How enclosure affects community characteristics of the sandy grassland in semi-arid areas of northwestern China

Using line transect and quadrat sampling methods, vegetation was investigated in the semi-arid areas with different enclosure periods (5, 10 and 20 years) and one non-enclosed area (contrast area) in Yanchi County of Ningxia Province, northwestern China in 2013. The patterns of population niche on the community of sandy grasslands with different enclosure periods were revealed after analyzing the importance value, Levins index of niche breadth and Pianka index of niche overlap. The results showed that perennial herbs or subshrubs were dominant in the enclosed areas and annual herbs were dominant in non-enclosed area. There was a high niche overlap among the species with high niche breadth; however, the maximum niche overlap emerged among species with low niche breadth, which implied that there was no direct linear correlation between the niche breadth and niche overlap. The research showed that there was a significant vegetation recovery in the areas with 5 and 10 years of enclosure, but value of quantitative characteristics, i.e., the number of population in the area with longer enclosure time of 20 years decreased, which might closely relate to the soil crust. Crust test showed that its coverage was negatively correlated with vegetation coverage and soil infiltration depth. In the area enclosed for 20 years, the coverage and thickness of crust were much higher than those in other enclosed areas. The biological crust prevented water to penetrate deeper, and hindered the effective precipitation infiltration, which led to the recession of deep-rooted shrubs and the spread of shallow-rooted subshrubs and annual herbs. Biological crust competed for water, fertilizer and living space with surrounding plants, and dense biological crust was not conducive to the emergence of seedling and thus inhibited plant growth. Long-term succession and ecological process of competitive exclusion among population resulted in a certain degree of niche differentiation and fluctuation of inter-specific relationships and community structure. It is suggested that appropriate human disturbance should be used in this region to promote plant growth.