Response Mechanisms Of Terrestrial Ecosystem Carbon And Water Flux To Environmental Water Condition And Coupling Variation Characteristics

As the terrestrial ecosystem plays an important part in global carbon and water cycles,it is of great significance to recognize characteristics of the ecosystem carbon and water coupling and understand its responses to environmental water conditions,which can provide scientific support to the modelling of global carbon and water cycles and climate changes.Starting from the mechanism of environmental water regulations on ecosystem conductance,this study applied both model simulations and theoretical analysis to investigate the relationship between ecosystem conductance and environmental water conditions,and deeply examine subdaily hysteretic characteristics of ecosystem carbon and water fluxes,as well as environmental water impacts on ecosystem water use efficiencies.The results of this work can provide a comprehensive perspective for better understanding of ecosystem carbon and water coupling variation characteristics.Based on the Penman-Monteith framework,an inverted averaged leaf-surface vapor pressure deficit?VPD_l?at the canopy scale was introduced to reflect the real water stress sensed by the vegetation,and a general ecosystem conductance model based on VPD_l was proposed.The results showed that the proposed model can better describe the relationship between ecosystem conductance?G_s?,ecosystem gross primary production?GPP?and external water conditions,i.e.VPD_l,and the response of G_s to VPD_l is between Medlyn's model(i.e.VPD_l^-0.5 depdendence)and Medlyn's model(i.e.VPD_l^-1 depdendence).Through analyzing the soil moisture impact on ecosystem conductance,G_s can be quantitatively devided into two parts:a soil conductance mainly affected by soil moisture and a canopy conductance that is independent of soil moisture variations.The hysteretic characteristics of ecosystem evapotranspiration?ET?,GPP and different environmental variables at the subdaily sacle were systematically evaluated.Combined with the variance decomposition method and the simulation results of the general ecosystem conductance model,the intrinsic mechanism of GPP hysteretic characteristics and the subdaily variation characteristics of ecosystem stomatal regulations were revealed.The results showed that ET increases in the afternoon,which changes the hysteretic patterns of VPD_l.GPP was found to decrease in the afternoon,and the decline of light use efficiency induced by decreased leaf water potential is the important reason for the GPP depression in the afternooon.Ecosystem conductance tends to be lower in the afternoon,with higher responses to VPD_l,which further affects the variations of ET and GPP.Meanwhile,the ecosystem marginal water cost is higher in the afternoon.The study explored the influence of environmental water conditions on ecosystem water use efficiency from two aspects:variations of ecosystem marginal water cost and that of water use efficiency coefficient?WUE?,which is also analyzed at the extreme arid region in the downstream of the Heihe River basin.Principal component analysis and multiple regressions were applied,and the results showed that both soil moisture and leaf area index have positive marginal correlations with the ecosystem marginal water cost,while the effect of CO₂ concentration is not significant.The results of a logarithmic sensitivity model showed that the sensitivity of WUE to soil moisture is far lower than that of VPD_l and CO₂ concentration,but significantly increases in arid regions.Analysis of WUE in the downstream of the Heihe River basin showed that the marginal water cost in this extreme arid region is high,indicating a lower WUE,and soil moisture has a stronger regulation on variations of ecosystem WUE.