Vegetation in a Post-Ecological Threshold State May Not Recover after Short-Term Livestock Exclusion in Mongolian Rangelands

In the present study, we tested the potential irreversibility of vegetation dynamics in Mongolian rangelands using well-studied plant communities that exist along grazing gradients, in which ecological thresholds (defined as the points or zones at which disturbance should be limited to prevent drastic changes in ecological conditions) exist in terms of the compositional changes along these gradients. To accomplish this, we removed livestock grazing impacts by establishing exclosures along a grazing gradient at two study sites located in Mandalgobi and Bulgan, Mongolia. Each exclosure was established in the summer of 2004 at a location with either a post-ecological threshold state or a pre-ecological threshold state. We examined general patterns of temporal change in vegetation for the permanent plots inside and outside each exclosure at each site between 2005 and 2010. The trajectories of floristic composition in the permanent plots outside and inside each exclosure were similar from 2005 to 2010, indicating that the trajectories were mainly associated with annual rainfall and annual phenological changes in the plant communities. Post-threshold states at both sites did not reach their respective target community for restoration, indicating the lack of restorability despite livestock exclusion. Moreover, ordination separated the trajectories of floristic composition for the permanent plots inside exclosure in the post-threshold state from those in the pre-threshold state. Thus, our results suggest that vegetation in a post-threshold state may not recover after short-term livestock exclusion in Mongolian rangelands.