Effect of Grazing Pattern on Primary Productivity

Grazing usually increases the relative growth rate (RGR) of plants. However, it is not clear under which conditions this will result in a positive effect on primary productivity. We tested the hypothesis that the effect of grazing on community productivity depends on the level of competition between grazed and neighboring ungrazed plants, and that the level and outcome of that competition depends on the size of the grazed patch. The hypothesis predicts that as the size of a grazed patch increases, the area of influence of the neighboring plants becomes less important in relation to the area of the patch. Thus, grazing will more likely increase the productivity of large patches. We performed two field experiments in a grassland community of the Flooding Pampa, Argentina under two different weather conditions (non-flooded year and flooded year). We analyzed the productivity of undefoliated patches, defoliated patches of different sizes, and a neighboring area around the defoliated patches. We found that (I) the effect of defoliation on productivity was affected by patch size: defoliation had neutral to positive effects on intermediate and large patches and negative effects on very small patches, and (2) defoliation had no effect on the productivity of the area surrounding the defoliated patches. Our results also suggest that flooding limits the ability of plants to compensate for tissue removal.