A Study On Ecosystem Carbon/Water Fluxes And Water Use Efficiency In A Warm-Temperate Oak(Quercus Aliena Var.acuteserrata) Forest

Oak（Quercus aliena var.acuteserrata）forest is a representative type of deciduous broad-leaved forest in the Qinling Mountains,the transitional ecozone of south to north,which is from subtropical to warm temperate area of China.It is one of the main tree species in the warm temperate zone.The oak forest has an important influence on the CO2,H2O and energy budgets.However,we know little about the variation characteristics of CO2,H2O and energy fluxes,and their environmental and biological control mechanism.From October 2016 to December 2019,CO2/H2O flux and various environmental factors such as photosynthetically active radiation（PAR）,air temperature,soil water content in a 65-year-old natural oak forest at Baotianman natural reserve were continuously measured by an open path eddy covariance system（Li-7500,Gill）and a micrometeorological measurement system.This paper analyzed the variations of carbon and water fluxes and their controlling mechanism of the oak forest.This study helps us to understand the carbon sink function and its environmental controlling mechanism in the oak forest in central China.The main results are shown below:1)The annual average of gross ecosystem production（GEP）,ecosystem respiration（ER）and net ecosystem carbon exchange（NEE）were 1096±86.6,506±26.4 and-590±69 gCm^-2 y^-1.Observations on tree growth within the footprint of the eddy flux tower showed that 407 gCm^-2year^-1 was contributed by biomass increment.Our data suggested that the natural oak forest in central China was a strong carbon sink.Moreover,the interannual variation of GEP was greater than that of ER.Net ecosystem productivity（NEP）was positive in the growing season（May—October）as a carbon sink,and was negative in the remainng months as a carbon source.The change of carbon exchange was mainly regulated by air temperature.The anomalous high temperature inside the forest in May 2018 significantly increased monthly ecosystem respiration and contributed to earlier peaks in NEP and GEP in 2018.2)Diurnal and seasonal variations of water vapor flux in the oak forest ecosystem showed obvious single peak pattern and evatranspiration was mainly regulated by net radiation.Evapotranspiration（ET）was decreased obviously in dry year,because canopy conducrance was more easily inhibited by high VPD under soil water deficit.3)Ecosystem water use efficiency of the oak forest was mainly regulated by vapor pressure deficit（VPD）and PAR.ET of the oak forest was reduced obviously while GEP was not affected under soil water deficit（the relative extractable soil water content in soil<0.1）.These results showed that GEP of the oak forest could be maintained by reducing ET and increasing ecosystem WUE under soil water deficit.