| The northeast region of Yanshan Mountain is the transition from Yanshan mountainous region to the plateau region and the ecotone between the plateau froest-grassland and Yanshan mountain forest. Based on the investigation data, the species diversity of five kinds of plant community were studied in the paper, such as the Spiraea shrub, the Spiraea-Hazel shrub, the Hazel shrub, the natural Aspen-Brich forest and the artificial larch forest. The conclusions are follows:(1) For the herbaceous and shrub's species diversity, the species diversity of the Hazel shrub, the natural Aspen-Brich forest and the artificial larch forest were obviously higher than the Spiraea shrub and the Spiraea-Hazel shrub. The primary factors which can affect the species diversity of five kinds of plant community were the slope aspect and disturbances.(2) As to the composition of the flora, the flora composition of the Hazel shrub was the most abundant, which contained 127 species belonging to 36 families and 91 genera. The natural Aspen-Brich forest contained 107 species, which belonged to 42 families and 85 genera. The species richness of the artificial larch forest were lower than the natural Aspen-Brich forest, it contained 106 species belonging to 40 families and 80 genera. The Spiraea-Hazel shrub contained 83 species, which belong to 27 families and 66 genera. The flora composition of the Spiraea shrub was the lowest in five kinds of plant community, which contained 79 species belonging to 24 families and 66 genera.(3) The five kinds of community could be divided into two groups according to community similarity. diversity. The Spiraea shrub and the spiraea-hazel shrub belonging to the first group, had highest species similarity;There was higher species similarity, among the Hazel shrub, the natural Aspen-Brich forest and the artificial larch forest, and they belonged to another group.(4) The spatial patterns of species diversity in different plant communities were obviously complex. In the downslope-midslope-upslope gradient, the evenness index and diversity index of The Spiraea shrub increased gradually, the richness did not obviously change; three kinds of diversity index of the Spiraea-Hazel shrub displayed the obvious decreasing tendency; in the hazel shrub, the richness index and diversity index dropt in first, and then began to rise; in the natural Aspen-Brich forest, the richness index showed the dropping tendency, the evenness index and diversity index began to rise slightly after dropping in downslope; three kinds of multiple index of the artificial larch forest did not display the obvious changing tendency.(5) The proportion size of the forest gap in three kinds of community was the Hazel shrub>the artificial larch forest>the natural Aspen-Brich forest. Generally, comparing with the non-forest gaps, the herb diversity of forest gaps in different communities was higher, but the difference between them was various. For the natural Aspen-Brich forest, the species diversity of forest gaps was higher than that of the non-forest gaps remarkly(P<0.01).(6) The sample area influenced the community species richness, species diversity index and evenness markedly. Both species richness andα-diversity indices increased as the sample area increased, and the evenness decreased with the increasement of sample area. The sample area of 1m2 which is used generally led to a lower value of species richness andα-diversity indexes and a higher value of evenness. Comparatively, the measure of species richness needs a bigger sample area than the measurement ofα-diversity indexes. The sample area had a distinct influence on the degree of variation of species diversity, and enlarging the sample area properly would decrease the oversize variation between different parallel samples due to the small sample area.
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