Mechanisms Of Community Change Impact On The Fine Wood Decomposition In Subtropical Forests

Background Woody debris is the key component maintaining forest biodiversity,and closely related to climate change via its decomposition dynamics.Previous studies have revealed three major drivers of decomposition: substrate quality,decomposers and environments.Yet,there has not been a unified understanding of how community change(e.g.tree diversity,canopy density and forest fragmentation)impact decomposer and wood decomposition,even though woody debris mainly originates in forests.Meanwhile,termites and fungi are widespread and important wood decomposer taxa in(sub)tropical forests.On the one hand,they show different preference for substrate quality and environments.On the other hand,they could closely interact with each other,e.g.competitive exclusion,with termites probably outcompeting fungi in competition and decomposing deadwood.However,there is scarce effort in predicting the effects of community change on wood decomposition via the biotic interactions and different habitat preference(for substrates and environments)among termites and fungi.AimThis thesis aims to test if community changes pose uneven filtering effects on termites and fungi by altering understory environments and substrate quality,which in turn leads to the changes of wood decomposition rate.Three specific hypotheses are as follows:1)Tree species diversity would promote the temporal and spatial niche differentiation between termites and fungi.This would alleviate the competition intensity for resources and thus lead to the decrease of wood decomposition rates.2)Canopy density would have positive effects on predatory ant diversity and humidity but have negative effects on air temperature.These environmental changes would favor fungi against termites,and thus lead to the decrease of wood decomposition rates.3)The relationship between habitat fragmentation and decomposition rate would depend on the affinity between decomposers and substrate quality/environments.Meanwhile,the effects of habitat fragmentation on decomposer and decomposition rate would be altered by the interactions(positive or negative)between termites and fungi.MethodsThis research was conducted in two study systems: plantation forests with tree diversity gradients in Xingangshan Town,Jiangxi Province,and land-bridge island archipelago in Thousand-Island Lake in Zhejiang Province.Fine woody debris from seven and three common tree species,respectively,was used as decomposition substrates.After 2-year(Xingangshan)and 1-year(Thousand-Island Lake)decomposition,we analyzed how fine wood decomposition rate responded to tree species diversity,canopy density,island size and isolation,based on the structural equation modelling.Results1)Termite feeding intensity depressed fungal growth and reproduction.After 1 year,tree species diversity decreased fungal fruitbody number and termite feeding intensity by increasing the community-weighted mean of leaf tannin content and decreasing air temperature,respectively.After 2 years,tree diversity had positive effects on fruitbody number,hyphal cover and termite colonization,with the latter two indexes negatively correlated with decomposition rates.Under species mixtures,the correlation between termite and fungal colonization shifted from negative at early decay stage to positive at advanced decay stage.No significant correlation was found under monocultures.Furthermore,termites preferred lower wood density and hot-dry microclimate,while fungi showed higher fitness with higher wood density and wet-cool microclimate.2)Tree species diversity and community-weighted mean of specific leaf area promoted the increase of canopy density.In tum,canopy density increased predatory ant diversity and humidity but decreased air temperature.Predatory ant diversity had negative effects on termite abundance but positive effects on fungi abundance;air humidity depressed termite abundance.Termite abundance and plot-level termite occurrence drove the wood mass-loss rates of three tree species.In contrast,fungi abundance and plot-level fungi abundance showed no positive but negative effects on the wood mass-loss rates of seven tree species.In the denser forests in Site A,canopy density showed negative effects on the wood mass-loss rates of five tree species;in the less dense forests in Site B,canopy density showed no impact on wood mass-loss rates of any tree species.3)Island size showed positive effects on termite feeding intensity but negative effects on fungal diversity.Island isolation had no impact on termites,but had negative effects on fungal diversity and positive effects on wood decomposition rate.Island size altered decomposers by changing tree species composition and understory environment: tree abundance of Loropetalum chinense,which is least preferred by termites,decreased with island size;woody debris diversity,increasing with island size,showed positive effects on termite abundance.Besides,soil moisture,which decreased with more L.chinense trees,had positive and negative effects on termite abundance and fungal diversity,respectively.Termite feeding showed negative effects on fungal diversity.Further analyses showed that for two tree species preferred by fungi(L.chinense and Quercus serrata),wood decomposition constant decreased with larger island size if it is not accessed by termites;for two tree species preferred by termites(Pinus massoninana and Q.serrata),wood decomposition constant increased with island size if it is accessed by termites.Conclusion1)Tree species diversity depresses the utilization of woody debris by decomposers,and thus leads to the decrease of wood decomposition during early decay stage.This in turn enables the more stable and long-lasting coexistence among decomposer taxa in woody debris.The contrasting response to substrate quality and environment between termites and fungi indicates the spatial niche partitioning,which suggests that tree diversity can increase the spatial heterogeneity of resources and environment and thus increase the diversity of decomposers.2)The increase of canopy density could alter understory biotic and abiotic environment,and shift the dominant decomposers from more efficient(i.e.termites)to less efficient taxa(i.e.fungi).This suggests that canopy closure retards wood decomposition in the young regenerating forests;3)The relationship between island size and decomposition rate depends on the affinity between decomposers and substrate species,and on the interactions(e.g.competitive exclusion)between decomposers.Therefore,it is not sufficient to understand the effects of habitat fragmentation on wood decomposition based solely on island biogeography theory and the positive BEF hypothesis.In conclusion,this thesis tests if two major decomposer taxa had different preference on substrate quality and understory environment,and detected the complex interactions between decomposers(facilitation and mostly competitive exclusion).Knowing these ecological processes is critical to predict the impact of community change in subtropical forests on wood decomposer communities and decomposition rates.Our research provides the theoretical supports for the scientific management on biodiversity conservation and carbon emission control.