Vegetation change along gradients from water sources in three grazed Mongolian ecosystems

Abstract

Foliar cover of plant species; grass, forb and total herbaceous biomass; soil P, K, N and C; and percent coarse fraction of soils were sampled over two years along grazing gradients from livestock water sources in three grazed Mongolian steppe ecosystems of varying productivity. Samples within each of the three systems (mountain-steppe, and desert-steppe) were classified into plant communities using TWINSPAN and species-environment relationships in each system were examined using CCA. Community classifications were driven by the presence/absence of ruderal species and highly palatable grasses in the steppe and mountain-steppe and by the presence/absence of salt-shrub or Caragana shrub species and associated Iris species in the desert-steppe. Ordinations were largely driven by soil nutrient concentrations, particularly P and K, in all three zones. Bulk density and percent coarse fraction to 10 cm were also important in the desert-steppe. Distance from water, which we assumed to be inversely related to grazing pressure, was a significant driving factor in steppe and mountain-steppe ordinations, and was negatively correlated with P and K. We speculate that elevated nutrient concentrations near water sources result from livestock redistributing nutrients in the landscape by voiding urine and feces in the areas where they congregate. Livestock may thus influence species composition in these systems both through the direct effects of defoliation and trampling, and the indirect effects of nutrient enrichment and depletion over the broader landscape. This hypothesis deserves further testing under controlled conditions.