Effect Of Forest Canopy On The Structure Of Microbial Community In Decomposing Foliar Litter In A Subalpine Forest Of Western Sichuan, China

It is widely accepted that litter decomposition is affected by the decomposing environment, decomposer community and litter substrate quality. Theoretically, tree canopy might control the decomposition process in materials cycling and energy fluxes on the forest floor through two essential approaches. On one hand, tree canopy determines the litter production, litterfall dynamics and litter substrate quality, and in turn runs the litter decay process. On the other hand, tree canopy manipulates the decomposing environment and the structure and function of decomposer community on the forest floor by redistributing the light and precipitation, and forming heterogenous moisture and temperature conditions on the forest floor. Consequently, studying the effects of tree canopy on the structure and function of microbial community in decomposing litter will be helpful in understanding the mechanisms on material cycles and energy transfer in forest ecosystems, and to provide scientific information for sustainable management of forest ecosystems. As yet, the effects of tree canopy on the structure of microbial community in decomposing litter on the forest floor remain unknown. A 5-year litter decomposition experiment was conducted in a subalpine coniferous forest ecosystem in the upper reaches of the Yangtze River, in the eastern Qinghai-Tibet Plateau. This forest plays significant and irreplaceable ecological roles, it is mainly regenerated by natural forest gap and is greatly affected by seasonal snow cover. The litterbags with freshly foliar litters of larch (Larix mastersiana), Minjiang fir (Abies faxoniana), cypress (Sabina saltuaria) and birch (Betula albo-sinensis) were in-situ incubated on the forest floor in closed canopy, extended gap, canopy gap, north and south gap centers. Consequently, the abundance, structure and groups of bacterial and fungal communities in decomposing foliar litters were measured at five critical periods (the onset of freezing stage, deep freezing stage, thawing stage, early and late growing seasons) from 2010 to 2015, using the method of qPCR and DGGE in combination with clone sequencing.(1) The gap which was formed by the lack of canopy changed the hydrothermal dynamics in the forest floor. The response of hydrothermal dynamics in winter and growing stage was entirely different, and is influenced by seasonal snow cover. There was an obviously increasing tendency in the average temperature which followed the depth of snow cover; it increased from the closed canopy to the gap center by the effect of canopy in winter. The increase in the average temperature of the forest floor in growing season was caused by differences of solar radiation from the closed canopy to the gap center. Which means that; the environment of decomposition was significantly different and is caused by the forest canopy in the growing and non-growing season in the area which that was influenced by seasonal snow cover, thereby affecting the dynamics and structure of decomposer in the litter.(2) The number of bacterial 16S rDNA copies and fungal 18S rDNA copies in four foliar litters were significantly affected by canopy. There was an obviously decreasing tendency in the abundance of bacteria and fungus with the decomposition of litter. The abundance of microbial was higher during growing season compared with the non-growing season. In the different winter critical stages, the abundance of microbial was the highest during the deep freezing stage, but the abundance of bacteria and fungus were the lowest during the thawing and early freezing stage, respectively. The ratio of bacteria and fungus was the highest in winter compared with the growing stage and the lowest during the early freezing stage, with these values increasing to a maximum in the deep freezing stage during winter; which suggests that, the bacteria in foliar litter had a better tolerance and adaptability to resist the low temperature during winter times. The abundance of microbial was the lowest in the closed canopy during winter times but was the highest in growing season, the ratio of bacteria and fungus was lower during the early freezing stage and higher during the deep freezing and thawing stage in the closed canopy, the ratio of bacteria and fungus varied non-significantly with growing season. These results indicated that microorganisms respond significantly to the shift in the forest canopy, but the response was different between bacteria and fungus.(3) There were significant differences in microbial community structures and diversity indices at various stages during litter decomposition among forest canopies. The microbial community structures varied greatly, and variable shifts in diversity indices were observed as the decomposition proceeded. Compared with the non-growing seasons, higher Shannon-Wiener indices and lower Simpson indices were observed during winter times, when the Shannon-Wiener indices decreased and then increased whereas the Simpson indices rose and declined later, suggesting that the harsh environmental conditions during winter altered microbial community structure and therefore improved the dominance but constrained the diversity of microorganisms. Lower Shannon-Wiener and higher Simpson indices of microbes in foliar litters were determined under closed canopy and in extended gap at all stages except for early growing season, when no regular pattern in microbial diversity was observed. These results indicated that; the shifts in ambient environment during winter significantly affected the community structures of microorganisms and thereby contributed to litter decomposition.(4) The taxonomies of microorganisms, especially the dominant taxa, varied greatly at the different stages of litter decomposition. The dominant bacterium during winter times were Gammaproteobacteria and Sphingobacteria, whereas the abundant bacterial communities were observed during growing the seasons and there were no recognizable dominant types. For fungus, the dominant species were Dothideomycetes during winter and Dothideomycetes and Agaricomycetes during growing seasons. These results implied that the environmental conditions during severe winter changed the taxonomies of microorganisms in foliar litter, as a result the groups with better tolerance and adaptability are the dominant communities during winter. Although there were significant differences from closed canopy to the gap center, the microbial taxonomies were related to mass losses and nutrient release in foliar litters. Gammaproteobacteria was positively associated with phosphorus release from foliar litter while Flavobacteria and Actinobacteria were related to lignin degradation. Moreover, Agaricomycetes was associated with cellulose degradation, and Leotiomycetes was related to the decomposition of lignin and cellulose. These results suggested that; different taxonomies of microorganisms played various roles in litter decomposition, and thus significantly contributing to the potential understanding on materials cycling and energy fluxes in the subalpine forest ecosystems.In conclusion, the micro-environment of decomposing foliar litter varied by the shift of canopy in the subalpine forest, which affect the abundance and structure of microorganism during the different stage of foliar litter decomposition process. The result of this research therefore, provided some information for understanding the ecological process of decomposing foliar litter in the subalpine forest which was characterized by complicated and variable climatic conditions such as the seasonal snow cover and freezing-thawing cycles.