The influence of active restoration of tropical rainforests on ecosystem carbon sequestration: potential links with ectomycorrhizal fungi

Active forest restoration in tropical forests of Southeast Asia may alter mycorrhizal community structures with consequences for carbon sequestration at the ecosystem scale. While tropical rainforests are generally dominated by arbuscular mycorrhizal fungi, restoration efforts in Southeast Asia often entail the planting of tree species from the family Dipterocarpaceae (short: dipterocarps), which form associations with ectomycorrhizal fungi. With increased cover of dipterocarps, we expect a concomitant increase in ectomycorrhizal fungi and in turn altered forest biogeochemistry. In particular, an increase in the occurrence of ectomycorrhizal fungi could boost ecosystem carbon sequestration in actively restored compared to naturally regenerating forests via the suppression of decomposition belowground and enhancement of aboveground biomass. We tested this hypothesis at a restoration site in Sabah, Malaysian Borneo, which was selectively logged in the 1980s – 1990s and partly restored with enrichment planting and accompanying silvicultural interventions. In contrast to expectation, our results show that a higher density of trees forming symbioses with ectomycorrhizal fungi is associated with lower soil carbon stocks and altered biodiversity of soil fungi. These results highlight the need to better understand how active restoration of tropical rainforests may alter the net potential of these ecosystems to sequester carbon, and that fungi, not trees alone, can control carbon storage outcomes.