| Chinese fir(Cunninghamia lanceo late)is the main afforestation and timber species in southern China,which plays an important role in balancing CO2,H2O and energy expenditure.However,few studies have been reported on the characteristics of sensible heat flux(H)in Chinese fir plantations,especially at the long-time scale.The study of H is helpful to further reveal the energy balance and water balance process of Chinese fir plantation ecosystem.In this paper,the flux data and conventional meteorological data were used to study the variation characteristics of H in the ecosystem at different time scales and its relationship with environmental factors,as well as the energy balance closure,measured by open-circuit eddy covariance system and automatic meteorological gradient observation system in the subtropical Chinese fir plantation ecosystem from 2008 to 2018.The purpose of this research is to provide a scientific basis for accurately evaluating the material and energy exchange between ecosystem and atmosphere.The main results are as follows;(1)H had obvious diurnal and annual variation characteristics.The H was negative at night and the range of variation was very small,while the H was positive in the daytime and reached the maximum around noon,showing obvious unimodal diurnal variation.The diurnal range of H was significantly greater in summer than in spring and autumn,and the minimum in winter.The mean daily total amount of H was larger in summer than that in spring and autumn,and the smallest in winter.Due to the different times of sunrise and sunset in different seasons,the length of day and night is different,and the time for the positive value of H appearance was different,as well as the duration of positive value.The earliest appearance and longest duration of positive value was in summer,followed by spring and autumn,and the latest appearance and shortest duration of positive value was in winter.There was also a slight difference in the peak time of each season,with the earliest in summer,the second in autumn,and the latest in winter and spring.The mean annual variation of H showed a unimodal change characteristic of high summer and low winter,and the total monthly H of the ecosystem was positive in each month,with the maximum in July and the minimum in January.The inter-annual variation of H showed a fluctuating trend as a whole.(2)At the half-hourly and daily scales,the H had an extreme significant linear relationship with net radiation(Rn),air temperature(Ta),soil temperature(Ts),precipitation,vapor pressure deficit(VPD),and soil water content(SWC).At the monthly scale,the H showed a very significant linear relationship with Rn,Ta,Ts,VPD and SWC,while a significant linear relationship with precipitation.At the annual scale,only VPD showed a highly significant quadratic relationship with H.Stepwise regression analysis showed that Rn,Ta and VPD were the main factors affecting the variation of H in the Chinese fir plantation ecosystem.(3)Energy closure had obvious diurnal and annual variation characteristics.During the day,the energy closure reached its maximum value shortly after sunrise and then decreased gradually.The diurnal energy closure was greater in summer than in spring and winter,and the smallest in autumn.The mean annual variation range of energy closure was relatively flat,with the best energy closure in July-August and the worst in November-December,and the energy was not closed in every month in Chinese fir plantation ecosystem.The least square method was used to carry out linear regression of half-hour turbulent energy and effective energy of the ecosystem,with a slope of 0.66,an intercept of 4.50 and a correlation coefficient of 0.81.The energy closure of the ecosystem was calculated to be 0.68 using the energy balance ratio method,which was slightly better than the least square method.Compared with other sites,the energy closure degree of Chinese fir plantation in Huitong was at the average level. |
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