Water Use Efficiency Of A Poplar Plantation In Daxing District Of Beijing

Global warming and the storage of water resources are two main environmental issues for the sustainable world development. As the largest terrestrial ecosystem, forest ecosystems play a vital role on carbon and water balance. Therefore, studies on relationship between carbon and water flux are helpful to understand and quantify how much water the plants need to assimilate a certain amount of carbon and determine how future climate warming induced hydrological changes will impact carbon budgets of poplar plantation ecosystems. An open path eddy-covariance (EC) system and a microclimate monitor system were employed to continuously measure the carbon, water, and energy exchange between a poplar (Populus euramericana) plantation and the atmosphere in Daxing District of Beijing, China, during2006to2012. The objectives of our study were to quantify the diurnal and seasonal variability of ecosystem water use efficiency (WUE) of poplar plantation, and determine the effects of environmental factors on the ecosystem WUE at different time scales.We found that the poplar plantation in Northern China was a strong carbon and there was a significant relationship between gross ecosystem productivity (GEP) and evapotranspiration (ET) under different soil water. Besides, the results show that the WUE changed diurnally which peaked in early morning and reached the minimum at the2pm to3pm because the limitation of photosynthetically active radiation (PAR) and saturated vapor pressure deficit (VPD) regulating stomatal opening and closure. Unlike the diurnal variation, there was apparent day-to-day variation but no substantial seasonal variation of WUE because the influence of both PAR, precipitation and soil water condition.We examined the effects of environmental factors influencing WUE under different soil moisture conditions and found that when relative extractable soil water (REW) was less than0.1, both GEP and ET maintained at a low level and did not changed with the variation of environmental factors.GEP and ET increased as soil water contents (VWC) increase, but the greater effect of VWC on ET over GEP resulted in negative correlation between WUE and VWC when0.1<REW<0.4. When REW was higher than0.4, climate factors showed a significant influence on the variation of GEP and ET through stomata. Air temperature (Ta) and photosynthetically active radiation (PAR) were the main factors for the variation of GEP while air temperature (Ta) and saturated vapor pressure deficit (VPD) showed a significant effect on changes of ET. WUE decreased with the increasing VPD due to the higher sensitivity of ET to VPD than GEP. Overall, the water received always is biologically effective for vegetation when volumetric soil water contents (VWC) was between5.2%and8.8%(0.1<REW<0.4) in this fast-growing poplar plantation. In addition, the influence of environmental factors on GEP, ET and WUE was covered by disturbance of human activities.3PG+model was used to simulate carbon, water flux and WUE and the simulative results showed that the poplar plantation in our study was still a strong carbon sink under human disturbance, such as deforestation and replantation. But WUE was lower under human disturbance because the higher effects of deforestation on GEP than ET. Besides, the simulative results of the age-related variation of WUE showed that the carbon sequestration and water loss is always effective for poplar plantation when felling trees at18years old.