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

The impact of domestic livestock on soil properties and perennial vegetatio n is greatest close to water points and generally decreases exponentially w ith distance from water. We hypothesized that the impact of livestock on an nual-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 abunda nt in severely disturbed areas and also in area:; that art: known to have h igh soil nitrogen content. Amaranthus palmeri was abundant in half of the t ransects in the zones nearest the water points. The relationships of annual -plan; abundance and species richness with distance from water points and w ith perennial-plan: cover were not significant (R-2 < 0.1). The number of b oundaries and sizes of zones varied with distance from water points, with s easons, and with duration of grazing. The first biotic zone (most severely impacted by cattle) ranged from 75 to 795 m radius for winter-spring annual s 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 corre lated with the frequency and intensity of disturbance, with landscape posit ion, and with patchiness of soil features. There were fewer and larger zone s of summer annuals than of winter-spring annuals. Boundary analysis of liv estock disturbance gradients provided a method with replication for assessi ng the impact of long-term livestock grazing on annual-plant communities. L ivestock create nutrient-rich patches near water points by mixing dung with soil by hoof action. These nutrient-rich patches support species of annual s that are rare or absent in areas where soils are subjected to low-intensi ty disturbance.