A survey was conducted to study the structure and tree species diversity of a subtropical evergreen broad-leaved forest in Xiaokeng,Laoshan Nature Reserve,Anhui Province.A total of 26 tree species(DBH≥3.0cm)were recorded within 5 plots in the forest,which belong to 15 families and 21 genus.Fagaceae and Hamamelidaceae were dominant families.The vertical structure of the forest was simple,without obvious shrub and herb layers.The stand density ranged from 4152(18-year-old)to 2492(65-year-old)stems·hm-2.The trees with DBH≥20cm accounted for 3% of the total number in the young stands(18-and 36-year-old)and 30% in the medium-aged stands(over 40-year-old),while the basal area of large trees occupied 8.8%~20.6% and 61.9%~70.7% of the total basal area for different successional phases of young(18-and 36-year-old)and medium-aged(over 40-year-old)stands,respectively.The diversity index and evenness index of tree species in medium-aged stands averaged 1.79 and 0.53.The dominant family for the forest was Fagaceae in terms of importance value,and the dominant tree species were Castanopsis eyrei,C.sclerophlla,Lithocarpus harlandii,and Cyclobalanopsis glauca.In view of successional trend,C.sclerophlla and L.harlandii were development type and successional mid-phase species,while C.eyrei and C.glauca were growing type and climax species.

Litter decomposition and related nutrient release are fundamental process of nutrient cycling in forest ecosystems,and an important component of global carbon budget.A thorough understanding of decomposition process is essential in understanding the structure and functioning of forest ecosystems.Decomposition predictors can be divided into three types,including environmental index(e.g.actual evapotranspiration(AET)),litter physical and chemical quality indices(e.g.leaf tensile strength(LTS),C/N,lignin/N and C/P ratios etc.).The mechanism of nutrient release in decomposition is extremely complicated.There are three types of dynamical patterns of nutrients during the decomposition process such as leaching-release,leaching-accumulation-release,and accumulation-release,which vary with litter types,decomposition phases,and elements with respect to both the mobility of nutrients and limitations of nutrients on decomposer organisms.The decomposition of mixed-species litter exhibits different responses such as synergistic,antagonistic or null response,depending largely on species composition and the proportion of individual litter types.Mixtures of different litters likely result in both chemical diversity and microhabitat complexity,which correlated with the change in abundance and physiological activities of the decomposers.Emerging researches suggested that in the mixed-litter experiment,the representation of litter types in proportions should determined by those in the natural setting.Elevated CO2 can affect the litter chemistry and biotic and abiotic environment including soil resource availability,soil fauna community and the activity of soil enzymes responsible for the decomposition of litter.However,elevated CO2 do not alter the relationship between litter chemistry and decomposition.The results from most emerging researches implied that changes in the species composition of the plant community under elevated CO2 have a much larger effect on the long-term rate of nutrient cycling than changes in atmospheric CO2 alone.