| Deadwood is structural component in most forest ecosystem,which it vital to biodiversity,soil organic matter formation,nutrient cycle,water conservation and soil erosion reduction.The decomposition process is mainly controlled by substrate quality,forest environment and decomposer communities.Thus,the factors affecting substrate quality,forest environment and decomposer communities could also greatly affect the deadwood decomposition process.Forest is usually disturbed by canopy gap,which could influence the decomposition of plant residuals by re-regulating light and precipitation,consequently altering the ecosystem process.Whereas,mostly of current studies are focused on the effects of canopy gaps on litter decomposition,little attention has been received on the deadwood decomposition.Subalpine in western Sichuan is the second largest forest region in China,which is characterized by shallow soil,simple forest structure and frequent geological disasters.It is fragile and sensitivity to global climate change.The existence of deadwood on the forest floor could greatly increase the ecosystem stability in this region.Whereas,whether and how forest regeneration affects deadwood decomposition process? Whether there is a strong difference in the response of different structural components(i.e.bark,sapwood and heartwood)decomposition to forest locations(i.e.canopy gaps)? Whether there is a strong difference in the response of deadwood decomposition with different decay classes to canopy gaps and epixylic vegetation removal? None of the previous studies have answered those questions which are important to increase our knowledge about deadwood decomposition.In our study,Abies faxonania deadwood(30±5cm)with decay classes I-V were in situ incubated in the forest canopy gap,canopy edge and closed canopy.We also combined the in situ decomposition study with decomposition vector and epixylic vegetation removal method.We collected the samples of bark,sapwood and heartwood separately for microbial community composition and substrate quality analysis.Decomposition vector i.e.deadwood with decay class I-III decomposed for 5 years was applied in our study to achieve the best decay model of bark,sapwood and heartwood at a long decay time scale.However,the commonly used density loss could underestimate the real mass loss as it could not estimate the volumetric loss as decay advanced.Accurately predicating the decay rates requires the advances in methodology.In our study,we found that:(1)Bark,sapwood and heartwood all had strong volumetric loss during decomposition,but in different ways.Hereby,we assessed bark mass loss per stem surface area and wood mass loss based on volume-corrected density loss based on their characteristics of volume loss.This method could be widely used in diverse climate regions across various tree species,but still need empirical study.(2)The best-fit decay model showed that the decay mode and decay rate of deadwood varied with structural components and location within forest.Bark and sapwood decay followed Sigmoid decay curve under closed canopy,while heartwood decay followed single exponential decay curve in all forest locations.Bark always decays faster than sapwood and heartwood.Prediction of time to lose 25% of bark original mass under closed canopy,in canopy edge and gap are 3.73-year,29.73-year and 18.35-year,respectively.Whether sapwood decayed faster than heartwood depend on the location within the forest.We found a faster decay rate of sapwood to heartwood under closed canopy,while a faster decay rate of heartwood to sapwood in canopy edge and canopy gap.(3)Forest canopy gaps greatly altered the decay mode and decay rate of different structural components,which indicated the variation within stem along cross section should also be considered to inform model regression to get a better prediction.(4)The result that location within forest significantly altered the decay mode and decay rate of different structural components suggests that considering radial variation within stem would greatly increase the power of model prediction.Total microbial biomass,Grampositive bacteria biomass,fungal biomass and ratio of Gram-positive bacteria to Gramnegative bacteria in decay class I of deadwood,and total microbial biomass,Gram-negative bacteria and fungal biomass in decay class III of deadwood were greatly affected by heterogeneity within forest.The highest value of Gram-positive bacteria to Gram-negative bacteria(6.71)in decay class I of deadwood was observed in forest gap,while the lowest value was observed in canopy edge(2.86).The changes in the value of Gram-positive bacteria to Gram-negative bacteria indicates that heterogeneity within forest could regulate the microbial community composition and carbon cycling via affect the carbon availability.We also found a profound change in community composition within the clades of Grampositive bacteria,Gram-negative bacteria and fungi,indicating a significant effect on the biodiversity in deadwood although had little effects on the decay rate.(5)The absence of epixylic vegetation on deadwood had a profound and direct effects on the microbial community composition in bark,while it had indirect and complex effects on that in sapwood and heartwood.The effects of epixylic vegetation on microbial community composition also greatly varied with sampling seasons and locations within forest.The absence of epixylic vegetation stimulated the total microbial biomass in deadwood with decay class I under closed canopy both in snow-covered season and snow thawing period,and it depressed the total microbial biomass in canopy gap in snow-covered period.No significant changes were found in canopy edge in these two sampling seasons.Inconsistent with total microbial biomass in deadwood with decay class I,the total microbial biomasses in deadwood with decay class III were always depressed during the whole years sampling except in snow thawing period.(6)Generally,bark had the highest total PLFAs microbial biomass with 77.84 nmol/g,while heartwood had the lowest total PLFAs microbial biomass with 22.50 nmol/g.The total PLFAs microbial biomass in bark decreased as decay advanced.The total PLFAs microbial biomass in sapwood and heartwood always in low level except in decay class I and V.The microbial community structure varied with structural components and decay classes.We found the biggest variation of microbial community structure between bark and heartwood.The similarity between bark and heartwood/sapwood increased as decay advanced.(7)The composition of carbon,nitrogen,phosphors,p H,lignin and cellulose significantly varied with structural components and decay classes.The changes in microbial community composition were significantly correlated with the changes in the chemical composition(spearman coefficient 0.38).The content of carbon,phosphors and lignin best explained the major clades microbial community composition(spearman coefficient 0.46).The content of carbon was significantly(P<0.05)negatively related with Gram-positive bacteria,Gram-negative bacteria and other bacteria.The contents of phosphors and lignin were both significantly positively related with these three clades microbial organisms.In conclusion,deadwood decomposition is a long and complex process,and the dominant drivers could change as decay advanced.Thus,prediction at the long-term decomposition based on the first few years’ observation could result in big biases.Benefit to the methods applied in our study,we could monitor the whole decay process by including wide ranges of decay classes from freshly decayed to advanced decayed deadwood.We can also predict the response of deadwood decomposition to heterogeneity within forest and epixylic vegetation removal within a short-term study by detecting the microbial community dynamics which is always fast and sensitive to environmental changes.Furthermore,the model regression derived from decomposition vector method greatly increased our understanding of deadwood decomposition dynamics in different scenarios.This thesis showed that deadwood bark,sapwood and heartwood all follow different decay modes and the respond to disturbance also varied in direction and strength.Different structural components could play different roles in carbon and nutrient cycling,that is fast decayed bark could be an important source of carbon and nutrient,and decay resistant sapwood and heartwood cloud act as long-term carbon sink in the forest ecosystems. |
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