Impacts of tropical forest modification on soil microbial communities, function and resilience

Tropical forests are hotspots of biodiversity and have global significance to the terrestrial carbon cycle. However, forest disturbance, clearance for plantation agriculture and future climate change are threatening their ability to sustain crucial ecosystem services. Ecosystem processes in tropical forests are underpinned by soil microbial activity, but knowledge of the principal drivers of microbial community dynamics and relationship to ecosystem functions is lacking. An improved understanding is therefore essential to better predict the response of tropical forests to a changing environment. The overarching aim of this thesis was to determine environmental drivers of soil microbial communities in tropical forest and explore the effects of forest modification (degradation and conversion) on soil microbial community composition, functioning and resilience to climate perturbations. Using a combination of field survey, in situ ecological manipulations and controlled laboratory experiments I found that fungal communities had stronger local covariance with standing tree communities and were thus more strongly influenced by tropical forest disturbance than bacterial communities. Moreover, the relative abundance of ectomycorrhizal fungi was strongly associated with the standing biomass of dipterocarp trees. In old growth forest, ants were found to modulate fungal growth rates and termites influenced the abundance and distribution of wood degrading taxa. Suppressed ant and termite activity also restructured microbial networks with properties associated with reduced resilience to climate extremes. As logging alters the functional diversity of ants and strongly reduces termite abundance in tropical forest, these findings indicate that invertebrate – microbial interactions may control the fungal contribution to soil C storage, rates of wood decomposition, and the resilience of soil microbial decomposers to future climate extremes across OG and SL tropical forests. Microbial communities in SL forest did have lower resilience to experimental drought – rewetting relative to those in OG forest and oil palm plantations whilst communities in old growth tropical forest had broader functional abilities, degrading both forest and oil palm litters faster than communities from logged forest and oil palm plantations. Taken together, these findings suggest that logging of tropical forests reduces the function and resilience of soil microbial communities, by modifying tree community composition and invertebrate activity. This may influence the size and stability of soil C stocks under future climate change