Dynamics Of Net Ecosystem Productivity And Its Controling Mechanism In A Deciduous Broad-leaved Forest In Songshan,Beijing

Deciduous broad-leaved forest ecosystem plays an important role in improving the ecological environment.Songshan in Beijing is located in the transition zone between the warm temperate zone and the middle temperate zone,and is widely covered by temperate deciduous broad-leaved forests.Examining the dynamics of net ecosystem productivity（NEP）of forests in this area is an effective supplement to the research field of forest carbon cycle community,and is of great significance for assessing the carbon sequestration capacity of forest ecosystems and improving the management of carbon sink functioning.In this study,a typical deciduous broad-leaved forest ecosystem in Songshan Mountain in Beijing was used as the research sample.The relative greenness index extracted from the digital image by phenological camera over the site was used to fit the vegetation growth curve,with which the phenological period was divided.Net ecosystem exchange between the site vegetation and atmosphere was measured in 2019 using eddy covariance technique.The micro-meteorological variables were measured simultaneously.The path analysis and regression analysis were used to examine the relationship between NEP and environmental factors at different time scales.The main findings and conclusions are as follows:（1）Based on the analysis of the phenological camera image data,the phenophases were quantified.The expanding leaf period was from day 110 to 152,the expanded leaf period from day 153 to 269,and defoliation period from day 270 to 300.（2）Diurnally,NEP showed a single-peak curve.The diurnal NEP curve was the highest in the expanded leaf period,followed in the expanding leaf period,and the lowest in the defoliation period.Due to the difference in the duration of light in different phenophases,there was a significant difference in time from net CO₂uptake to release.The time from net CO₂release to uptake was the earliest in the expanded leaf period,being at 8:00 Beijing time,and the latest was the defoliation period at 9:00.The time from net CO₂uptake to release was the earliest in the defoliation period at 17:00,and the latest in the expanded leaf period at 19:00.NEP was mainly affected by PAR during day time,and mainly by T_sduring night time.Seasonally,NEP pattern showed a single-peak curve,with net CO₂uptake days accounting for 42%,and NEP in the expanded leaf period was significantly greater than that in the expanding leaf period and defoliation period.The daily cumulative NEP and its partitioning ecosystem respiration（R_e）and gross ecosystem productivity（GEP）.The maximum NEP,R_e,and GEP were 3.0 on day 151,8.7 on day 230,and 9.2 g C m^-2day^-1on day 192,respectively.The monthly cumulative NEP,R_eand GEP had the similar changing trend.The largest monthly NEP was in April with the value of 41.54 g C m^-2mon^-1,while the largest monthly Re and GEP both were in August with the values of 154.15 and 172.66 g C m^-2mon^-1,respectively.The annual values of NEP,R_eand GEP were 111,555,and 666 g C·m^-2·a^-1,respectively,and the overall performance was a weak carbon sink.NEP was affected by different dominant factors at different phenological stages.In the expanding leaf period,the NEP was mainly affected by SWC,and the SWC accounted for 52%of the seasonal variation of NEP（P<0.01）.T_aand PAR were the main factors affecting the expanded leaf period.T_aand PAR accounted for 17%and12%of NEP seasonal variation,respectively（P<0.01）.T_saccounted for 40%of the seasonal variation of NEP（P<0.01）.The results showed that the variation of NEP at the seasonal scale was mainly controlled by temperature,photosynthetically active radiation and vapor pressure deficit.Swc had an effect on NEP only at the expanding leaf period.In conclusion,the sampled forest ecosystem acted as a carbon sink in 2019 as a whole.The dominant factors controlling NEP differed with time scales.Diurnally,NEP was mainly controlled by PAR during the day and T_sat night.Seasonally,NEP was mainly controlled by PAR,T and VPD.The results of this paper are helpful to better understanding the forest ecosystem carbon cycle process and its driving factors under the background of climate change,and provide support for more accurate estimation of carbon sequestration of temperate deciduous broad-leaved forest ecosystems.