Driving environmental factors and the role of grazing in grassland communities: A comparative study along an altitudinal gradient in Western Mongolia

In order to address questions relating to the driving factors in grassland vegetation, the potential role of grazing, and how the importance of these factors may vary along an altitudinal gradient of decreasing aridity, we studied grassland communities of desert steppe, mountain steppe, and alpine vegetation in Western Mongolia. Based on vegetation, soil parameters, and additional environmental data collected along grazing gradients, the variables of highest explanatory power for variation in vegetation were identified and ranked using a complementary approach of unconstrained and constrained ordination techniques. In desert steppe soil moisture deficiency is the main driving factor. While grazing is of minor importance in desert steppe, in mountain steppe it leads to substantial floristic and edaphic changes. In the alpine belt, a complex suite of driving factors involves thermal supply, water saturation, energy cycling, and grazing. Our study demonstrates how the role of vegetation as a site-affecting factor rises with increasing precipitation and productivity. It furthermore reveals how ecosystems may change from primarily abiotically driven to both abiotically and biotically driven systems. Finally, the study provides empirical evidence that the severity of grazing impact is coupled with the magnitude of importance of vegetation as a site-affecting factor; an aspect that has until now not been incorporated into the non-equilibrium paradigm of rangeland dynamics.