Studies on Species Diversity of Plant Community of the Converting Farmland in Beichuanhe Basin

The study was carried out in Beichuanhe basin of Datong county,Qinghai Province,where there is the demonstration site for Converting Cropland to Forest.Based on different communities in this area,the authors choose some representative areas,and set 24 standard localities according to the site factors,vegetation and land utility mode.And species richness,ecological dominance,species diversity and evenness of community are comprehensively calculated by richness index,Gleason index,Simpon index,Shannon-Wienner index and evenness index.The results are as follows.(1)According to different converting time,the community is becoming more and more steady as time goes on.The Shannon-Winenner index and richness index can be showed as asmall-big-smallprocess and Simpon and evenness index showed asbig-small-big.(2)According to different places of origin,the level of the species diversity can be arranged from top to bottom as following:nature forestconverted farmlandcultivated farmland.The diversity of converted farmland is close to that of the nature forest after some years.