Effect of Grazing on the Population Biology of Phalaris aquatica

Abstract

To examine the effect of grazing and potential interactions among grazing and biological traits of plants, we studied patch dynamics, seed production, and seedling survival in a Mediterranean population of the perennial grass Phalaris aquati ca L. in grazed and ungrazed plots in southern Spain. Grazing by cattle induced an important (70%) decrease in the abundance of plants over 4 growing seasons. In the ungrazed plots, abundance of plants remained stable. Within these plots there was some (30%) spatial replacement of plants. However, replacement was by tillering and not by genets, genetically different individuals produced from seeds. The lack of genet replacement within the ungrazed plots agrees with results on mortality of young plants that were obtained from an independent field experiment, in which 85‐95% of plants in different cohorts died within 1 to 3 growing seasons. This mortality of seedlings and young plants was concentrated in summers, especially when drought was prolonged. In contrast, seed production was apparently not a limit ing factor for plant recruitment in ungrazed plots as seed output of the perennial grass (25,312 ± 3,255 seeds m -2 ) was of the same magnitude as seed output in annual grasses that were abundant in the study site. Intensive grazing limited tiller production, patch size, and a summer drought limited recruitment of new adult individuals. These factors resulted in a low tolerance to episodes of high stocking rates from which the perennial grass was unable to recover.