|DE SOYZA, AMRITA|
|Van Zee, Justin|
Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/1998
Publication Date: N/A
Interpretive Summary: The impact of domestic livestock on soil and perennial vegetation is greatest near water points and decreases with distance from water--is it the same for annual vegetation? We used a 1.0 x 0.5 m sampling quadrat placed at 30 m intervals extending away from water points. Using boundary analysis mathematical models we located vegetation boundaries based on annual plants. Tansy mustard was abundant in severely disturbed areas and areas with high soil nitrogen content and Pigweed was most abundant near water points. The number of vegetational boundaries and the sizes of the vegetational zones varied with distance from water points, season of measurement and season of grazing. Thus, boundary analysis is a reliable method for assessing the impacts of long-term grazing on annual plant communities.
Technical Abstract: 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 estimated abundance of annuals in 0.5m**2 quadrats placed at t30 m intervals on 10 livestock disturbance gradients originating at water points. We used multivariate analysis and semivariograms to locate boundaries and to determine the number and width of different annual-plant- zones (biotic zones) on long-term livestock disturbance gradients in the northern Chihuahuan Desert. Tansy mustard was abundant in severely disturbed areas and in areas which are 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 plant abundance eand species richness with distance from water points and with perennial plant 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 m to 795 m radius for winter-spring annuals and from 165 m to 1065 m radius for the summer annuals. Variability in the number and size of biotic zones along grazing gradients was spatially correlated with frequency and intensity of disturbance, with landscape position and patchiness of soil features. There were fewer and larger zones of summer annuals than winter-spring annuals. Boundary analysis of livestock disturbance gradients provided a repeatable method for assessing the impact of long-term livestock grazing on annual plant communities.