The Net Ecosystem Carbon Exchange And Its Response Mechanism Of Vegetation Ecosystem In Typical Tropical Coastal Plain

The unique tropical coastal sandy ecosystem is important not only for its high production but also for ecological safety.A long-term ecological observation was launched in a typical coastal sandy forest of Wenchang,China to investigate the dynamic and characteristics of the carbon flux.Temperature,precipitation,radiation monitoring and other meteorological factors was also measured simultaneously.The flux data from January to December 2016 was used to analyze the energy balance,dynamic changes of meteorological factors and CO2 flux on the daily,monthly,and seasonal scales.The response of ecosystem carbon flux under the influence of different environmental factors in the tropical coastal sand vegetation ecosystem in Hainan Island was also included and discussed.It provided basic data for revealing the regulatory mechanisms of the dynamic changes in carbon exchange of coastal sandy vegetation ecosystem.The results of the study are as follows:(1)The annual average temperature of the typical tropical coastal sand vegetation ecosystem in Wenchang,Hainan,was 23.9?.The highest average temperature in July was 28.2? and the lowest average temperature in February was 16.8?.The annual total precipitation was 1703.1 mm,the highest monthly rainfall(October)was 499.4 mm,and the minimum monthly(December)rainfall was only 8.5 mm.The daily average changes of photosynthetically active radiation(PAR)and vapor pressure difference(VPD)showed a similary unimodal curve,and the rainy season value at the same time was always greater than the dry season;The average monthly change of soil temperature at 5 cm is similar to that of air temperature,but the soil temperature of 5cm is always higher than the temperature because the soil has a certain insulation capacity;The annual change of soil water content of 5cm was generally "U" type.The soil water content in the dry season(November to April)was significantly higher than the rainy season(May to October).(2)The diurnal variation characteristics of the energy flux of vegetation ecosystem in typical tropical coastal plain showed a similar inverted "U" curve,and net radiation determines other energy flux processes.The average degree of closure calculated by the EBR method was 0.745,and there was 25.5%of the energy unclosed.Using the OLS method to force the regression line to pass through the origin,the slopes of the whole-day,daytime,and nighttime slopes of the ecosystem were 0.691(R2=0.78),0.694(R2=0.81),and 0.40(R2=0.03),respectively,and the energy closed was highest in daytime.The annual Bowen ratio was less than 1 and the range of variation was between 0.19 and 0.78.This indicates that the annual net radiation of energy was mainly used for latent heat transport in tropical coastal sand vegetation ecosystem.(3)The pattern of diurnal variation of net ecosystem exchange(NEE)of vegetation ecosystem in typical tropical coastal plain was inverted "U" shape,and the valley of carbon ecosystem exchange(maximum carbon absorption)appeared at 12:00?2:00.The maximum daily exchange value occurred in April,with a maximum of-0.655 mg Cm-2·s-1;and the cumulative carbon budget in April also reached the monthly maximum of-91.817 gC.m-2 month-1.The total carbon budget in 2016 was-400.959 gC m-2·y-1,indicating that sand vegetation in the study area had a strong ability to absorbed CO2.(4)At half an hour scale,based on the correlation analysis between NEE and PAR,VPD,Ta and Ts,the results showed that on a short-term scale,changing of daytime NEE were largely depended on photosynthetically active radiation,and PAR could explain 55%of daytime NEE variability.However,the nighttime NEE(ecosystem respiration)has a better fitting relationship with temperature,and the change trends of NEE with air temperature and 5cm soil temperature were basically the same,and the correlation coefficients were 0.327 and 0.315,respectively.According to the relationship between respiration and with air temperature and 5cm soil temperature,the basal respiratory rate R0 were 0.265?mol·m-2.s-1 and 0.285?mol.m-2·s-1 respectively,and the temperature sensitivity coefficient(Q10)of respiration were 2.86 and 2.57 respectively.There was a strong correlation between NEE and VPD,and the correlation coefficient R2 in the rainy season(R2=0.455)was significantly greater than dry season(R2= 0.182).