| Due to the rapid growth and higher productivity and biomass, poplar plantation, which is also with the higher water consumption, plays an important role in carbon sinks forestry in northern China. Under the background of the climate change and the water scarcity in the region, studies on the characteristics of water consumption of poplar plantation and its potential effects on the water availability of the region’s ecosystems and livelihoods, and evaluating the adaptability of poplar to semi-arid environment and predicting the responses of carbon and water fluxes to the future climate change, have a very important significance in responding rationally to climate change, developing a long-term forestry planning and implementation of scientific management. In this study, on the basis of continuous long-term measurement (during 2006 to 2012) of carbon, water and energy fluxes between the poplar(Populus euramericana cv. "74/76") plantation ecosystem and the atmosphere by using an eddy-covariance (EC) and a micro climate monitor system in Daxing District of Beijing, China, the characteristics of energy partitioning and evapotranspiration (ET) of poplar plantation ecosystem and their biophysical controlling mechanism were analyzed under different climatic conditions. At the same time, the sensitivity analysis and optimization of eco-physiological parameters in Biome-BGC model were primarily made to evaluate the applicability of Biome-BGC model on stimulation of carbon and water fluxes of poplar plantation.Main results of the study were as follows:(1) On the annual scale, in response to meteorological drought, there was a significant difference in energy partitioning and ET of poplar plantation between dry and wet years, the partitioning of available energy (Rn-G) to latent heat flux (LE) decreased from 0.62 to 0.53 under mediated meteorological drought by irrigation applications, correspondingly; average daily ET in dry years (2.23±1.30 mm day-1) was 17% lower than that in wet years. A concomitant increase in sensible heat flux (H) resulted in the increase of Bowen ratio (β) from 0.83 to 1.57. While, on the seasonal scale, the energy partitioning and ET were mainly affected by water supply (sum of precipitation and irrigation) due to the seasonal drought stress.(2) Similarly, the surface resistance (Rs) responsed to meteorological drought with Rs,:LAI in dry years 50% higher than in wet years (71.2 s m-1 LAI-1), however, there were no differences in climatological resistance (R,) and aerodynamic resistance (Ra) between dry and wet years, with their average of 74.6 s m-1 and 23.2 s m-1, respectively. While the drought stress affected the Rs and Ri significantly on the seasonal scale, with their values higher in drought stress periods than non-stressed period.(3) As indicated by the low decoupling coefficient (Ω) and results of partial analysis,Rs was the dominant factor controlling the β and daily ET even in wet years,β and ET were positively (SOCC:0.905～ 0.965) and negatively (SOCC:-0.518～-0.293) related to Rs, separately. Meanwhile, the controlling of Rs on β and ET were greater in wet years than in dry years, especially, and the controlling effect of Rs, on ET was much stronger under the better soil moisture. The effect of Ri on β was negatively and much higher in dry years (SOCC:-0.667～-0.614), while Ra had a significant negative impact on β only in wet years, but not in dry years. However, the controlling effect of Ri and Ra on ET were much less than Rs and changed with soil moisture conditions.(4) Low LE/LEeq in both dry (0.68) and wet (0.81) years indicated that dry surface conditions dominated the poplar plantation in both wet and dry years. ET consumed nearly all of the precipitation in wet years and even further depleting groundwater through irrigation in dry years. Therefore, the widespread poplar plantation-the water-intensive species-in this water-limited region, it could potentially compromise the region’s ecosystems and livelihoods in the long term.(5) Sensitivity analysis for eco-physiological parameters revealed that SLA, k, LWT, LWC:TWC, FLNR, CGP, SC:LC, gsmax, wint, and LFG in Biome-BGC have significant effects on stimulation of carbon and water fluxes in poplar plantation ecosystem. With model calibration, the accuracy of carbon and water simulation was shown as ET> NEE> GPP> ER, and the accuracy of ER and NEE were highly and poorly improved, respectively; While the degree of fit between simulated and measured values were shown as NEE> GPP> ET> ER, the degree of improvement was ET> ER> GPP> NEE.(6) Simulation results indicated that rising air temperature (+2,+4,+6℃) in the future will cause increased MR and Tr, while lead to reduced GPP, GR, HR and Es of poplar plantation; increased CO2 (up to 650 ppm and 860 ppm) will exert a positive effect on GPP, MR, GR, HR, NPP and Es, but a negative effect on Tr; increased precipitation will improved all of the carbon and water fluxes of poplar plantation. The individual effect of increased CO2 on carbon and water fluxes of poplar plantation is higher than that of rising air temperature and increased precipitation, the response of carbon and water fluxes to climate change is based on their combining effects. Compared to carbon and water fluxes of poplar plantation under the current climate condition, all of them will improved under the climate change scenarios with increased CO2, air temperature and precipitation. |
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