COMPETITIVE INTERACTIONS BETWEEN PALATABLE AND UNPALATABLE GRASSES NATIVE TO A TEMPERATE SEMIARID GRASSLAND OF ARGENTINA

The argument that selective grazing leads to competitive replacement o f palatable grasses by unpalatable grasses is based upon the assumptio n that the competitive ability of the palatable species is higher than the one of unpalatable species in the absence of grazing. In order to test this hypothesis we have compared the competitive ability of Stip a clarazii (palatable) and S. trichotoma (unpalatable) under held cond itions, and S. clarazii and S. gynerioides (unpalatable) under greenho use conditions. The three species are native to a temperate semi-arid grassland of central Argentina. In the held experiment, plants of both species were grown either independently or in pairs (palatable + unpa latable), protected from grazing. Shoot and seed production were measu red at the end of the growing seasons of 1993, 1994 and 1995. In the g reenhouse experiment, plants of both species were grown in pots, eithe r in monoculture or in mixture, under conditions of high and low water and mineral nutrient availability. Total biomass and seed production were measured at the end of the experimental period. In both experimen ts the presence of the unpalatable species did not affect (P > 0.05) t he productive responses of the palatable species. On the contrary, the presence of the palatable species significantly reduced (P < 0.05) th e productive responses of the unpalatable species. Our results support the assumption, on which most interpretations of floristic changes in duced by grazing are based, that the competitive ability of palatable grasses is higher than the one of unpalatable grasses in the absence o f grazing.