Regenerating secondary forests following anthropogenic disturbance compose one quarter to one half of the forest cover in much of the Neotropics and are becoming ever more important reservoirs of plant and animal diversity as further destruction of old-growth forests occurs. However, there is a major debate about how quickly, if ever, secondary tropical forests converge on old-growth forests over time in terms of species or functional trait composition. We examine secondary forests ranging between 20 and 100 years since land abandonment as well as old-growth stands of central Panama to test whether species composition of seedlings, saplings, and trees ≥ 5 cm diameter in secondary forests become more similar to old-growth forests over time. In addition, we test whether community-weighted mean values for wood density, one important functional trait related to survival in the shade, increase over stand age as would be predicted if secondary forests become functionally more similar to old-growth forests with time.
Results/Conclusions
Species composition of trees in older secondary forests was more similar to old-growth composition than younger forest, but 100-year-old forest stands were still fairly dissimilar to old-growth. In contrast, seedling and sapling composition of secondary forests was always fairly dissimilar to old-growth seedling and sapling composition and did not become more similar to old-growth forests over forest age. In addition, seedlings and saplings were poor indicators of the tree community composition within their own stand across forest age. Community-weighted mean values for wood density increased with forest age for trees, but not for seedlings or saplings. All of these results are consistent with predictions that shade-tolerant species become more abundant in the canopy of secondary forests over time and that seedlings, by 20 years, are already composed of a shade-tolerant subset of the tree species pool. Overall, both species and functional composition convergence in the tree layer appears to be occurring in these secondary forests, but even by 100 years species composition of trees has not converged on old-growth. High diversity and stochasticity in species assemblages may preclude convergence in tree species composition.