Successional trajectories of regenerating tropical forests in central Panama

Background/Question/Methods

Across the tropics, secondary and degraded forests now cover a larger area than undisturbed old-growth forest. However, processes determining successional trajectories in regenerating secondary forests remain poorly understood. From chronosequence studies, we know that secondary forests rapidly accumulate diversity and complex structure, but much less is known about compositional changes over regeneration and how well chronosequence results can predict dynamic changes within forest stands. We examine tree species composition and functional traits in secondary forests, ranging from 20 to 100 years since land abandonment, and nearby old-growth forests. Specifically we investigate 1) if species composition becomes more similar to old-growth with forest age across the chronosequence, 2) how species composition has changed within each forest stand over the last 16 years and 3) how these trajectories relate to trends seen across the chronosequence. Additionally, we test whether the proportion of shade-tolerant species and the community-weighted mean wood density increase over stand age as secondary forests functionally converge on old-growth forests.

Results/Conclusions

Across the chronosequence, tree species composition became more similar to old-growth with increasing forest age. However, species composition increased in similarity to old-growth very slowly. For example, mean similarity between 100 yr-old forest and old-growth was only 0.41. These trends were reflected in the dynamic data from some secondary forest stands; in contrast, some stands exhibited idiosyncratic successional trajectories that may be related to land-use history, length of land-use and composition of the surrounding landscape. Across the chronosequence, shade-tolerant species became more abundant, and the community-weighted mean wood density increased with increasing forest age. Within secondary forest stands, shade-tolerant species increased in abundance over the 16 year census period and the biggest increases in shade-tolerant species were seen in the youngest forests. Overall, both species and functional composition of these secondary forests appear to be converging on old-growth composition. However, functional composition converged on old-growth more rapidly than species composition, which may be delayed due to high diversity and stochasticity in species assemblages.