Vegetation variations under different exclusion measures and their correlation to soil factors
半干旱沙地封育草场的植被变化及其与土壤因子间的关系

Exclusion has been applied as a main measure for revegetation all over the world.This paper,by comparing the results of year-round,seasonal and non-exclusion,quantified the vegetation variations under three different exclusion measures and their correlation to soil factors.The analysis results for community species component and plant diversity using MRPP showed that exclusion did change the species component and increase plant diversity remarkably,while the period of exclusion had no significant influence on these two community features.The indicator species analysis and calculation of similarity indices indicated that community for year-round exclusion is becoming xerophytization and unpalatability,and showed highly spatial heterogeneity of plant species distribution,whereas community for seasonal exclusion is under stable non-equilibrium condition.DCA and DCCA results of relationship between plant species and soil variables demonstrated that soil moisture is a controlling factor for plant species component,microbiotic soil crust cover,soil organic matter,and soil bulk density have significant effects on soil moisture,among which microbiotic soil crust is a leading fact due to its limitation to rainfall infiltration on one hand,and its constraints to entrance of herbaceous seeds into soil or to germination of soil seeds.As result of long-term removal of animal grazing,crust keeps intact in year-round exclusion community,which is one main reason of community xerophytization.It is also obvious from ordination results that some importantly environmental factors such as tempo-spatial change of rainfall and corresponding tempo-spatial change of soil moisture are neglected during direct gradient analysis. In addition,biodiversity has close relation to soil nutrients besides to soil moisture condition(soil water content and crust cover),biodiversity has positive relation to available N,and negative relation to available P.Higher soil N has advantage to non-leguminous plants growth on nutrition-poor sand land definitely.The impact of P to community component is unclear and should be studied from plant physiology.