Defoliation tolerance and ammonium uptake rate in perennial tussock grasses

Abstract Stipa 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. ...