Threshold changes in vegetation along a rangeland gradient in the eastern Mongolian steppe

Rangeland ecosystems are undergoing rapid change in terms of vegetation composition, species diversity, and productivity, primarily due to increased grazing pressure. Assessing rangeland condition on which a reliable classification of the degree of degradation can be based is required for grazing management. To achieve this objective this study identified species indicator species of degradation of grazing land and quantified their threshold responses to along a rangeland condition gradient in the Eastern Mongolian steppe. Plant species richness, composition, abundance, and the cover of canopy and litter were measured at 52 sites along a rangeland condition gradient in six years between 2013 and 2023. Rangeland condition scores were calculated for each site using the Rangeland Condition Scoring Method (RCSM). Thereafter, we used threshold indicator taxa analysis based on the frequency and abundances of 102 plant species across sites to detect community thresholds at which many species simultaneously and abruptly change in abundance in response to rangeland degradation. At the community level, there were three threshold changes in rangeland condition. Forty-five species showed significant change points, defined as abrupt shifts in abundance along the gradient, and 13 indicator species that are sensitive to rangeland degradation were identified. Species richness, Shannon diversity index, and canopy cover of plant species (p = 0.001) exhibited significant differences across rangeland condition categories, while total abundance of plants did not differ significantly (p = 0.9). PERMANOVA analysis confirmed significant compositional differences among the rangeland condition categories (p < 0.001). This study provides valuable insights into the points of condition change in rangeland ecosystems and highlights sensitive indicators of change in the vegetation. Our findings demonstrate how plant species richness, diversity, and cover in rangeland plant communities impacted by overgrazing can be used for condition monitoring for adaptive management.