Species Coexistence On The Community Of Fagus Engleriana And Cyclobalanopsis Oxyodon In Shengnongjia Area,Hubei, China

Understanding the mechanisms that allow species to coexist is a central enquiry in community ecology. Many theories have been put forth to explain species coexistence and the more influential ones include the following: 1) Competitive exclusion principle, 2) Nutrition competition theory, 3) Intermediate disturbance hypothesis, 4) Compensatory death (Density-dependent hypothesis). The question of species coexistence involves a large number of factors, such as environmental heterogeneity, natural disturbance, and human activities. In this research we detected the spatial pattern, topographic heterogeneity, soil nutrient heterogeneity and the influence of predator (pathogen) on species coexistence on the community of Fagus engleriana and Cyclobalanopsis oxyodon to research the mechanism of species coexistence of from the aspect of resource heterogeneity. The research plot is located in Shennongjia area of Hubei province (31°19′4″ N,110°29′44″ E), which is in the north part of Fagus distribution region. This area crossed mid-subtropical and north-subtropical zone of China, and also is a transition area of landform. The unique geographic position caused its particular plant species diversity and antiquity. The community totally maintained 46 woody plant species, belonging to 22 families and 27 genera. The dominant families were Fagaceae, Aceraceae, Ericaceae, Cornaceae and Rosaceae. Tree layer was divided into three sub-layers. In the first layer, deciduous trees were 80.7%, and evergreen species was 19.3%, dominant species were Fagus engleriana and Cyclobalanopsis oxyodon. In the second and third layer, evergreen species increased. Cyclobalanopsis oxyodon and Rhododendron hypoglaucum were dominant ones. Distribution pattern were analyzed of the two dominated species respectively. Fagus engleriana and Cyclobalanopsis oxyodon population were all clumped. It may relate to their biological and recruitment characters. The clumped pattern of dominated species may provide the coexistence of other species. Topography is a primary factor that creates microenvironment heterogeneity in a landscape. In the previous studies, topographic factors, such as slope and aspect, were measured as discrete data and had limited use in analysis. In this paper, a digital elevation model (DEM) was used to analysis the influence of topographic heterogeneity on the distribution patterns of wood plants in community level by calculating digital topographic values and creating a digital description of the landform. The slope varied from 18.2 to 74.6 degrees in the 9 600 m2. Twenty four species were chosen to calculate correlation coefficient between slope, aspect and position. The topographic adaptation of these species was different, for example Fagus engleriana was positively associated with slope, and Sorbus alnifolia was positively associated with aspect and so on. According to their response to topography, these species were classified into four groups using hierarchical cluster analysis. Soil nutrition is a key factor influencing on species combination in a community, but it was clearly scale-dependent heterogeneity even in plot level. In this study, the semi-variograms were calculated with N, P, K, Ca and Mg for example. The result showed that there existed space autocorrelation in the entire five chosen elements. In this case, the scales of autocorrelation of those elements ranged from 15 m to 200 m. CCA method was used to investigate the relationship between plant species in this community and soil properties. It seems that five of nine species are correlated with Total Nitrogen, K, Ca, Mg, and pH. Two are correlated with P and Organic C. The Janzen-Connell hypothesis provides a mechanistic explanation to account for the high species diversity in tropical rainforests. In this study, we tested the hypothesis in a subtropical, mixed evergreen-deciduous broadleaved forest. The two dominant species were not regulated by the Janzen-Connell effect. To test the relationship between seed densities and species distributions, 18 seed traps were set in half of the plots. The number of seeds was not significantly related to the number of seedlings, saplings or adults of the two dominant species at various distances from the seed traps. Two of nine species were regulated by the Janzen-Connell effect. Two other species showed the density depended pattern in adult or sapling stage. Overall, our results indicated that the Janzen-Connell density-and distance-dependent effect was not an important mechanism influencing the species composition of this forest community. However, around an individual adult tree, non-conspecific trees were far more abundant than conspecific trees. Although the mechanism for this pattern is not known, it provided space for the survival of other species and thus promoted species coexistence. All the four parts of spatial pattern, topographic heterogeneity, soil nutrient heterogeneity and predator had their contribution to the species coexistence, but none of them could give a satisfaction explanation for the biodiversity of the community. For the subtropical broad leaves evergreen and deciduous community, the high diversity may determinate by the species pool of this area.