The impact of herbivores in a Mongolian forest steppe

Herbivores play a key role in the sustainability of steppe grasslands through their influence on the structural and functional properties of the ecosystem. Their grazing often leads to changes in vegetation structure, species composition and abundance, plant productivity and plant nitrogen cycling. \n \n \nThe aim of this thesis is to gain insight in the impact of different types of herbivores, a large and a small, on the steppe vegetation. We conducted an exclosure experiment on the Stipa steppe, in which we examined the effects of 3 years of exclusion of Tahki (Przewalski horse) and Siberian marmots on plant biomass and plant N-concentration, and plant species abundance. \n \n \nGrazing significantly decreased the plant standing crop (which consists of aboveground biomass and standing dead material) of the Stipa vegetation (chapter 3). The exclusion of takh and marmots led to a higher plant standing crop in the exclosure plots, and the intensity of grazing by takh and marmots was sufficient to cause significant differences between the different types of exclosures. A successive increase in the amount of standing crop on fully grazed, marmot-grazed and ungrazed plots reflected a successive decrease in grazing intensity.\n \n \nOur results on plant species abundance showed a higher frequency of the rhizomatous grass Leymus chinensis under a higher grazing intensity, and a decrease of the caespitose Stipa krylovii.\n \n \nWe addressed the question of whether shifts in species dominance that occur with increased grazing, can be explained by differences in growth responses to clipping. In a greenhouse experiment we tried to establish a link between the species dynamics in the Mongolian steppe and physiologically based plant characteristics. We compared the potential for compensatory growth of Leymus and Stipa, using a recently developed technique of growth analysis. Leymus exhibited a much stronger compensatory growth and increase in RGR after clipping than Stipa, and Leymus plants maintained its productivity levels, while productivity in Stipa dropped. These results suggest that the greater grazing tolerance of Leymus results from the much stronger compensatory growth after clipping than Stipa. \n \n \nFor the estimation of the carrying capacity, which is an important index for sustainable pasture use, one should account for the effect of compensation. Without this it would lead to a considerable underestimation of the carrying capacity of Leymus grasslands, while for Stipa grasslands it would lead to an overestimation.\n \n \nOn the Stipa steppe we studied the impact of marmot disturbances. Burrowing activities of marmots lead to important changes in the vegetation of the Stipa steppe. They create mounds that differ greatly in vegetation cover; there are mounds covered with bare soil, mounds covered by a vegetation more or less similar to the surrounding Stipa vegetation, and mounds dominated by the grass Leymus chinensis or the herb Artemisia adamsii. It is likely that the different types of mounds represent a series of succession, considering the growth habit of the species dominating the different types of mounds. A dynamic and patchy vegetation cover is created, which can easily regenerate into the more stable Stipa steppe.