Testing a non-equilibrium model of rangeland vegetation dynamics in Mongolia.
The extent to which the non-equilibrium persistent (NEP) model of rangeland vegetation dynamics applies to three distinct Mongolian rangeland ecosystems, the desert-steppe, steppe and mountain-steppe was investigated. 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 precipitation. In the desert-steppe, biomass, functional group cover, richness and diversity did not vary along grazing pressure gradients, but all vegetation variables except the cover of weedy annuals and unpalatable forbs varied significantly between years. Vegetation dynamics in this zone largely conformed to the NEP model of rangeland dynamics. In the mountain-steppe, grass and total biomass, total vegetative cover, the cover of grasses, weedy annuals and unpalatable forbs, and richness and diversity varied along grazing pressure gradients. With increasing grazing pressure, grasses decreased and forbs and weedy annuals increased, as the conventional range condition (RC) model predicted. Interannual variation in precipitation influenced total vegetative cover, species and functional group cover, and richness and diversity. In the steppe, forb biomass, grass, forb, unpalatable forb and weedy annual cover, and diversity varied along grazing pressure gradients. Grass biomass and total vegetative cover responded interactively to rainfall and grazing. Forb biomass, grass, forb and weedy annual cover and richness varied among years. Grasses decreased and forbs and weedy annuals increased with increasing grazing pressure, conforming to the RC model. Ecosystem response to rainfall and grazing is considered complex, and interpretation of the response depends on the specific variables examined. It is suggested that the recent paradigm shift in rangeland science from the RC model to non-equilibrium models has been embraced with such enthusiasm by some that the concept of non-equilibrium rangelands may be as much in danger of being misapplied as equilibrium-based models have been.