Effects of grazing exclusion on vegetation community characteristics over 22 years in the Zoige alpine meadows from China

The grazing exclusion is a core policy implement applied by governments to rehabilitate degraded grasslands in China. However, the effects of grazing exclusion on species diversity, interspecific association, and niche overlap values changes in the Zoige alpine meadows are still unclear. The purpose of the paper aims to answer the following questions: (1) How do species diversity, interspecific association, and niche overlap values changes in grazed sites with time? (2) When is the optimal exclusion in the Zoige alpine meadows? Based on the method of space-for-time substitution, we investigated the amount of plant species in seven grazing exclusions established separately in 1996, 2007, 2009, 2011, 2012, 2016, and 2017 (representing seven succession stages of 22, 11, 9, 7, 6, 2, and 1 years after grazing exclusion, respectively), compared with free grazing. Aboveground plant biomass remarkably increased with time under short-term fencing, but no differences were found under long-term fencing. As for underground plant biomass, it obviously increased with time under short-term fencing, but significantly decrease under long-term fencing. Moreover, plant species richness firstly decreased and then increased with time under short-term fencing, while it significantly decrease under long-term fencing. The analysis of variance test showed that all species pairs in short-term fencing have a positive correlation, while negative correlation in long-term fencing. The niche breadth values of total species ranged from 0 to 0.75. These results indicated that short-term fencing for less than two years is beneficial to vegetation recovery in the Zoige alpine meadows.