Temporal Variations And Environmental Control Of Carbon Dioxide Exchange Of A Natural Secondary Forest In Northeastern China

Long-term observations of CO₂ flux between the vegetation and the atmosphere contribute greatly to further understanding on the role of tcrrestrial ecosystems in global carbon cycle.In ecosystem scale,eddy covariance(EC) technique has become the leading means of CO₂ flux measurcmcnts。Secondary forest in Northeastern China plays an important role in both regional and national carbon budget.In this study,CO₂ exchange of a typical stand of the secondary forests was observed by EC method,in Maorshan,Hellongjiang Province,and initially discusscd appicativc issues in long-term flux measurements in the complex terrain,and evaluated the influences of different correcting stratigies to CO₂ exchanges in the valley,then analyzed environmental control of ecosystem photosynthesis and respiration.The results indicatcd that:(1) CO₂ concentration(molar fraction) had distinct diurnal variations,with the much larger amplitude for the growing season.Spacially,the highest value occurred near the forest floor,and decreased with height increasing for both the growing and dormant season. Temporally,for every given height,the CO₂ peaked in the early morning,decreased sharply after sunrise,indicating a marked release of CO₂,then reach the lowest point in the afternoon,a rapid accumulative cause began after sunsct and ended at the early night.This diurnal pattern was mainly controllcd by the development of atmospheric boundary layer,characterized by air tcmpcraturc stratification.The diurnal amplitude decreased from mid-growing season to winter, which indicated an essential influence of the activity of vegetation(carbon mctabolism);(2) The canopy storage flux(F_s) could contribute 29%to the half-hour NEE,thus it could not be ignored in short time scalc,i.e.for the diurnal variations of CO₂ flux.F_s derived only from the height of eddy covariance system might systematically underestimate the accumulation and rclease of CO₂ in thc canopy,compared that derived from the CO₂ profile measurements;(3) When air flows up the hill or up along the valley,the vertical velocity is positive,and it is ncgativc when air flows down the slope or down along the valley,although the tilt correction tended to decrease the effect of wind direction.This was mainly caused by the mesoscale circulation.Because the difference between the top and the mean canopy concentration was always negative,with a large diffcrcnce in the nighttime than in the daytime,the vertical advection flux(F_vad) was positive in the nighttime and negative but small in the daytime. Howcvcr,in the dormant season,the magnitude of F_vad Was smaller,and the diurnal pattern was much less distinct.Overall,F_vad trended to make night flux more positive and make day flux more negative,and consequently increased the ecosystem photosynthesis(P_eco) and respiration, but dccrcased thc net ecosystem carbon sequestration;(4) Compared the two data fliter strategies of the night flux,rejecting the negative flux at night seemed to underestimate night respiration,but removing the data below soil respiration seemed to overestimate respiration. The different standard substantially influenced the estimates of the annual fluxes,the former strategy might underestimate annual ecosystem respiration(R_eco,only positive value,indicated carbon release),overestimate annual gross primary production(GPP,only negative value, indicated carbon uptake),and overestimate annual net carbon exchange(NEE,negative value indicated carbon uptake).However,the latter one might perform the opposite;(5) In the growing season,P_eco was driven by photosynthetically active radiation,but was also significantly influenced by vapor pressure deficit in the canopy.However,the canopy leaf-off finally ended the P_eco.R_eco was dominated by soil temperature(at 10 cm depth),but was also significantly affected by volumetric soil water content(at 20 cm depth);(6)The estimated annual NEE indicaed that this forest was a net carbon sink,although there was a large uncertainty.GPP uptook about 1176 g C m^-2 yr^-1,and R_eco released 972 g C m^-2 yr^-1,with a net carbon sequestration of 204 g C m^-2 yr^-1.