Seed survival in tropical pioneer trees is unrelated to conspecific distance but linked to fungal infection

Plant demography is heavily influenced by individual survival at local scales, with host-specific natural enemies potentially reducing the survival of seeds and seedlings in high density patches near conspecific adults, favoring heterospecific recruitment, and ultimately increasing local plant diversity. Natural enemy pressure should differ 1) between sites directly beneath and away from their hosts' crowns, 2) between sites where their hosts are abundant versus rare, and 3) among potentially competing plant species. Assessing the potential pathways by which natural enemies affect diversity at local scales requires coupling experimental approaches that track plant performance with characterization of their natural enemy communities. Here, we focused on the fate of individuals at the seed stage, a critical demographic bottleneck in the plant life cycle. We tested how seed germination is affected by fungal infection, distance from the adult tree, burial duration, and host tree identity. We conducted a seed burial experiment on Barro Colorado Island, Panama, using four pioneer tree species (Jacaranda copaia, Cecropia insignis, C. peltata and C. longipes). Seeds were buried beneath and 30 m away from adult C. insignis and J. copaia trees and retrieved after 3 and 12 months. We measured germination, cultured seed-infecting fungi, and evaluated fungal communities using a culture-independent approach. Germination rates of all tree species declined with burial time. Germination of seeds buried below versus away from focal trees also did not differ. Fungal isolation frequency and richness increased after burial but did not change with distance or identity of the adult tree. Fungal communities in seeds differed markedly among tree species, but distance, burial duration, and seed viability status had little effect on community composition. Consequently, we found limited evidence that distance from conspecifics drives seed mortality in the soil. Nonetheless, seed exposure to the soil increases fungal infections and decreases seed survival, highlighting the role of fungi in seed demography. We recommend that future research explore the mechanisms by which host-generalist soilborne fungi may drive seed mortality in plant communities.