Rangeland monitoring in northern South Australia using a grazing gradient method

Abstract

The grazing gradient method (Pickup et al., 1994) allows grazing effects on vegetation to be separated from those due to rainfall and local landscape variability by examining patterns of cover change with increasing distance from water. Vegetation cover tends to increase with distance from watering points as grazing intensity decreases, producing a grazing gradient. Where this cover gradient persists after large rainfalls, it indicates a degree of land degradation. Grazing gradient analyses were conducted for the regional assessment and reporting of landscape function (cover) in northern South Australia, in areas used for extensive grazing by domestic cattle. A time series of Landsat TM images were analysed to compare average cover levels at increasing distance from water in October 1988 (dry conditions), July 1989 (after a large rainfall event) and April 1997 (following another large rainfall event). The study area covered 50,000 km/sup 2/ in the Marla-Oodnadatta Soil Conservation District. Regional summaries of vegetation cover changes with distance from water were produced for 70% of the study area (30,000 km/sup 2/), covering 19 land types in 10 land systems. The remaining area was not suited for application of the grazing gradient as the landscape was too heterogeneous. Application may have been improved through improved landscape stratification however it was preferred to exclude these areas from the analyses, as they are not used extensively for grazing. Grazing gradient plots were interpreted to determine the condition of landscape types. Thresholds of change, indicating where landscape function may be severely disrupted, were identified through inflection points on the plots.