Insect herbivores and fungal pathogens alter seedling performance, abundance, and diversity in mature forests. Their role in secondary succession, i.e., rainforest recovery, remains however understudied. Interactions between plants and their natural enemies could drive successional shifts in seedling community composition if they vary with plant functional type (PFT) and species’ local abundance or favor unrelated successors. We present two research projects on the impact of natural enemies on successional seedling community dynamics across two Panamanian chronosequences.

First, we conducted greenhouse experiments to assess how soil successional age (0-, 15-, 25-, 115-yrs) affects fungal-mediated plant-soil feedbacks (PSF) on germination and survival of seven tree species. Tree species varied in their association from early- to late-successional forests and their phylogenetic distance to each other. We found lower susceptibility of late-successional species, suggesting a potential decrease in importance of PSFs with increased forest age. Overall, species experienced more positive PSF in soil ages at which they peak in abundance, indicating that species-specific mutualists promote establishment of species at their associated successional stages, potentially slowing down successional turnover. Negative heterospecific PSF decreased with phylogenetic distance, favoring unrelated successors and possibly accelerating diversification.

Second, an ongoing field experiment explores the effect of insects and fungi on seedling communities in seven forests regrowing for 9, 49, 69, or 99 years. In each forest, three treatments (insecticide, fungicide, water-sprayed control) are applied to 26 1m2 plots each. All woody seedlings (5-130 cm) are identified to species level and grouped into PFT based on growth form (tree, liana) and demographic strategy. We monitor seedling survival and growth biannually for two years.

Integrating findings of our greenhouse and field studies will advance our understanding of successional and PFT-specific variation in the effects of natural enemies on seedling communities, which play a crucial role in shaping future forest composition and diversity.