Observation And Simulation Of Carbon Flux Of Hippophaer Hamnoides Plantation In The Loess Hilly Region

The Loess Hilly region is a typical ecologically fragile region.Vegetation restoration and construction in this region is an important ecological restoration project in northern China.Hippophaer hamnoides plantation is one of the most important tree species in the Loess Hilly region.Understanding the stability of its carbon flux dynamics under current environmental conditions and future climate change is of great significance to the management and construction of the plantation in the Loess Hilly region.At the same time,the development of Biome-BGC ecological process model provides a reliable method for the study of regional ecosystem carbon cycle.In this study,H.hamnoides plantation in the Loess Hilly region was used as the research subject to analyze the seasonal dynamic characteristics of H.hamnoides plantation ecosystem carbon flux from 2016 to 2019,to clarify the impact of environmental factors on carbon flux,and to evaluate the response of the ecosystem carbon budget to environmental factors at the present stage.At the same time,combined with the Biome-BGC model,the EFAST sensitivity analysis method was used to determine the sensitive parameters affecting the total ecosystem productivity（GEP）,ecosystem respiration（RE）and net ecosystem productivity（NEP）of H.hamnoides plantations based on the optimization of model independent parameter estimation method（PEST）.The first order and total sensitivity index of each parameter were calculated to identify the high and medium sensitivity parameters affecting the carbon flux of H.hamnoides plantation.Then,the optimized model was applied to analyze and predict the carbon flux response of H.hamnoides plantation ecosystem under the RCP4.5 climate change scenario,and to estimate the carbon sequestration capacity of the H.hamnoides plantation ecosystem in this region,and judge its carbon source and sink function.The main conclusions are as follows:（1）H.hamnoides plantation ecosystem in the Loess Hilly region showed an annual carbon sink from 2016 to 2019,with the annual NEP of four years were 264.79g Cm^-2yr^-1,205.13g Cm^-2yr^-1,226.83 g Cm^-2yr^-1,and 216.29 g Cm^-2yr^-1,respectively.The annual scale GEP of four years was 1075.74 g Cm^-2yr^-1,981.16 g Cm^-2yr^-1,1086.29 g Cm^-2yr^-1,and1037.57 g Cm^-2yr^-1,respectively.The annual scale RE of the four years were 811 g Cm^-2yr^-1,774.04 g Cm^-2yr^-1,859.99 g Cm^-2yr^-1 and 821.28 g Cm^-2yr^-11,respectively.The interannual differences of GEP,RE and NEP were small.GEP,RE and NEP had obvious seasonal variation pattern,showing obvious"double peak"variation characteristics.The photosynthetic capacity and respiration intensity of plants were weaker in the early and late growing season,but stronger in the vigorous growing season.（2）The regression analysis showed that the daily and monthly NEP were mainly affected by air temperature（TA）and saturated vapor pressure difference（VPD）.The daily and monthly GEP are mainly affected by air temperature and photosynthetically active radiation（PAR）.The daily RE is mainly affected by air temperature and PAR,while the monthly RE is mainly affected by air temperature and precipitation（PPT）.（3）Path analysis showed that TA could enhance the negative effect of VPD,reduce the positive effect of SWC,and reduce the positive effect of PAR to affect GEP and RE.PAR could enhance the negative effect of VPD to affect GEP and RE,while PPT could enhance the negative effect of VPD and enhance the positive effect of SWC to affect GEP and RE.On the other hand,VPD can change the stomatal opening and closing of plant leaves to affect GEP and RE.VPD can increase stomatal closing and weaken photosynthesis and respiration of plants.SWC also affects plant GEP and RE by affecting stomatal opening and closing of leaves.（4）According to the sensitivity analysis results of physiological and ecological parameters,the ratio of fine root carbon to leaf carbon（FRC:LC）and the ratio of shady leaves to sunny leaves(SLA_shade:sun)had the same effects on the GEP,RE and NEP of H.hamnoides plantation in the Loess Hilly region,and the FRC:LC was a significant negative effect.SLA_shade:sun had a significant positive effect,and FRC:LC had a strong independent and interactive effect on GEP,RE and NEP of H.hamnoides plantation.FRC:LC indirectly affects photosynthesis and respiration by affecting the relative content of carbon and nitrogen in leaves and fine roots,while SLA_shade:sun affects the area and intensity of light received by leaves to affect photosynthesis of plants.Meanwhile,changes in leaf surface temperature caused by SLA_shade:sun also affect respiration.（5）Under the RCP4.5 climate change scenario（In the early period（2020-2040）,the temperature will increase by 0.82℃,and the precipitation will decrease by 3.27%;In the middle period（2041-2070）,the temperature will increase by 1.65℃,and the precipitation will increase by 5.71%.In the late period（2071-2100）,the temperature will increase by2.35℃,and the precipitation will increase by 8.07%.The CO₂ concentration will be set at395ppm in the early period,430ppm in the medium period,and 538ppm in the year2100.）the carbon flux of H.hamnoides plantation ecosystem will show an increasing trend and will reach the maximum value at the end(GEP will reach 1357.62 g Cm^-2yr^-1,RE will reach 1067.907 g Cm^-2yr^-1,and NEP will reach 289.714 g Cm^-2yr^-1,respectively),and the carbon fluxes will show a fluctuating increase in the early,middle and late stages,with a small increase.With climate change,the H.hamnoides plantation ecosystem still acts as a large carbon sink.In conclusion,based on the carbon flux of H.hamnoides plantations,this study analyzed and predicted the carbon sequestration capacity and carbon source and sink function of H.hamnoides plantations in the Loess Hilly Region under current and future climate scenarios.The results showed that environmental factors and physiological and ecological parameters can directly affect the carbon flux,or the positive and negative effects of the increase or decrease of other factors on the carbon flux affect the carbon flux.With climate change,the H.hamnoides plantation ecosystem showed a large carbon sink,which was beneficial to the reconstruction of the ecosystem and the sustainable development of vegetation restoration in this region.