Suitable exclosure duration for the restoration of degraded alpine grasslands on the Qinghai-Tibetan Plateau

Exclosures erected to prevent grazing have been widely adopted as a measure for the recovery of degraded grasslands. Many studies have explored the effects of exclosure use on vegetation and soil; these studies, however, were carried out over relatively short-time periods. Given the length of time required for a grassland to fully develop, short-term studies result in a limited understanding of exclosure effectiveness. To address this challenge, this study will use 5-, 13-, 22-, and 39-year exclosure times in an alpine grassland on the Qinghai-Tibetan Plateau, China, to evaluate the influence of exclosure time on species number, aboveground biomass, including both living and dead biomass (i.e. standing dead biomass and litter), and soil water content. To explore the differences between traditional grazing activity and exclosures, seasonal grazing served as a control measure. The results show that species richness was at its lowest after a 39-year exclosure period while the most dominant species remained invariant for both seasonally-grazed and exclosed plots. The greatest increase in living biomass occurred after a 13-year exclosure time; in contrast, the greatest increase in dead biomass occurred after a 39-year exclosure time. Furthermore, the aboveground biomass did not vary with exclosure time. In terms of soil depths ranging from 0-0.30 m, there was little difference in soil water content between seasonally-grazed and exclosed plots. While the soil water content recorded at the 0-0.15 m layer fluctuated somewhat. At the 0.15−0.30 m layer it presented a consistent decreasing trend as exclosure time increased. A 13-year exclosure period proved to be a suitable duration for alpine grassland recovery since, at this stage, living biomass, species richness and soil water content were greater than that of seasonally-grazed grasslands.