Effects of selective defoliation on the competitive interaction between palatable and unpalatable grasses native to a temperate semi-arid grassland of Argentina

Floristic changes induced by grazing frequently involve the replacement of palatable grasses by unpalatable grasses in native grasslands. It is hypothesized that selective defoliation of palatable species allows unpalatable species to realize a competitive advantage. To test this hypothesis, we quantified growth of two unpalatable grasses (Stipa trichotoma and S. gyneriodes) in the presence of non-defoliated and defoliated plants of the palatable grass S. clarazii under field and greenhouse conditions. Basal area, end-of-season aerial biomass, seed production, total green blade length per tiller, tiller recruitment and tiller dry mass of S. trichotoma and S. gynerioides were higher (p 0.05) in the presence of defoliated plants of S. clarazii than in the presence of non-defoliated plants of the same species. These results support the hypothesis that selective defoliation of palatable species confers a competitive advantage to unpalatable species, which could ultimately lead to replacement in grasslands.