Seedling Dynamics Of Secondary Masson Pine Forests On The Islands Of Thousand Island Lake:Effects Of Functional Traits And Island Spatial Attributes

Secondary Masson pine(Pinus massoniana) forests, mostly in small patches, are widely distributed in the subtropical region of China, and they represent an important regional forest type. Masson pine forests are mostly the product of early stage secondary succession, of which the end stage might be the typical subtropical evergreen broad-leaved forest of this region. However, due to the potential effects of fragmentation on forest succession, it is necessary to investigate the dynamics of fragmented secondary Masson pine forests, which would be beneficial to forest management, biodiversity conservation and the maintenance of forest ecosystem functioning of this region.Xinanjiang reservoir (i.e. Thousand Island Lake) harbors more than one thousand land-bridge islands, most of these islands are covered with secondary Masson pine forests formed since the late 1950s, which is suitable for investigating the effects of habitat fragmentation and succession on dynamics of secondary Masson pine forest. Here, to explore the regeneration dynamics of the fragmented secondary Masson pine forests, we sampled 29 islands in the Thousand Island Lake, and we systematically set up seedling quadrats in the forests on these islands. From 2011 to 2014, we conducted a seedling survey every half a year in spring (April) and fall (October), we identified every woody seedling (diameter at breast height<1 cm) inside the seedling quadrats. We classified species into shade tolerant or intolerant, evergreen broad-leaved or deciduous broad-leaved or needle leaved, tree or shrub, papery leaved or leathery leaved or thick leathery leaved, according to shade tolerance, leaf habit, growth form and leaf texture, respectively. We performed Chi-squared test, linear regression and correlation analyses, etc., to investigate the association between traits, the seedling dynamics of functional groups, dynamics of seedling functional diversities (α and β diversities) and the island spatial attributes effects on the dynamics of seedling functional diversities, etc.With respect to the association between traits, we found shade tolerance was highly associated with leaf habit and leaf texture (P<0.001 in both cases), but not with growth form (P>0.05), which implies that shade tolerance might be partly determined by leaf traits. Growth form showed no associations with leaf habit and leaf texture (P>0.05), imply that growth form to be independent from leaf traits. With respect to temporal seedling dynamics of functional groups, we found the abundance of evergreen broad-leaved seedling was declining, deciduous broad-leaved seedling was stable, while needle leaved seedling was increasing. We found thick leathery leaved seedling abundance was increasing, leathery leaved seedling was decreasing, and papery leaved seedling was stable. We also found tree seedling abundance was increasing, and shrub seedling was declining. With respect to the dynamics of seedling functional diversities and island spatial attributes effects on them, we found the seedling a diversity (i.e. island) was stable, and β diversity (i.e. between island) was increasing. Island area and shape index showed no significant effects on a seedling functional diversities of seedlings, but they showed significant effects on β seedling functional diversities; isolation showed no significant effects on the both a and β seedling functional diversities.In this study, we found that leaf traits might to some extent represent the shade tolerance property of plants, and leaf traits could be independent from growth form. Meanwhile, the dynamics of seedling abundance differed between functional groups, which might change the relative importance of different functional groups in the long term. However, the overall seedling functional diversity at the island level was stable, although functional composition between islands were becoming less similar, which might be partly due to island area and shape index. In all, we found that the seedling regeneration dynamics were related to functional traits, and divergence in seedling functional composition between islands, which might influence functional community composition at the landscape scale and the divergence in successional directions among islands in the long term, respectively.