Abstract:

Studies of simulated grazing of Elymus nutans, a common species in subalpine meadows in Gannan, were conducted to assess the effects of population density and fertilization on the plant compensatory responses to mowing. We subjected Elymus nutans to different types of mowing treatment, early or late (3 June or 28 July, 2001) and light or heavy mowing (4 cm or 2 cm from the ground), to different levels of soil fertility (unfertilized or fertilized), and to different levels of intraspecific competition (five designed sowing densities: 100, 400, 800, 1 600 and 3 200 seeds·m-2).Plant responses to these treatments were measured as changes in plant aboveground biomass and inflorescence dry weight, and the degrees of compensation were measured as changes in compensation indixes that related plant aboveground biomass or inflorescence dry weight of mowed to control plots. Our results showed that population density, soil fertility and mowing treatments significantly affected plant growth, and there were no interactions between two factors and among three factors (density × soil fertility × mowing).Mowed plants growing in early and light mowing treatment plots were more or less enhanced, and grew better than in other treatment plots. Further, early and light mowed plants in the low-density (100 seeds·m-2) and unfertilized plots showed enhanced growth (overcompensation), and the number of tillers and plant compensatory ability decreased with increasing density. These results indicated that plant compensation ability was directly related to the number of tillers. The compensation may be considered as an indirect consequence of growth of low tillers when apical dominance is removed.Mowed plants in fertilized plots showed enhanced growth irrespective of the population density. Our results support the view that rapid plant regrowth was more likely to occur under low-stress conditions. We also found that plant vegetative growing than reproductive growing was enhanced when the soil was fertilized. We suggest that overcompensation should not generally be considered as an adaptive response, but rather as a maximal end in the continuum of plant responses to herbivory.