Secondary forests of central Panama increase in similarity to old-growth forest over time in shade tolerance but not species composition

Abstract

Question: Does species and functional composition of secondary tropical forests more closely follow a deterministic or a stochastic model of succession? Location: The Barro Colorado Nature Monument (BCNM), central Panama. The BCNM comprises Barro Colorado Island (BCI) and five adjacent mainland peninsulas and is a mosaic of tropical old-growth (OG) and secondary forests (SF). Methods: We studied the species and functional composition of seedling, sapling and adult tree communities in a chronosequence of SF stands ranging in age from 20 to 100 yrs since abandonment, as well as two OG stands for comparison. Functional composition was defined as the representation of species with different degrees of shade tolerance. We also examined patterns of species diversity and dominance to better understand processes driving compositional changes. Results: Species diversity recovered rapidly, and, by 20 yrs, species richness for all size classes was equivalent if not higher in SF compared to OG. Species diversity tended to be lower and species dominance higher for seedlings than saplings or adults. Seedling, sapling and adult tree species composition did not clearly increase in similarity to OG with forest age. Instead, differences in species composition among stands reflected variation in dominance by particular species. Light-demanding species became less common and shade-tolerant species more common over stand age, with the functional composition of older secondary forests converging on that of OG. The seedling and sapling communities across the chronosequence were composed of a shade-tolerant subset of those species found in the stands as adults. Conclusion: Our results suggest that recruitment into the seedling community of SF is determined by both predictable (e.g. selection for shade-tolerant species) and unpredictable factors (e.g. species-specific reproduction events and dispersal limitation). Recruitment into the sapling and tree communities reflects stronger, and potentially compounded, selection for shade-tolerant species. Therefore, changes in the species composition of regenerating forests in this area appear to be unpredictable, even though the functional composition follows a more deterministic and predictable trajectory with convergence on OG over time.