Grazing exclusion altered the pattern of the soil seed bank but not the aboveground vegetation along an altitudinal gradient in alpine grassland

Grazing exclusion by fencing is the principal option to restore degraded grassland, but its effects on recovery and succession of vegetation are not clear, especially in high mountain areas. In this study, we aimed to fill this gap by assessing the effect of seven-year grazing exclusion on vegetation and soil seed bank along an altitudinal gradient in northern Tibet, China. Species richness, Shannon-Wiener index and Simpson's diversity index of vegetation and soil seed bank, seed density, and the Sorensen index all displayed a mid-domain (hump-shaped) pattern with increasing altitude, except for the Shannon-Wiener and Pielou indices of soil seed bank in fenced grasslands. These results demonstrated that the impact of grazing exclusion on the patterns of vegetation and soil seed bank change with altitude but are not synchronous. Grazing exclusion increased vegetation coverage but decreased soil seed bank density, and species diversity of vegetation and soil seed bank. The Sorensen index (0.03-0.2) and non-metric multidimensional scaling (NMDS) analysis indicated that the species composition differed between vegetation and soil seed bank, but no difference was found between fenced and grazed grasslands. The low similarity suggests that the soil seed bank contributes little to vegetation recovery in alpine grassland. Seven-year grazing exclusion had no effect on either vegetation succession or species composition of soil seed bank in alpine grassland. Overall, our findings suggest reconsidering the use of long-term grazing exclusion to restore degraded grassland at high altitudes (above 4400 m above sea level) on the Tibetan Plateau.