area: (1) North- and east-facing “ transport-limited” slopes that form primarily in response to lower soil temperatures and enhanced soil moisture, conditions favoring development of 2 to 3-dm thick weathering profiles and the establishment of piñon (Pinus edulis) and (2) South- and west-facing “detachment-limited” slopes with significant areas of exposed bedrock have warmer and drier soil conditions, favoring minimal piñon establishment and weathering. These slope form-aspect relations strongly suggest that the position of the transition between these two end-member slope types has shifted during the late Holocene. These hillslopes have been (and may be currently) changing on decadal to centennial time scales—within the lifetimes of the plant communities associated with the slopes. We identified two sub basins that encompass the full range of aspects and characterized the vegetation along 18 50-m long transects. Three different plot sizes capture all vegetation, bedrock and bare ground area, and variation in mantle depth. In contrast to south-facing slopes, north-facing slopes have substantially thicker mantles, less exposed bedrock and maximum tree cover. Cliffrose (Cowainia stansburiana), spatially abundant on south-facing slopes, may be a key indicator species in areas undergoing rapid slope transition. The preliminary data also reveal other significant relations between vegetation, weathering and erosion, not all of which may be aspect-related. Future studies, including detailed mantle/soil descriptions, tree age distributions and slope analysis using LiDAR technology, will further elucidate the biogeomorphic responses of these slopes to past and future climate changes.