GEOLOGIC, ECOLOGIC, CLIMATIC, AND LAND-USE INTERACTIONS IN THE HIGH DESERT OF THE CENTRAL COLORADO PLATEAU (USA)—THE PAST 40,000 YEARS

Studies in the high desert of the central Colorado Plateau (USA) are elucidating relations among geologic substrates, hydrology, vegetation communities, soil crusts, past and current land-use history, wind-erosion vulnerability, and recent effects of severe drought. Geologic, palynologic, and geochronologic studies reveal climatically controlled episodes of aeolian and alluvial activity separated by periods of stability and paleosol formation, all spanning the past 40,000 years. Holocene deposits of aeolian sand that contain far-traveled atmospheric dust (as much as about 30 %) are important substrates for modern vegetation communities and surfaces stabilized by biologic soil crust (BSC). Aeolian and slope processes, acting on different bedrock substrates, have redistributed sand, silt, and clay in surficial deposits with resultant strong influence on soil respiration, soil moisture, as well as contemporary spatial patterns and temporal dynamics of plant nutrients.

Undisturbed surface sediments were compared to those of previously grazed surfaces (before 30 years ago) with respect to BSC, chemistry, particle size, mineralogy, and microbial activity to evaluate their respective nutrient status and physical properties. BSC on the previously grazed surfaces has lower diversity of species and is less dense than on undisturbed surfaces. The disturbed surfaces show significantly lower contents of some nutrients such as P, N, C, Na, and Mn, as well as lower rates of soil respiration. We attribute the lower amounts of nutrients to wind-driven soil erosion on poorly stabilized surfaces degraded by grazing. Particle-size distribution and content of magnetite (a mineral-dust component in this setting) differ among surfaces in patterns that are consistent with such wind erosion.

Monitoring of saltation and dust deposition at sites having different land-use histories, including two currently grazed settings, show a range of aeolian activity, with most wind erosion at one of the currently grazed sites. Extreme drought in the region during 2002 greatly exacerbated vegetation loss, wind erosion, and dust-event frequency at this site. The other sites experienced far fewer effects of drought.