Effects of grazing on plant community structure and aboveground net primary production of semiarid boreal steppe of northern Mongolia

We studied the effects of grazing on plant community structure and total plant biomass across the landscape while taking account of nutrient gradient, from wet and nutrient-rich sites (north-facing steppe) to dry and nutrient-poor sites (south-facing steppe) in semiarid steppe of northern Mongolia. Livestock grazing increased species richness of wet and nutrient-rich sites, while no significant change was observed in dry and nutrient-poor sites. The species richness increase in the wet and nutrient-poor sites was explained by local colonization of grazing-tolerant species. Species that adapted in the wet and nutrient-rich sites were driven to local extinction as a consequence of competitive exclusion. At a large spatial scale, livestock grazing can have a potential negative effect on a regional species pool, as it excludes species adapted in wet and nutrient-rich sites. Although grazing did not affect species richness in the dry and nutrient-poor south-facing steppe, plant communities under grazing shifted to dominance by short and prostrate forb species. A spatial difference of the total plant biomass across the landscape was higher in non-grazed landscape but this difference lessened in grazed landscape. The greatest percentage reductions of the total plant biomass due to grazing were in wet and nutrient-rich sites. In conclusion, at a community level, plant communities responded differently to livestock grazing, but at an ecosystem level, the total plant biomass decreased under grazing across the landscape of the semiarid boreal steppe. The livestock grazing in the wet and nutrient-rich sites resulted in the disappearance of moss cover, which is the main insulator of permafrost. The loss of moss cover could potentially accelerate a thawing of permafrost and warming of this region. Also, we found different results on dominance of Artemisia frigida Willd. from those reported in the steppe of Inner Mongolia. At a regional scale, this species might not be considered as an indicator species of livestock grazing.