Secondary Succession in Tropical Dry Forests: Drivers and Mechanisms of Forest Regeneration

Secondary succession is a complex process involving numerous factors acting across scales. Understanding secondary succession in tropical dry forests is important for the conservation and restoration of this highly threatened biome. My research aims to improve knowledge of the trajectories and drivers of secondary succession in this biome, and the underlying mechanisms. I used a combination of literature synthesis, observational and experimental approaches to study plant-plant interactions and community changes during succession. Through review of published studies, I showed that established trees have a mainly positive effect on the seed dispersal, survival and germination of the subsequent generation of woody plants. However, the balance between positive and negative effects is more complex at the seedling establishment stage and can be influenced by the precipitation regime. Metaanalyses of chronosequence studies showed an increase in tree and shrub species richness with succession and a slow convergence of successional forest species composition with that of old-growth forests. Using survey of young woody plants establishing under isolated trees in pastures, I showed that the attributes of the trees influence the functional composition of the regeneration assemblages but are only weakly related to their taxonomic composition. The position of isolated trees in the landscape is also influential, but this is complex and sitespecific. Through extensive sampling of leaf functional traits of sapling communities in secondary forests of different successional age, I found that community functional composition shifts from conservative towards acquisitive strategies of resource economics, through both species turnover and intraspecific variation of trait values. Five of the measured traits also showed directional changes with tree ontogeny. Lastly, an experimental test of seed fate showed that leaf litter reduced seed removal in successional forests. Seed germination rate was higher in successional forests compared with open sites and generally benefited from the presence of litter. Overall, this research suggests a higher predictability of successional trajectories when studied through functional rather than taxonomic omposition. It also shows heterogeneity in successional trajectories among tropical dry forests that require further study.