WOODY AND HERBACEOUS ABOVEGROUND PRODUCTION OF A PATAGONIAN STEPPE

Aboveground net primary production (ANPP) of the Patagonian steppe in southwestern Chubut (Argentina) was estimated using a harvest technique to assess the herbaceous (mainly grass) component and a double sampling technique to evaluate shrub production. The latter requires the mesurement of plant dimensions and the harvest of shrub biomass in small plots. This technique, by virtue of having an explicit biological model which considers both shrub size and production per unit surface of plant, allows comparisons among years, sites, and treatments. Detailed estimates of ANPP yielded a value of 79 g of dry matter (DM) m-2 yr-1 (SE = 19 g DM m-2 yr-1) for an annual rainfall of 191 mm. Our estimates fits (+/- 17%) predictions of 4 models relating primary production to annual precipitation. Two thirds of production were accounted for by perennial grasses and one third by shrubs. A less detailed method, which uses only peak biomass, gave ANPP estimates for 4 additional years ranging from 21 to 75 g DM m-2 yr-1 while annual precipitation during this period ranged from 55 to 167 mm. There was a large reduction in ANPP during a year of extreme drought; however, there were no increases in ANPP during years with above-average precipitation. This suggests that the carrying capacity for the Patagonian steppe may not be linearly related to precipitation.