Spatial species turnover maintains high diversities in a tree assemblage of a fragmented tropical landscape

The fragmentation process has many negative effects on communities, particularly for plants. This process can generate two distinct scenarios: homogenization of species composition, due to assemblage nestedness, or flora differentiation, due to spatial species turnover. The aim of this study was to answer the following questions: (1) Is the tree canopy and understory community of a highly fragmented landscape (9% of forest cover) the result of species nestedness or turnover? (2) Is the pattern of additive partitioning of diversity similar between the understory and canopy tree communities? and (3) Are landscape characteristics responsible for diversity partitioning of the tree assemblage? The studied area has low remaining forest coverage (similar to 9%), caused by deforestation that started in the 18th century, and a very heterogeneous matrix around forest patches. Within this landscape context, we hypothesized that the tree assemblage (both canopy and understory strata) in the studied fragments would be homogeneous, as a consequence of a nested subset. The study was carried out in nine fragments of submontane semideciduous Atlantic Forest. All individuals with a height >1 m in 10 (200 m(2)) plots in each studied fragment were sampled, measured, and divided into two strata (canopy and understory individuals). The study found that the high beta diversity among plots and among fragments in both strata was due to species turnover (avoiding species homogenization) and that the landscape characteristics tested were not responsible for this result. These fragments present many rare and exclusive species and are not dominated by only a few species. In this scenario, it is necessary to conserve as many fragments as possible to protect most of tree assemblage because each fragment has a unique species composition.