The Observations On CO₂,H₂O Fluxes And Atmospheric Water Vapor Stable Isotope Composition Of An Old Evergreen And Deciduous Broad-leaved Mixed Forest In Tianmu Mountain

Research on CO2, water and heat fluxes in terrestrial ecosystem is a hot and difficult issue, playing a key role in the understanding of ecosystem change, predicting the trend of global climate change and evaluating the carbon sequestration of ecosystem. New research shows that the subtropical forest ecosystems in Asia plays an essential role in the global carbon cycle and carbon sink function. The regional forest type diversity, research on old growth forest are less. It is very important to establish long-term continuous observation of the flux tower, to reveal old-growth forests source and sink process, to calculate the regional scale characteristics of forest ecosystem carbon balance research based on these observation data. In Tianmu Mountain of Zhejiang Province, we build up old-growth subtropical evergreen and deciduous broad-leaved mixed forest observation tower, set up three dimensional ultrasonic anemometer, H2O/CO2 infrared analyzer, and multi-layer gradient of the temperature and humidity, wind speed, the radiation of the conventional meteorological instruments, from December 2012 observation to current. Based on these data which is observed from July 2013 to June 2014, we analyzed the dynamics the carbon and water flux components in different time scales and calculated the carbon source/sink properties and strength of the ecosystem. Meanwhile, we also analyzed the influence factors of atmospheric water vapor stable isotope composition.The major conclusions are summarized as follows:(1) The ecosystem had a negative monthly CO2 flux that played role of a carbon sink except for December and February. The average monthly net ecosystem exchange were-61.52 g C?m-2, the monthly carbon sequestration showed a double-peak curve, the maximum carbon sink was-149.40 g C?m-2 occurred in June, then in October; while the maximum carbon source was 23.45 g C?m-2occurred in February. The average diurnal change of the monthly CO2 flux varied greatly from-0.35 mg?m-2?s-1(December) to-0.98 mg?m-2?s-1(June). The net ecosystem exchange(NEE) at the time point of positive and negative conversion had typical seasonal characteristics. The yearly net ecosystem exchange(NEE), ecosystem respiration(RE), and gross ecosystem exchange(GEE) were-738.18 g C?m-2、931.05 g C?m-2、-1669.23 g C?m-2. The carbon fixation is larger compared to the other young forest ecosystems in similar latitude location. This shows that old-growth forests of Tianmushan mountain in subtropical China had a strong capability in carbonsequestration.(2) Annual water vapor flux was positive value, which discovered that the ecosystem was the source of water vapor; as well as the water vapor flux had distinct diurnal and seasonal variations. The maximum monthly water vapor flux occurred in July, and the maximum daily water vapor flux reached 0.115g?m-2?s-1.While January has the lowest monthly water vapor flux value, the maximum daily water vapor flux in January was only 0.029 g?m-2?s-1.The maximum quarterly water vapor flux occurred in summer, while January has the lowest quarterly water vapor flux value. The annual total evapotranspiration was 721.25 mm, which was about 51.46% of the annual precipitation, while water vapor flux for all other months was less than the precipitation. It is at a low level in different types of forest ecosystems in similar latitudes. The yearly net radiation of the ecosystem was 3305.65MJ?m-2.Rn was the main driving force of evapotranspiration and was a key factor on the energy balance. Through this study of regression relationship between water vapor flux and different net radiation analysis, there were a high correlation between them. And the squares of correlation(R2) were 0.500 in Summer,0.318 in Spring,0.232 in Autumn and 0.125 in Winter.(3) Based on the data which is observed from August 2013 to September 2013,this paper analyzed the atmospheric water vapor stable isotope composition influencing factors and their relationship, and accordingly found that: the atmosphere of rainfall, air temperature, soil temperature at 5 cm in soil depth, soil water content at 5cm in soil depth were important environmental factors in the forest ecosystem in Tianmu Mount. Correlation between the average wind speed, relative humidity, net radiation and atmospheric water vapor stable isotopic composition is not significant. Compared with the agricultural ecosystems, water vapor stable isotope composition influencing factors in forest ecosystem are different.