Classification of plant communities and species diversity responses to changes in the permafrost depths of the north slope of the Great Khingan Mountain valley of Northeast China

The Great Khingan Mountains,located in northeastern China,feature zonal permafrost. Lying on the southern edge of the Eurasian permafrost boundary,the permafrost region of the northeast portion of the Great Khingan Mountains is one of the plant communities most sensitive to the effects of global warming. Permafrost is gradually degrading from south to north,because of the effects of human activity and environmental changes such as global warming. The degradation of permafrost leads to changes in the characteristics of permafrost plant communities,such as species composition,species diversity,and biomass. The impacts of permafrost on vegetation have become a key research topic in the field of climatechange research. However,previous studies of permafrost have mostly focused on the distribution of permafrost,and so the mechanistic connection between permafrost and environmental factors, and the impacts of permafrost degradation on community composition and structure have not been the focus of much quantitative research. Thus,the plant communities of30 plots on the north slope of the Great Khingan Mountains were classified according to permafrost depth,using two-way indicator species analysis( TWINSPAN) and canonical correspondence analysis( CCA). Accordingly,the effects of permafrost depth on plant species diversity were analyzed using these plots. The results indicate the following.( 1) Plant communities were categorized into three plant associations,and as the permafrost depths changed along a gradient from shallow to deep,plant associations changed from Betula fruticose + Ledum palustre var. angustum-Carex subpediformis to Betula fruticosa-Carex subpediformis and Betula fruticosa + Salix rosmarinifolia-Carex subpediformis. These relationships were validated by CCA ordination. In addition,permafrost melting depth( PMD),topography,and soil moisture could together explain 38% of the community change. Their combined effect was the biggest explanatory factor for community change.( 2)The species diversity index increased trend initially( in shallow permafrost) and decreased as the permafrost melting depth increased. It reached its maximum value at about 50-150 cm deep. When PMD [< or =] 50 cm and PMD 150 cm,the species richness and diversity index are low,and the significant difference. Patrick index and melting permafrost depth has significant correlation( R~2= 0.58,P 0.58). When PMD [< or =] 50 cm,the species richness of the communities was higher,at 23.83+ o