Plant production along a grazing gradient in a semiarid Patagonian rangeland, Argentina

Abstract

Patagonian rangelands have been grazed by sheep since early twentieth century. However, there is still a degree of uncertainty regarding how production of grass and shrub species changes along a grazing gradient. The study was undertaken in Northeastern Patagonia, Argentina. The characteristic vegetation in the area is a mosaic of herbaceous steppe with shrubs (HSS) and shrub steppe (SS). Grazing intensity was estimated through sheep paths density. Individual plant production and plant density were used to determine grass and shrub production per unit area, in both plant communities over three years. Community production was obtained as the sum of grass and shrub components. Differences were explored in shrub and grass production among communities and years, and linear regressions between sheep paths density and the proportion of each plant community along the transect were performed. Mean community production was lower in SS than HSS; shrubs did not compensate for the decline in grass production in spite of the increase in shrub density. SS presented the highest community production in the most humid year, while HSS production peaks in the year of average precipitation. We found that as the number of sheep paths increases (i.e., higher grazing intensities), the SS community replaces HSS, resulting in a reduction of forage for sheep. The results showed that changes in vegetation structure as a result of grazing strongly influenced above-ground production. Results also indicate that the response of vegetation to changes in annual precipitation is community specific.