Edge Effects On Diversity And Dynamics Of Woody Plant Seedlings On Land-bridge Islands In The Thousand Island Lake

Land-bridge islands formed by dam construction are considered to be "experimental" systems for studying the effects of habitat fragmentation and edge effect, offering many distinct advantages over terrestrial fragments. The Thousand Island Lake (TIL) in Southeast China is one such land-bridge system with more than1000islands. In this study,29islands in the TIL were selected to investigate woody plant seedlings, microclimate and soil conditions. We divided the plots into5×5m quadrats, and categorized the quadrats into different edge gradients according to the distance (by10m) from the forest edge. At least6quadrats of each edge gradient on each island were set up. I assigned the plants into different functional groups, including light tolerant and intolerant, deciduous and evergreen, as well as tree and shrub. Then I analyszd the data using tests such as ANOVA, MANOVA, Spearman correlation analyses and Pearson correlation analyses, etc. The results showed that:(1)I recorded a total of5397seedlings belonging to55species,42genera and25families on the29study islands, with34light tolerant and21light intolerant species,,30deciduous and25evergreen species,24tree and31shrub species).(2) There was no difference in species diversity or structure of the seedlings between the different edge gradients. But we found that the seedling density was negatively related to the island area class.(3) The seedling mortality rate and recruitment of all seedlings and seedlings in different functional groups were not significantly affected by edge effects. Furthermore, the mortality and recruitment rates of evergreen, shade tolerant and shrub seedlings were higher during fall and winter than spring and summer.(4) The seedlings in different functional groups responded differently to the microenvironment. For example, the recruitment rate of evergreen, shade tolerant and shrub seedlings were positively related to the mature tree abundance, while the deciduous and shade intolerant seedlings were negatively related to mature tree abundance. In addition, the mortality rate of shade tolerant seedlings were negatively related to the light intensity, understory air temperature, soil temperature, while the shade intolerant seedlings showed no significant correlation with the environmental factors.(5) The envirionmental factors were significantly influenced by the edge effects. The forest edges were high in air temperatures and light intensity, but low in air humidity. An interesting finding was that the microclimate variables were not monotonically decreasing with the increase of the distance from the forest edge. For example, with the increase of the distance from the edge, the air temperature firstly decreased, reached its minimum at10-30m, then went up. The light intensity firstly increased, reached its maximum at30-40m, then decreased from forest edge to interior. Soil temperature firstly decreased, reached its minimum at10-30m, then increased. Soil moisture firstly increased,reached its maximun at20-40m, then decreased with the distance from the forest edge.Our results indicated that the woody seedling diversity and dynamics were not significantly influenced by the edge effects on the study islands at TIL. This phenomenon may be due to the forests on these islands are still under the early succession after only50-years dam construction, and the canopy of the dominant vegetation-Masson pine forests has not largely closed, thus there are not remarkable changes of canopy structure and microenvironments along the edge-to-interior gradients. Further, it is likely due to the mature trees on the islands are not significantly affected by edge effects as well.