不同土壤水分条件下杨树人工林水分利用效率对环境因子的响应

运用涡度相关(Eddy covariance,EC)开路系统和微气象观测系统,于2007年对位于北京市大兴区永定河沙地杨树(Populus euramertcana)人工林与大气间碳、水和能量交换进行了连续测定.通过分析总生态系统生产力(GEP)、蒸发散(ET)以及水分利用效率(WUE=GEP/ET)随相对土壤含水量(REW)的变化趋势,探讨杨树人工林不同土壤水分条件下水分利用效率对气象因子以及下垫面因素的响应,为杨树人工林经营管理提供理论依据.研究结果表明:当REW＜0.1时,GEP和ET受到严重水分胁迫的影响维持在较低水平,环境因子对GEP、ET和WUE的影响较小；当0.1＜REW＜0.4时,GEP和ET随着土壤体积含水量(VWC)的增加而增大,WUE随VWC的增大而减小；REW＞0.4时,气象因子是影响碳固定和水分损耗的主要原因,由于ET对气象因子变化的响应较GEP更为敏感,因此,WUE随空气饱和水汽压差(VPD)的增大而减小.沙地土壤保水能力较差,不能保证土壤水分被植物有效利用,因此当VWC处于5.2%-8.8％(0.1＜REW＜0.4)范围时,碳固定与水分消耗达到最高效率.研究表明杨树人工林WUE随降水变化而变化,未来气候变化和变异有可能影响杨树林耗水和生产力之间的关系.

Environmental controls on water use efficiency of a poplar plantation under different soil water conditions

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 during the growing season of 2007. The study site was located on dried flood plain dominated by sandy soils in Daxing District, southern Beijing, China. The objectives of our study were to a) qualify the dynamics of carbon, water fluxes and water-use efficiency (WUE) at an ecosystem level under various soil water conditions and b) examine the effects of environmental factors influencing WUE under different soil moisture conditions. We found that there was a significant relationship between gross ecosystem productivity (GEP) and evapotranspiratoin (ET) under different soil water. When relative extractable soil water (REW) was less than 0.1, 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 when 0.1<REW<0.4. When REW was higher than 0.4, climate factors showed a significant influence on the variation of GEP and ET through stomata. Air temperature (T_a) and photosynthetically active radiation (PAR) were the main factors for the variation of GEP while air temperature (T_a) 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. VWC was between 5.2% and 8.8% (0.1<REW<0.4) in this fast-growing poplar plantation, vegetation could utilize soil water efficiently and reached high rates of water use efficiency and carbon sequestration. We concluded that soil water condition was a key factor influencing WUE of the poplar plantation. Assessment of forest carbon sequestration and water consumption under a climate change environment must consider future precipitation regime in the study region.