| Nine Karst Tiankengs with forest covers were selected for the present study. Based on the vegetation investigation and the data from sampling survey, the minimum sampling area of mid-subtropical evergreen-deciduous broadleaf-mixed forest in Tiankeng was estimated by using methods of species-area curves and importance value-area curves. The floristic composition structural characteristics of Tiankeng forest communities were analyzed by the importance values of arbor. Species diversity was also analyzed by various kinds of diversity indices, such as the species richness index, Shannon-Wiener index (H), Simpson index (D), PIE and Pielou evenness index (J). Furthermore we detected the effects of habitat heterogeneity on plant community diversity by the principal component analyses. The results are as follows: (1)The minimum sampling area of evergreen-deciduous broadleaf-mixed forest in Tiankeng is 800m~2, which is less than typical evergreen-deciduous broadleaf-mixed forest in mid-subtropical Karst region. (2)According to the Chinese Vegetation Classification System and the importance values fundamental, Tiankneg forest vegetation in Dashiwei Tiankeng group is mainly classified 5 types: the evergreen-deciduous broadleaf-mixed forest, the lower subtropical evergreen broadleaf forest, the warm coniferous forest (including the coniferous and broadleaf-mixed forest), the deciduous broadleaf forest and the open forest, which includes 14 major formations . And the species diversity, evenness of 14 formations in Tiankeng forest varies from high to low as follows: Form. Neolitsea confertifolia, Manglietia aromatica; Form. Machilus glaucifolia, Acer tonkinense; Form. Lindera glauca, Handeliodendron bodinieri; Form. Neolitsea confertifolia, Carpinus polyneura; Form. Pseudotsuga brevifolia, Keteleeria fortunei, Platycarya longipes or Carpinus polyneura; Form. Lindera pulcherrima, Ilex macrocarpa; Form. Cinnamomum burmannii, Platycarya longipes; Form. Neolitsea confertifolia, Handeliodendron bodinieri; Form. Choerospondias axillaris, Handeliodendron bodinieri; Form. Lindera pulcherrima, Alangium chinense; Form. Machilus calcieola, Handeliodendron bodinieri; Form. Lindera pulcherrima, Platycarya longipes; Form. Cinnamomum burmannii, Rubus spp.; Form. Pseudotsuga brevifolia. And the stability of different vegetation types in Tiankeng forest varies from high to low as follows: the lower subtropical evergreen broadleaf forest; the evergreen-deciduous broadleaf-mixed forest; the warm coniferous forest (including the coniferous and broadleaf-mixed forest); the deciduous broadleaf forest; the open forest;. (3)From the top to down the vertical zonation of the vegetation in Tiankeng forest as follows: the warm coniferous forest(including the coniferous and broadleaf-mixed forest) (1400-1300m Alt.), the evergreen-deciduous broadleaf-mixed forest (1360-1120m Alt.), the lower subtropical evergreen broadleaf forest (1240-900m Alt.), or the deciduous broadleaf forest (1150-1045m Alt.), or the open forest (1155-1095m Alt.). (4)From high to low variation of diversity indices and evenness indices in relation to layer in Tiankeng forest community as follows; In the evergreen-deciduous broadleaf-mixed forest: tree layer, shrub layer, herb layer or tree layer, herb layer, shrub layer. In the lower subtropical evergreen broadleaf forest, diversity indices: tree layer, herb layer, shrub layer, but evenness indices: tree layer, shrub layer, herb layer. In the coniferous forest: herb layer, shrub layer, tree layer; but in the coniferous and broadleaf-mixed forest and open forest: shrub layer, herb layer, tree layer. In the deciduous broadleaf forest the values of diversity indices change as: tree layer, shrub layer, herb layer, and the values of community evenness change as follow: herb layer, shrub layer, tree layer. (5)According to comparison of the species diversity indexes in relation to different layer in the reserve, the varies from high to low as follows, In the tree layer: the evergreen-deciduous broadleaf-mixed forest; the lower subtropical evergreen broadleaf forest; the warm coniferous forest (including coniferous and broadleaf-mixed forest); the deciduous broadleaf forest; the open forest. In the shrub layer: the lower subtropical evergreen broadleaf forest; the evergreen-deciduous broadleaf-mixed forest; the open forest; the coniferous forest (including the coniferous and broadleaf-mixed forest); the deciduous broadleaf forest. In the herb layer: the lower subtropical evergreen broadleaf forest; the evergreen-deciduous broadleaf-mixed forest; coniferous forest (including the coniferous and broadleaf-mixed forest); the open forest; deciduous broadleaf forest. (6)Results indicate that diversity index shows a great difference in different forest communities caused by variation of habitat attributes. In Tiankeng forest, community diversity order correlates with topography attributes from high to low was: bottom, upside, middle, top; southern exposure, northern exposure. Diversity index shows a tendency that value of diversity index decreases with increasing elevation, though linear correlation between them is not clear. Appropriate disturbance can increase and keep community diversity. Effects of rock exposure on Tiankeng forest community diversity is not clear. (7)Results of principal component analyses show that topography characteristics, human disturbance and moisture are the important factors as the first component have effects on community diversity, and its cumulated contribution is up to 40.588%. The second principal component (cumulated contribution is up to 31.815%) may reflect elevation gradient correlated with community diversity.
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