Characteristics Of Carbon Flux Changes In Fir Secondary Forests And The Influencing Factors

Forest ecosystem is one of the parts of terrestrial ecosystem,and the carbon and water cycle of forest ecosystem is the basic process of material cycle and energy exchange in forest,therefore,in the context of global warming,it is of great significance to study the effect of different meteorological factors on carbon fluxes.In this study,carbon fluxes were observed in a secondary fir forest in Jinzhai Mahailing Nature Reserve,Anhui Province,China,by eddy correlation techniques,and meteorological observations and carbon fluxes were obtained for two years from August 2020 to July 2022.The carbon fluxes in the study area were analyzed at different time scales,and the influence of meteorological factors on carbon fluxes was obtained through path analysis,which clarified the relationship between carbon fluxes and meteorological factors and explained the influence of meteorological factors on carbon changes in fir secondary forests.It can also provide theoretical data and basis for prediction models of carbon fluxes in forest ecosystems with similar climatic conditions.The main results of the study are as follows:（1）The analysis of the variation characteristics of meteorological factors showed that the daily variation of air temperature（Ta）and 10 cm soil temperature（Ts）had a consistent seasonal pattern,with the highest monthly average temperature of Ta and Ts occurring in July and August,and the lowest monthly average temperature occurring from December to February.the magnitude of both Ta and Ts was summer>autumn>spring>winter;the overall trend between Ta and Ts was Ta>Ts in summer and Ta<Ts in winter.Ts in summer and Ta<Ts in winter.The saturated water vapor pressure difference（VPD）,water vapor flux,and soil volumetric water content（SWC）were consistent with precipitation,with lagging characteristics compared with precipitation,and VPD was significantly higher in summer than in the other three seasons,and water vapor flux and SWC content had the same seasonality（summer>spring≈autumn>winter）;sensible and latent heat fluxes both increased slowly in spring,peaked in summer,and then decreased slowly in winter to a wave.After that,it slowly decreases,and in winter,it decreases to the trough,and the average value of each month of sensible heat flux is smaller than the average value of each month of latent heat flux.（2）The CO₂fluxes at different time scales have significant daily and seasonal variation characteristics.The daily cumulative CO₂fluxes ranged from-20.08 to 12.11 g?m^-2?d^-1,with 310 days per year for carbon fixation（negative CO₂fluxes）and 54 days per year for carbon release（positive CO₂fluxes）.The average daily minimum value of CO₂flux occurs in July at-0.59 mg?m^-2?s^-1,which is a"carbon sink",and the average daily maximum value of CO₂flux occurs in December at 0.25 mg?m^-2?s^-1,which is a"carbon source".The peak absorption is summer>spring>winter,and the cumulative time of CO₂absorption in each season is summer>spring>autumn>winter.（3）The analysis of meteorological factors of fir secondary forest was obtained by fitting the Michaelis-Menten model to conclude that diurnal CO₂flux decreases with increasing PAR;on the daily and monthly time scales,diurnal CO₂flux decreases with increasing Ta;on the seasonal time scale,when Ta≥25.0℃the effect of Ta on diurnal CO₂flux gradually decreases and finally plateaus.VPD affects the relationship between Ta and diurnal CO₂flux,and the effect of Ta on diurnal CO₂flux reaches the maximum when VPD is 1～1.5 k Pa.SWC also affects the variation of diurnal CO₂flux,and the value of diurnal CO₂flux is highest when SWC reaches 0.33 m^-3?m^-3.The most important factors influencing the nocturnal CO₂flux are Ts and Ta.