Geomorphic Processes and Vegetational Change along the Meade River Sand Bluffs in Northern Alaska

Geomorphic processes within the region of sand deposits on the Alaskan Arctic coastal plain bring about changes in local environments, and consequently in local vegetation, through time. Geomorphic processes and vegetational patterns on bluffs are related to the directions which the bluffs face with respect to prevailing winds. Caribou and ground squirrels augment wind erosion of the bluffs by disturbing the vegetation while grazing, trampling or burrowing. Environmentally induced vegetational changes resulting from continued geomorphic and animal disturbances are more common than autogenic successions which, being generally accompanied by rising permafrost, help to stabilize sands. Vegetational sequences existing along the bluffs result from the interaction of both linear and cyclic changes in the ecosystem

Plants and the Ecosistem

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Vegetation and Moisture Controls on Soil C Mineralization in Semi-Arid Environments

Mechanisms of vegetation control on C mineralization in semiarid ecosystems are not well understood. We developed a series of model predictions for beneath the native shrub Wyoming big sagebrush [Artemisia tridentata (Nutt.) ssp. wyomingensis], the invasive annual grass cheatgrass (Bromus tectorum L.), and the exotic introduced perennial grass crested wheatgrass [Agropyron desertorum (L.) Gaertn.]. Soil samples (0–10 cm) collected biweekly for two growing seasons were analyzed in the laboratory for: water content, CO2 from intact soil cores and CO2 from soils sieved and wetted to 23%, total organic C, total N, and microbial biomass C. Our results suggest that different vegetation types in the Great Basin affect C mineralization primarily through modification of soil moisture and, secondarily, the amount of labile C. Soils beneath cheatgrass and sagebrush canopy retained more water after high‐and moderate‐intensity rainfalls than soils beneath crested wheatgrass and sagebrush interspace. Sagebrush canopy probably intercepts more incoming precipitation without significant throughfall to the soil surface below than cheatgrass or crested wheatgrass. At the same time, soils beneath cheatgrass had 8% more labile C and 36% higher C mineralization rates than sagebrush. Regression analysis showed that soil water content alone explained nearly 84% of the variation, and adding information on labile C accounted for nearly 88% of the variation in soil C mineralization rates. With increasing variability of precipitation in this region, the continuously increasing presence of cheatgrass in the semiarid and arid western United States may significantly impact the CO2 contributions to overall greenhouse gas emissions

A Bioeconomic Model of Cattle Stocking on Rangeland Threatened by Invasive Plants and Nitrogen Deposition

Across western North America, invasive plant species and elevated levels of nitrogen are threatening the productivity of rangelands. A bioeconomic model of stocking cattle on these rangelands is used to show that optimal stocking depends on the competition between native grasses and the invaders. However, nitrogen deposition is important in determining the ultimate rangeland species composition. Endogenous changes in plant successional thresholds are due to the interplay of nitrogen deposition and stocking practices. Nonoptimal overstocking can create ecosystem niches for invaders where they would not have occurred at lower stocking rates, although what constitutes overstocking depends on the nitrogen levels. Copyright 2008, Oxford University Press.