A ten-year record reveals the importance of tree species’ habitat specialization in driving successional trajectories on Hainan Island, China

Abstract

Tropical secondary forests play an increasing role in biodiversity conservation and climate change mitigation, but the underlying mechanisms driving the successional trajectory to old-growth forests are still not fully characterized. Species with different dispersal abilities or habitat preferences affect the compositional similarity of secondary forests to old-growth forests. We studied the dynamics of forest succession in twelve 1-ha plots of tropical secondary forests on Hainan Island, China, using ten years of census data (2010–2020). We compared the variations in demographic rates and community composition among three successional stages, with four plots each in young secondary forest (YS), old secondary forest (OS), and old-growth forest (OG). The effects of five species groups (generalists, rare species, and specialists in each of the three successional stages) on successional trajectory were explored using sensitivity analysis. Species richness in the OS only reached 65% of OG after 60 years of succession. The diameter growth, mortality, and recruitment rates were the highest in the YS and the lowest at the OG. Growth decreased and recruitment increased during the second census period (2015–2020) during which there were two extended droughts, but there was no significant change in mortality rate. The successional trajectories of YS and OS showed a slow but directional trend towards the OG, with a weak stochastic component. The five species groups made different contributions to the successional trajectory. Generalists contributed to the convergence of secondary forests to old-growth forests, while old-growth specialists contributed to divergence. Secondary species usually contributed to divergence with old-growth forest, and young-secondary specialists had a stronger effect than old-secondary specialists. The divergence effect of rare species on similarity occurred only when presence data were used. When based on abundance data, rare species had little effect on community similarity. Overall, our results indicated that the successional trajectories of our tropical secondary forests moved towards OG but the recovery rate was slow. Both deterministic and stochastic processes contribute to secondary succession, but deterministic processes dominated. Furthermore, it is necessary to emphasize the different contributions of species with different habitat specialization to the successional trajectory, in order to better predict successional outcomes and prioritize conservation targets.