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Defoliation tolerance and ammonium uptake rate in perennial tussock grassesStipa clarazii, Ball. has been shown to be more tolerant to defoliation and a superior competitor to S. tenuis Philo and S. ambigua Speg. 3 perennial grasses native to semiarid rangelands in central Argentina. Mechanisms contributing to its great defoliation tolerance and competitive ability, however, are largely unexplored. We examined tolerance to defoliation and ammonium uptake rates on defoliated and undefoliated plants of those species at 10, 25, and 50 ppm NH4+ using (NH4)2SO4 solutions containing 60 atom %15N excess. By mid-spring, greater regrowth following defoliation in S. clarazii than in S. tenuis or S. ambigua indicated greater defoliation tolerance in the first than in the other 2 species. Stipa clarazii had similar of higher ammonium uptake rates than S. tenuis and S. ambigua. Higher ammonium uptake rates in S. clarazii thus appear to be one of the mechanisms most likely contributing to its greater competitive ability and defoliation tolerance when compared to the other 2 species. Defoliated plants of all 3 species had similar or greater ammonium uptake rates than undefoliated plants. These results suggest that photosynthetic canopy reestablishment may be achieved without sacrificing root function in these perennial grasses, at least as long as carbon reserves do not become a limiting factor. Ammonium uptake rates increased when NH4+ concentrations increased in the labeled solutions in S. clarazii, S. tenuis and S. ambigua. This result demonstrates the capacity of the root system for increasing nutrient acquisition during periods of high resource availability.