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Large areas of Mongolia have been subject to semi-nomadic pastoralism for centuries, but recently increasing livestock numbers have raised concerns about possible degradation effects. We assessed the impact of grazing in southern Mongolian desert steppes (mean precipitation <150 mm) over eight years in order to capture the high interannual variation, as well as possible medium-term trends in the management regime. We assessed whether the impact of varying precipitation levels (coefficient of variation >30%) exceeds the effects of grazing on vegetation composition, species richness, plant flowering activity, biomass production, and soil nutrient content. The data presented here originate from a set of nine fenced exclosures established in 1999.

Plant community composition and species richness showed interannual changes over years of varying precipitation levels: mean species numbers on ungrazed plots ranged between 8 and 19 per 100 m(2). Grazing effects were apparent but light, as grazed plots had, on average, 1-2 species fewer than ungrazed plots. There was also no evidence of directed succession under grazing exclusion. Mean numbers of flowering species on ungrazed plots ranged between 2 and 14 over the years while numbers on grazed plots were slightly lower (range 1-12). In one moist year, mean biomass production in previously ungrazed plots was higher than in previously grazed plots (ca. 800 kg/ha vs. 250 kg/ha), but differences in a subsequent drier year were negligible. This was paralleled by soil phosphorus content, which tended to be higher inside the exclosures.

The overriding effects of precipitation variability are in line with the non-equilibrium theory of rangeland science and imply that the risk of direct degradation is low in southern Mongolian drylands. However, small but statistically significant differences between grazed and ungrazed sites suggest that herbivore impact is apparent and can alter plant reproduction and, ultimately, soil conditions and productivity. (C) 2010 Elsevier Masson SAS. All rights reserved.