Causes and consequences of ectomycorrhizal monodominance in the montane forests of western Panama

Most tropical trees form root associations with arbuscular mycorrhizal (AM) fungi. However, extensive patches of montane and premontane forest in Panama contain a small number of tree species in the oak and walnut families that instead associate with ectomycorrhizal (EM) fungi. Many of these patches are dominated by a single tree species: Oreomunnea mexicana at 1000 m elevation in the Fortuna Forest Reserve, and Quercus salicifolia and Q. costaricensis at different elevations on Volcan Baru. The ability of these EM tree species to achieve high relative abundance implies that they have escaped the population regulation mechanisms that are thought to maintain diversity in tropical forests. We tested three classical hypotheses for monodominance in Oreomunnea forest associated with (1) exceptional traits, (2) common mycorrhizal networks and (3) weakened negative plant-soil feedback. While none of these hypotheses received strong support, we found that both oak and Oreomunnea dominance is associated with greatly reduced inorganic nitrogen availability and the accumulation of a deep soil organic layer. We proposed that reduced nitrogen availability provides a key competitive advantage to EM trees as only EM fungi are able to access organic sources of N. The soil biogeochemical effects of EM fungi do not appear to be due to reduced litter decomposition rates, but instead likely reflect greater soil carbon inputs from EM trees and greater microbial nitrogen assimilation. Regardless of mechanism, the oak and walnut forests of Panama accumulate exceptionally high stores of soil and biomass carbon relative to other neotropical forests. The biogeochemical feedbacks that we describe also suggest that they may form a significant barrier to upslope migration of AM tree species under climate change.