Effects of Grazing Exclusion on Plant Functional Group Diversity of Alpine Grasslands along a Precipitation Gradient on the Northern Tibetan Plateau

摘要

Tibetan alpine grasslands are viewed to be sensitive to climate change and grazing disturbance. But it is not well understood how and to what extent grazing exclusion affects species assembly on the Northern Tibetan Plateau. We conducted a multisite transect along a precipitation gradient to detect species compositional changes at 25 grazed versus nongrazed paired pastures in summers of 2009 and 2010 in the Changtang Natural Reserve. Species richness and relative frequency were estimated for plant functional groups (PFGs: grasses, sedges, forbs, and legumes). Species richness at community level responds positively to precipitation and differs among vegetation types: meadow (22.63 ± 1.73) > steppe (11.23 ± 1.00) > desert-steppe (6.75 ± 0.63). Variations in species richness and relative frequency of PFGs are partly dependent on vegetation type. Three to four years of grazing exclusion has not significantly changed species richness or relative frequency at PFG level. Grazing exclusion has slightly changed PFGs' correlations in species richness but significantly altered their correlations in relative frequency within and across vegetations. Stepwise linear regressions indicate that PFGs respond to climate gradients in discrete ways. This study implies that specific adaptation mechanisms of different taxonomic groups to climatic change and grazing disturbance should be seriously considered in further studies.