Cessation of grazing stabilizes productivity through effects on species asynchrony and stability of shrub/semi-shrub plants in arid grasslands

Grasslands are globally abundant and important ecosystems, supporting considerable biodiversity and the livelihoods of herders. However, many grasslands have become degraded through overgrazing. Cessation of grazing can have significant effects on grassland biodiversity and ecosystem functioning, yet, to date, there is no consensus about how grazing cessation affects temporal stability of plant community productivity or the underlying mechanisms driving effects. In this study, we evaluated the effects of grazing cessation on the temporal stability of annual net primary productivity (ANPP) in the plant communities, using a 15-year field experiment conducted in an Inner Mongolian desert steppe landscape. Plant diversity, species asynchrony and the temporal stability of ANPP of functional groups were analyzed to explore the driving mechanisms of temporal stability of plant community ANPP during periods without grazing. The results showed that plant ANPP and its temporal stability of whole community, shrubs/semi-shrubs and perennial forbs were greater in areas when grazing had ceased compared with areas under continuous grazing. Grazing cessation increased both species asynchrony and species richness at the community level. Structural equation modelling (SEM) was used to better understand the mechanisms behind the result and showed that grazing cessation increased temporal stability of community ANPP by increasing species asynchrony and the temporal stability of shrubs/semi-shrubs ANPP. Temporal stability of whole community ANPP were not related to species richness. Our study also found that the effects of grazing cessation on temporal stability of community ANPP was largest during the initial five years, while the latter ten years of grazing cessation had few additional effects on temporal stability of community ANPP. Together, our results suggest that species asynchrony and the temporal stability of shrubs/semi-shrubs ANPP may be the key drivers affecting temporal stability of community ANPP during grazing cessation, and short-term grazing cessation may be more impactful than medium- or longer term grazing cessation.