Community Assembly And Stability In Degradating Process Of LYMUS Chinensis Grassland

The grassland ecosystems provided the livelihood of nomads and were used as pastures in Inner Mongolia. Today the issue of degradation in the grasslands of Inner Mongolia, China, is becoming increasingly serious, such as loss of biodiversity and productivity, decrease in ecosystem stability, and soil erosion, bringing about an urgent need to clarify the mechanism by which biodiversity and system stability are maintained in the face of degradation.In this study, we considered the impacts of multiple components (climate, soil, topographical, and human disturbance) on plant community assembly, community diversity, and stability at four different degradation levels of Lymus chinensis steppe at the river-basin scale, such as L. chinensis+perennial forbs, L. chinensis+Stipa, L. chinensis+C. squarrosa, and L. chinensis+A. frigida. Discussed the major maintenance mechanism of community diversity and stability derived from grazing degradation gradient by the way of community assembly process. The results showed:(1) The community diversity along the degradation gradientWe examined the three levels of diversity (species diversity, functional group diversity, and functional diversity) in community along the degradation gradient. Our results showed the following:(i) the species richness and community complexity increased with the degradation gradient, while the dominance of species decreased with the degradation gradient. The functional richness significantly decreased and functional evenness significantly increased with the degradation gradient.(ⅱ) there was also a considerable turnover in community structure along the degradation gradient. And there was a gradual change in the successful resource-use strategy from species with shorter leaf lifespans, and lower nutrient-use efficiency on rarely grazed and original communities to species with lower metabolic rates and higher nutrient-use efficiency on intense-grazing and degenerated communities.To sum up, these results indicated that the degraded communities own more inappropriate niche space, but the increase of species richness in such communities would more evenly utilize the niche space which already occupied. By contrast, the original communities possess more dominance species those could not evenly distributed in environmental space, thus might results in the phenomena of niche space that already occupied but not totally used up.(2) The community assembly along the degradation gradientOur work demonstrated the community assembly in the grassland degradation process by two-step null models modeling. Our results showed:(ⅰ) The patterns of trait distribution and filtering were strongly dependent upon the degradation gradient and trait types. Most traits were filtered intensely in degraded grasslands.(ⅱ) Plants with two different strategies showed contrasting trait-distribution patterns. The traits that were related to biological matter cycling showed divergent patterns in highly degraded grasslands, while convergent patterns along the overall gradient were demonstrated in traits associated with other plant strategies.This suggests that the coexistence of multiple ’biological matter cycling-related niches’might be a basic structuring pattern of plant communities in our study area, (ⅲ) The simultaneous occurrence of strong filtering and divergence revealed that environmental filtering does not necessarily prevent competition, and that lead to the trait divergence in the filtered range.(3) The community stability along the degradation gradientWe analyzed four stability components (resilience, resistance, structural variability, and functional variability) along the degradation gradient, and discussed the multidimensional stability for four degradation stages of L. chinensis steppe. Results showed:(ⅰ) the moderately degraded C. squarrosa communities have largest resilience and multidimensional stability, while the intensively degraded A. frigida communities have lower resilience, resistance and multidimensional stability, representing the self-protection mechanism of grassland under disturbance, such as 01</electronic-resource-num＞＜/record＞＜/Cite＞＜/EndNote>[⊥!! HYPERLINK\1 chinensis communities. That is, the ecosystem stability increased under certain disturbance. Nevertheless, the further intense of disturbance would leading to the collapse of ecosystem resilience and resistance.(ⅱ) the diverse relationships between four stability components remind us that would be misunderstood or underestimated the potential consequences of human disturbance on the ecosystem stability when we consider one or a few components. The 50 percent of multidimensional stability variance captured by first dimension, and 80 percent of multidimensional stability variance captured by first two dimensions. These results demonstrated that the effective dimensionality of stability can be comparatively low and collapsible to just one or perhaps two dimensions.(4) Community assembly and stability in the degradation processThe results of structural functional modeling for environmental variables, community assembly, community diversities, and community stability showed: community stability was positively related to species diversity, functional group diversity, and functional diversity, and mainly effected by functional diversity. That close path relationship between grazing intensity, trait distribution, functional diversity, and stability demonstrated that species niche differentiation and diverse response of species to environmental conditions are the main maintenance mechanism of community diversity and stability under disturbance.