Research on compensatory growth of Stipa aliena andimpact factors under different habitats

Grazing disturbance and resource availability can affect the compensatory growth of defoliated plants, but it is unclear which factor is the most important in inducing the over compensatory response to herbivores in a particular area. Our objectives were to examine how compensatory growth of Stipa aliena varied with different habitats in an alpine Kobresia humilis meadow and determine relative effects of these factors on compensatory growth. A field experiment was carried out at the Haibei Research Station of the Chinese Academy of Sciences from April to September 2007, which included three habitats (habitatⅠ: pen, grazedhigh nutrient availability; habitatⅡ: pass pasture, grazedlow nutrient availability and habitat Ⅲ: ungrazed pasture, null defoliation ratiolow nutrient availability) with three plots per habitat. In each plot, 12 quadrats (0.5 m0.5 m) were divided into two groups (6 caged and 6 uncaged). For each group, 3 of 6 quadrats were sampled in midJune and other 3 in midAugust, respectively. We used canonical correspondence analysis (CCA) with potential impact factors selected by the forward selection procedure for the compensatory growth of S. aliena to determine the relative effects of defoliation, water and soil nutrient variables on compensatory growth. The results indicated that full compensation occurred for roots at habitat Ⅱ and low compensation was found in leaves and inflorescence at habitat Ⅰ and Ⅱ. The ramet density, height and growth allocation of the S.aliena were higher at habitat Ⅲ. The ramet relative growth rate of the species was lower at habitatⅠ and higher at habitat Ⅱ. The results of CCA showed that the most important impacted factors of compensatory growth and ramet relative growth rate of the species were defoliation ratio by grazing. Although there were higher soil nutrient and water contents at habitat Ⅰ, but still did not blot out the negative effects of heavy deflolation and trampling by domestic animals on the species. The results above suggested that the most important factors affecting the compensatory growth of S. aliena was defoliation ratio rather than soil resource availability.