Influence Of Competition Between Artemisia Frigida And Cleistogenes Squarrosa On The Restoration Of Degraded Grassland Of A. Frigida In Inner Mongolian Steppe

Membership of a plant species in a community is regulated by many factors. Our assessment of the importance of many of these factors is complicated by their ability to potentially interact. The typical Inner Mongolian steppe is a well-studied system that is susceptible to multiple interacting factors, including grazing, urine and feces deposition, drought, and physical disturbance by hundreds of thousands of sheep. Numerous research projects have revealed that over-grazing and soil degradation associated closely with over-grazing have led to significant changes in plant composition, plant cover and, in some places, to the complete destruction of vegetation. Defoliation has frequently been shown to play a central role in the organization of plant communities. The increase of nitrogen (N) availability in grazing ecosystems may lead to shifts in the competitive balance between species. N availability can also interact with other environmental factors (e.g. grazing) to cause a change in species composition. Herbivory and N addition have been shown to influence competition between species. But relatively few experiments have examined the interactions between herbivory and N supply on the outcome of plant competition. Two species, Artemisia frigida Willd. (C3, semi-shrub and dominant on overgrazed sites) and Cleistogenes squarrosa (Trin.) Keng (C4, perennial bunchgrass and dominant or co-dominant on moderately grazed sites), were selected to study the succession mechanism of a grazing ecosystem in field and greenhouse experiments. We hope this information is helpful to understanding the mechanisms of grassland degradation and the potential restoration of degraded steppe grassland in Inner Mongolia. We tested three hypotheses: (1) that nitrogen is deficient on degraded grassland, so a species with high nitrogen use efficiency will become dominant, (2) the competitive ability of C. squarrosa increases with N addition and clipping intensities, and (3) the higher interspecific competitive ability of A. frigida, rather than its tolerance to herbivory, will determine its dominance on overgrazed sites. In contrast to our first hypothesis, low N levels were not observed after two years of restoration on previously heavily grazed treatments. Conversely, higher microbial biomass N and inorganic N were observed in that area. A. frigida has low nutrient use efficiency (calculated by the amount of nutrients are needed for the formation of 100 g of carbon), however, A. frigida is the dominant species on overgrazed sites. In partial support of our second hypothesis, C. squarrosa did not become dominant during four years of restoration in the field, and the density of the C. squarrosa population is still low. This finding is line in with the results from our greenhouse experiment, that is, the competitive ability of C. squarrosa is consistently inferior to that of A. frigida, regardless of the level of N addition or clipping intensity. The interaction of nitrogen fertilizer with clipping was significant, although the clipping effect on competition was not significant. The relative competitive ability of A. frigida over C. squarrosa significantly declined with increasing nitrogen fertilizer, regardless of clipping intensity. In support of our last hypothesis, the combination of tolerance to grazing along with the economical use of nitrogen by C. squarrosa allows it coexist with A. frigida under moderate grazing whereas the high competitive ability (high biomass, nitrogen yield, fertilizer uptake rate and recovery) of A. frigida associated with high gap colonization, root niche differentiation and higher resistance to water stress makes it dominant on overgrazed sites.