COS 144-2 - Investigating drivers of slow compositional recovery of tropical secondary forests

Abstract: Following disturbance, the recovery of tropical forests’ plant diversity, structure and functioning occurs rapidly. Plant compositional recovery, however, is remarkably slow. In this study, we examined the role of seed dispersal limitation and plant establishment limitation and biotic interactions in delaying compositional recovery, through the analysis of long-term plant abundance and field observations in old-growth and secondary forests in Sarapiquí, Costa Rica. Plots were monitored yearly for abundance of plants > 10 cm DBH, and a subset of those plots were used for monitoring seedlings and saplings. We conducted a multinomial species classification analysis to assign species to the following categories: old-growth specialist (OGS), generalist (GEN), second-growth specialist (SGS), and too rare to be classified. Then, we determined which species can be confidently categorized as absent from secondary forest plots, namely old-growth exclusive species (OGE). We found that 5,6% of all species that were present in old-growth plots were not present in second-growth as trees. However, more than half of these were found in second-growth as seedlings or saplings, meaning that only 2,1% of the species were totally absent. This suggests that compositional recovery is not limited by dispersal or establishment and may instead be related to factors that limit the transition from seedling and sapling to tree. We compared species interactions that may affect transition from seedling and sapling into tree, using four OGS and four GEN species. We measured herbivory and leaf pathogen infection in 6-10 individuals from each species in two old-growth and two second-growth plots. We expected that herbivory of specialist species would be higher in second-growth, and that it would be higher than generalist herbivory. However, there was no significant difference between OGS and GEN species in old-growth and second-growth plots. We only found differences in herbivory rates between species, but these differences were not consistent within specialization categories. Regarding leaf pathogen infection, OGS species had significantly more infection than GEN species in second-growth forests, while OGS and GEN were not significantly different in old-growth plots. Altogether, we found no evidence of differences in herbivory between OGS and GEN species in second-growth forests, but evidence that OGS species’ seedlings and saplings may have higher leaf pathogen infection in second-growth forests. The role of leaf pathogens in potentially limiting the transition between seedlings and saplings into trees in secondary forests should be further investigated to develop methods of assisting compositional recovery in tropical secondary forests.