Intensive grazing confounds the patterns of plant compositional change along a soil pH gradient, not an aridity gradient, in the Mongolian steppe

Abstract

Aridity, edaphic variables and livestock grazing are major drivers of plant community composition across arid rangelands. Accumulated knowledge exists on the impact of each driver on community composition. Although previous studies have demonstrated changes in direct grazing impacts on ecosystem functions and community composition at different aridity levels, ranging from mesic to arid grasslands, whether a regional-scale and continuous spatial pattern of plant compositional change along a gradient of aridity or edaphic variables is altered by grazing remains controversial. We compared the determinants and patterns of compositional changes with/without highly intensive grazing in the semi-arid/arid regions of Mongolia. The compositional changes based on Bray–Curtis dissimilarity were investigated using generalized dissimilarity modeling, including geographic distance, aridity, soil pH and soil texture as independent variables. Aridity consistently had a significant impact on community composition, regardless of the region and presence/absence of grazing. However, a difference in response patterns was observed between the regions. The compositional change was steeper at the upper (drier) limit of aridity than at the lower limit in the arid region. This pattern indicates the vulnerability of plant communities to aridity shifts owing to future climate change, especially in the desert steppe of Mongolia, although the predictions of shifts in aridity are not accurate. In addition, regardless of the region, the effects of soil pH on the community composition were eliminated by the presence of grazing. Grazing may homogenize community composition by not reflecting the spatial heterogeneity of soil pH or nutrient availability via selective herbivory. Despite the potential indirect impacts of climate change on community composition via soil pH, the observations for only plant communities under intensive grazing might overlook regional biodiversity changes caused by global change drivers.