Multiple drivers shaping tree regeneration in forest remnants within human-modified tropical landscapes

Forest regeneration is shaped by a complex array of ecological processes, many of which are increasingly affected by human activities. However, there is limited information on the relative importance of each process in shaping the resulting tree assemblages. Here, we used a dataset on adult and juvenile trees from twenty forest remnants in the Atlantic Forest, testing a model predicting that landscape-scale forest cover affects the local abundance and richness of juveniles of shade-tolerant species, both directly and indirectly. This influence is expected to occur through cascading effects of specific local changes including forest vertical structure, abundance and richness of adult tree assemblages (seed source), abundance of seed dispersers and abundance of shade-intolerant species. Using path analysis, we detected that local abundance and richness of juvenile shade-tolerant assemblages are positively affected by landscape forest cover. We also observed that only juvenile abundance is indirectly affected by forest cover through its effect on the abundance of the adult tree assemblage. This result demonstrates that distinct mechanisms are important for determining tree diversity metrics in tropical forests. In addition, we found no evidence that changes in the forest vertical structure, the abundance of shade-intolerant trees, or the abundance of seed dispersers explained abundance and richness of juvenile shade-tolerant species, indicating that other underlying mechanisms triggered by forest loss remain to be identified. Under the current scenario, the diversity of shade-tolerant assemblages in deforested landscapes is likely to increasingly diverge from that expected in conserved landscapes, due to disruptions in the regeneration process.