Divergent tree seedling communities indicate different trajectories of change among rain forest remnants

Aim To examine plant community composition within rain forest remnants, and whether communities in different fragments follow similar trajectories of change in composition. We investigate whether plant communities in rain forest fragments either diverge from, or become more similar to, plant communities in other fragments, in order to understand the biodiversity value of forest fragments. Location Rain forest fragments embedded within agricultural landscapes in Sabah, Malaysian Borneo. Methods We examined 14 forest fragments (39–120,000 ha) and five sites in continuous forest, and compared pre‐isolation (trees >5 cm dbh) and post‐isolation (seedlings <1 cm dbh) plant community composition. We used Chao‐Sørensen dissimilarity metric to compute beta diversity between all pairwise combinations of sites, and then used Non‐Metric Multidimensional Scaling to reduce 18 pairwise values per site to a single site value, which we used to test whether fragment area and/or isolation are associated with changes in plant communities. We compare analyses for trees and seedlings, and whether community changes arise from recruitment failure. Results Seedlings in fragments have diverged most from other communities, and divergence was greatest between seedling communities in small fragments, which have not only diverged more from tree communities in the same fragment, but also from seedling communities at other sites. This finding is partly associated with recruitment failure: the number of genera represented by both trees and seedlings is positively associated with site area. Main conclusions Seedling communities are diverging in forest remnants, associated primarily with reductions in fragment area, whilst tree communities have not diverged, possibly due to extinction debts. Divergence is likely to continue as seedling cohorts mature, resulting in communities in fragments following different trajectories of change. Individual plant communities in each fragment may become impoverished, but they can support different communities of plants and hence contribute to landscape‐scale diversity.