Keywords: 
• forest ecology /
• species composition /
• species diversity /
• plant community /
• conversion of farmland back to forests /
• Datong County of Qinghai Province

Abstract: Research on different plant communities of different plot types（conversion of farmland back to forests，natural secondary forest and farmland） was conducted in a project area that returned farmland to forests in Datong County，Qinghai Province from March to August in 2008. After analyzing species composition of the main plant communities（including 15 types，such as Picea crassifolia forests，Picea crassifolia-Betula platyphylla forests，Picea crassifolia-Hippohae rhamnoides ssp. sinensis forest，Larix principis-rupprechtii forests，Populus davidiana forests，and Sabina przewalskii forests），species richness index（R0），Gleason species richness index（I），Simpson ecological dominance index（D），Shannon-Weiner species diversity index（H），and Hill evenness index（E5） of the community were calculated. Results showed that （1） the level of the species diversity in the community was herb ＞ shrub ＞ tree. Also，（2） the level of the species diversity in mixed forests was higher than pure forests. （3） According to land use，species diversity was：natural secondary forest ＞ artificial forest on converted farmland ＞ cropland. However，after 27 years diversity of the converted farmland was close to that of the natural secondary forest. （4） As conversion time increased，the community became more and more steady-state. The H and I were small-big-small and the D and E5 were big-small-big. To promote stability and rejuvenation of plant communities when returning farmland to forests，logical distribution of species should be realized.［Ch，2 fig. 4 tab. 15 ref.］