An case study on vegetation stability in sandy desertification land:determination and comparison of the resilience among communities after a short period of extremely aridity disturbanc

Ecological stability,changes of eco-system under natural and human-induced disturbances such as fire,grazing,cultivation etc.,involved in how and to what extent human can utilize natural resources but without inducing eco-system degradation or collapse,is a problem worthy of study.From the reaction of an ecosystem to disturbances,it has been concluded that ecological stability could be defined as resistance and resilience.Because of many complicated factors such as the type,the intensity,the temporal scale and spatial scale of disturbances,it has been difficult to make qualified measurements for a real community.Therefore,researches related to resistance and resilience has been stopped at concepts,and studies on qualified resistance and resilience about a real community were scarce.In order to make a tentative example for study on this topic,6 communities in different desertification stage were sampled in a sandy desertified region,and community investigation was done one time for every half month from the beginning to the end of growth season.With the investigation data obtained at the same time in continual two years before and during a special period when climate was extremely arid,a resistance index,which indicates the resistance stability of the 6 communities,were defined and calculated as the ratio of vegetation data at the same time of two years in one study we have done in an earlier time.In this study,with investigation data of the same 6 sandy desertification communities after undergoing the same arid disturbance,a resilience index which indicates the resilience stability of communities was defined and calculated.It could be expressed as: resilience index = the vegetation data at the end of the year during which the aridity event took place / the vegetation data in the early year that was just before the aridity event took place.Results showed that,the resilience indice of the 6 desertified communities calculated with abundance,highness,coverage,and biomass were between 4.69—1.23,1.21—0.98,1.59—1.15,and 1.28—1.00,respectively,decreasing with enhanced desertification and showing that the 6 communities have the strong ability to keep stable when the precipitation recover to the normal standard after the short arid disturbance by high resilience.Furthermore analysis about source and constituents of the resilience showed that,the resilience was mainly from annual plants,as well as perennials.The resilience index calculated from abundance data was the most,indicating that there were abundant seed sources and a big soil seed bank to sustain the vegetation recovery in every community.It was also found that the dominant species in some communities were replaced quickly in the short time of two continual years,which could be explained as that it was one of the forms in which biodiversity played its roles.Furthermore,it could be concluded that biodiversity plays important roles for maintaining ecosystem function and its stability,and biodiversity is the fundamental characteristics of ecosystems.All results in this study and in the study which had been done in earlier time about resistance of these communities under the same arid disturbance showed that,both of the weak resistance and the strong resilience of the studied sandy communities were the reflection of the same fact that vegetation stability depended strongly on rainfall which could not be replaced.So,by the quantitative definition of vegetation stability with resilience and resistance,it could be concluded that,for a shorter temporal scale,vegetation is certainly unstable with the rainfall fluctuation,while for a longer temporal scale,vegetation has the ability and potential to maintain the stability of itself by the well resilience.