Community assembly in naturally regenerating wet tropical forests

Background/Question/Methods

Naturally regenerating tropical forests provide a promising avenue for recuperating biodiversity and ecosystem services in human-modified landscapes. They also provide a natural experimental setting to observe community assembly in action. To assess the role of deterministic and neutral processes during community assembly, we compared community-weighted mean (CWM) functional traits of trees ≥5 cm DBH in six secondary forests (10 to 47 years old) and two old-growth forests, in and around La Selva Biological Station, Costa Rica. In each plot, species composition was monitored annually for 7-15 years, and six leaf functional traits were measured for species accounting for >90% of the abundance and basal area of trees. If deterministic processes drive community assembly patterns during succession, we predicted a shift in abundance from species early in succession with traits that enable fast returns on tissue investment, to species later in succession with traits that lead to slow economic returns. We further compared alpha and beta functional diversity among plots. Deterministic processes would result in decreased alpha functional diversity along the successional gradient, and higher functional turnover (beta diversity) than expected based on species turnover.

Results/Conclusions

Our results suggest that environmental filtering influences community assembly during succession, but it is not the only factor. Community weighted mean traits were variable among plots, although many trends were consistent with our predictions. Along the successional gradient we found an increase in CWM leaf dry matter content (LDMC), leaf toughness (LTO), leaf density (LD), and leaf size (LS). There was little successional pattern in CWM specific leaf area (SLA), or leaf thickness (LT). Functional diversity decreased across the successional gradient, as predicted, suggesting that there is convergence in traits over successional time. Additionally, CWM traits of smaller trees in secondary forest plots were more similar to old-growth forests than the larger trees. Thus newer tree recruits in secondary forests have traits associated with the slow end of the leaf economics spectrum, showing functional convergence with old-growth forests in the region. We found differences in functional beta diversity among traits and pairwise plot combinations, but there was not a strong successional trend. Although we did see functional turnover along a successional gradient, most of the variation was caused by differences in species composition. We conclude that deterministic processes and random chance influence community assembly patterns during natural regeneration.