The restoration of community assembly processes in degraded tropical dry forests

Secondary forests, which regrow following conversion to non-forest land use, form through secondary succession. Community assembly processes shape the trajectory of secondary succession, but our understanding of these mechanisms is mostly based on research in intact wet tropical forests. Tropical dry forests are relatively understudied but represent a widespread and vulnerable ecosystem type. We lack a clear understanding of the environmental gradients driving dry forest succession and their effects on species turnover. Furthermore, secondary forests grow within human-modified landscapes, but the impact of landscape context on successional trajectories is unclear. This thesis addresses these knowledge gaps using novel data from a successional tropical dry forest in Mexico, offering insights into best practice for forest restoration. In Chapter 2 I examined functional and taxonomic changes in tree community composition at multiple ontogenetic stages along a successional chronosequence. I found that taxonomic composition appeared stochastic, but functional composition followed deterministic patterns, with key deterministic processes acting during tree recruitment. In Chapter 3 I characterised how variation in the abiotic environment responded to different forest restoration interventions and tested whether increased abiotic heterogeneity promoted greater tree diversity. I found that mean abiotic conditions, rather than heterogeneity, were more important for recruit diversity at multiple scales. To complement the observational research in Chapters 2 and 3, Chapter 4 describes a large-scale outplanting experiment in which I manipulated the abiotic and biotic conditions experienced by planted seedlings. This experiment demonstrated that abiotic processes alone determined seedling growth and survival early in succession but interacted with biotic processes later in succession. Overall, the results from my thesis reveal key differences between dry and wet forest secondary succession. They suggest that widespread recovery of ecosystem functioning in tropical dry forests is possible and can be assisted through targeted interventions addressing the mechanisms involved in tree recruitment.