Introduction: Forest restoration is a priority strategy for limiting biodiversity loss and mitigating climate change. Increasing forest cover through passive natural regeneration has the potential to sequester significant amounts of atmospheric carbon. Frugivorous birds are good allies to promote forest recovery because they can bring seeds from forests to open areas while moving between forest fragments. However, the contribution of birds to forest regeneration will vary depending on the tree species they interact with and the landscape configuration.
Objective: We evaluate how potential forest recovery and composition vary according to birds' dietary and movement preferences along a fragmentation gradient.
Methods: To simulate forest restoration potential in open areas within fragmented landscapes, we combined datasets that describe the seed dispersal process: frugivory interactions, bird movements, and gut passage time. Using data from the Southeast Atlantic Forest of Brazil, we fitted three hierarchical models with 1) 346 frugivore interactions between 30 birds and 41 plants, 2) bird movement tracks recorded across six fragmented landscapes, and 3) gut passage time of 30 species. We used traits from plants and birds as exploratory variables and bird phylogeny to account for phylogenetic relationships. Using species coefficients from the models, we simulated feeding, movement, and seed deposition events for 1400 birds in ten landscapes (1800 m2) that follow a fragmentation gradient. For each pixel, we evaluated the potential forest composition and carbon stock resulting from avian frugivore mediated seed deposition.
Results: We found that the contribution of frugivorous birds to forest recovery varied along the fragmentation gradient and was related to bird species' functional traits. Potential forest restoration decreased drastically in landscapes with less than 30% of forest cover and low connectivity (>100 m between fragments). Moreover, small birds with low fruit preference were more important for restoration in highly fragmented landscapes, but regenerating forests had low carbon storage. While, large birds, with a strong dependence on fruits, contributed to restoring forests with high carbon stock potential but only in landscapes with low levels of fragmentation.
Conclusions: Our results suggest that both the frugivore community and landscape configuration interact to determine natural regeneration of tropical forests. From our simulations, forests with high carbon storage potential can regenerate naturally in landscapes with > 30% forest cover. However, in highly degraded landscapes (< 30% forest cover) and those with impoverished frugivore communities, restoration interventions may be needed to regrow carbon-dense forests.