Livestock activity and Chihuahuan Desert annual-plant communities: Boundary analysis of disturbance gradients

The impact of domestic livestock on soil properties and perennial vegetation is greatest close to water points and generally decreases exponentially with distance from water. We hypothesized that the impact of livestock on annual-plant communities would be similar to that on perennial vegetation. We used multivariate analysis and semivariograms to locate boundaries and to determine the number and width of different annual-plant zones (referred as biotic zones) on long-term livestock disturbance gradients in the northern Chihuahuan Desert, New Mexico. We estimated abundance of annuals in 0.5-m(2) quadrats placed at 30-m intervals on 10 livestock disturbance gradients originating at water points. Tansy mustard, Descurainia pinnata, was abundant in severely disturbed areas and also in area:; that art: known to have high soil nitrogen content. Amaranthus palmeri was abundant in half of the transects in the zones nearest the water points. The relationships of annual-plan; abundance and species richness with distance from water points and with perennial-plan: cover were not significant (R-2 < 0.1). The number of boundaries and sizes of zones varied with distance from water points, with seasons, and with duration of grazing. The first biotic zone (most severely impacted by cattle) ranged from 75 to 795 m radius for winter-spring annuals and from 165 to 1065 m radius for the summer annuals. Variability in the number and size of biotic zones along grazing gradients was spatially correlated with the frequency and intensity of disturbance, with landscape position, and with patchiness of soil features. There were fewer and larger zones of summer annuals than of winter-spring annuals. Boundary analysis of livestock disturbance gradients provided a method with replication for assessing the impact of long-term livestock grazing on annual-plant communities. Livestock create nutrient-rich patches near water points by mixing dung with soil by hoof action. These nutrient-rich patches support species of annuals that are rare or absent in areas where soils are subjected to low-intensity disturbance.