Altitudinal pattern of grazing exclusion effects on vegetation characteristics and soil properties in alpine grasslands on the central Tibetan Plateau

Abstract

Grazing exclusion by fencing has been widely used for restoring degraded alpine grasslands, but rarely studies have considered the altitudinal difference of grazing exclusion effects on vegetation characteristics and soil properties. In this study, we focused our attention mainly on how alpine grasslands at different altitudes will response to long-term grazing exclusion. A 10-year altitudinal grazing exclusion experiment was conducted along a large altitudinal gradient (six altitudes, from 4400 to 5100 m) in alpine meadows on the central Tibetan Plateau. The vegetation characteristics and soil properties across grazing exclusion plots and free grazing plots were observed at each of six altitudes. Above-ground biomass (AGB), below-ground biomass (BGB), soil organic carbon (SOC), soil total nitrogen (STN), soil microbial biomass carbon (MBC), and soil microbial biomass nitrogen (MBN) showed a unimodal pattern along the altitudinal gradient from 4400 to 5100 m. A similar altitudinal pattern was found in sedges biomass but not in that of grasses, legumes, and herbs. Grazing exclusion practice increased vegetation cover, plant height, AGB, BGB, STN, SOC but has no significant effect on community diversity. Grazing did not alter the altitudinal patterns of AGB, BGB, SOC, and STN, but the stimulation impacts of grazing exclusion on these factors decreased with increasing altitude. The relative changes of MBC and MBN after grazing exclusion were significant related with that of BGB. Our study indicates that grazing exclusion by fencing is effective to restore vegetation and improve soil carbon and nitrogen properties in overgrazed alpine grasslands. However, the stimulation effects of grazing exclusion on alpine grasslands decreased with increasing altitudes. We suggest that in the high-altitude grassland ecosystems, especially where environment is harsh and stocking rate is relative low, grazing exclusion might not be efficient enough for restoring degraded grasslands.