Drought and grazing drive the retrogressive succession by changing the plant–plant interaction of the main species in Inner Mongolia Steppe.

Plant–plant interactions play a key role in the function and structure of communities. The combined effect of drought stress and grazing disturbance on shaping plant–plant interactions is still poorly understood, while this combination is common in semiarid ecosystems. Four species including Stipa grandis, which is dominant in the typical steppe, and Stipa krylovii, Artemisia frigida, and Cleistogenes squarrosa, which are dominant species in the S. grandis degraded communities, were selected as study targets. We conducted a competition experiment (uniformly dense monoculture or mixture, respectively) under controlled conditions, including both drought stress and mowing disturbance, and calculated the relative interaction index (RII) of tiller number and RII of biomass for each species under each condition. (a) Under the same condition, the RII of tiller number and that of biomass for the same species usually showed reverse trends. (b) Mowing disturbance rather than drought stress played a negative role in influencing S. grandis' or S. krylovii's RII of tiller number and played a positive role in influencing A. frigida's RII of biomass. (c) Drought stress rather than mowing disturbance played a positive role in influencing C. squarrosa's RII of tiller number. (d) Neighbor species significantly influenced S. grandis' RII of tiller number, S. krylovii's RII of tiller number, A. frigida's RII of tiller number and biomass, and C. squarrosa's RII of biomass. These results could provide an explanation for why S. krylovii, A. frigida, and C. squarrosa can replace S. grandis and become the dominant species when S. grandis communities undergo a process of degradation due to overgrazing or climatic drought in natural communities. The present study provided powerful evidences for species replacement in the typical steppe of Inner Mongolia and elucidated the driving mechanisms of S. grandis communities' retrogressive succession. The combined effect of drought stress and grazing disturbance on shaping plant–plant interactions is the key process of retrogressive succession in Inner Mongolia steppe, China. We conducted a competition experiment to explore how Stipa grandis is replaced by the other three species during the retrogressive succession. (a) Neighbor species could help Stipa krylovii to increase its tiller number but inhibit S. grandis's tiller number under mowing treatment. (b) Mowing disturbance played a positive role in Artemisia frigida expanding in medium–heavy‐degraded region. (c) The higher tolerance to drought stress and lower competitive capacity enabled Cleistogenes squarrosa to be dominant species in heavy‐degraded region but not in stable or light‐degraded region. The present study provided evidences for species replacement in retrogressive succession of S. grandis communities. [ABSTRACT FROM AUTHOR]