Testing a non-equilibrium model of rangeland vegetation dynamics in Mongolia

1. Few studies have tested the applicability of current non-equilibrium mod els of rangeland vegetation dynamics to a particular ecosystem, or across a range of systems that might be expected to respond differently to grazing. This study assessed the extent to which the non-equilibrium persistent (NE P) model of rangeland vegetation dynamics applies to three distinct Mongoli an rangeland ecosystems, the desert-steppe, steppe and mountain-steppe. 2. Standing biomass, vegetation cover and composition, and species richness and diversity were examined along grazing pressure gradients in ecological zones of differing productivity and interannual variability in precipitati on. 3. In the desert-steppe, biomass, functional group cover, richness and dive rsity did not vary along grazing pressure gradients, but all vegetation var iables except the cover of weedy annuals and unpalatable forbs varied signi ficantly between years. Vegetation dynamics in this zone largely conformed to the NEP model of rangeland dynamics. 4. In the mountain-steppe, grass and total biomass, total vegetative cover, the cover of grasses, weedy annuals and unpalatable forbs, and richness an d diversity varied along grazing pressure gradients. With increasing grazin g pressure, grasses decreased and forbs and weedy annuals increased, as the conventional range condition (RC) model predicts. Interannual variation in precipitation influenced total vegetative cover, species and functional gr oup cover, and richness and diversity. 5. In the steppe, forb biomass, grass, forb, unpalatable forb and weedy ann ual cover, and diversity varied along grazing pressure gradients. Grass bio mass and total vegetative cover responded interactively to rainfall and gra zing. Forb biomass, grass, forb and weedy annual cover and richness varied between years. Grasses decreased and forbs and weedy annuals increased with increasing grazing pressure, conforming to the RC model. 6. Ecosystem response to rainfall and grazing is complex, and interpretatio n of the response depends on the specific variables examined. The recent pa radigm shift in rangeland science from the RC model to non-equilibrium mode ls has been embraced with such enthusiasm by some that the concept of non-e quilibrium rangelands may be as much in danger of being misapplied as equil ibrium-based models have been.