毛白杨根系液流与水力再分配特征

Root sap flow and hydraulic redistribution of Populus tomentosa

为确定毛白杨(Populus tomentosa)根系是否存在水力再分配现象,并探究其发生特征与影响因子,该研究以四年生毛白杨为研究对象,利用热比率法对3株样树的共计7条侧根(R1–R7)进行长期液流监测,并对土壤水分以及气象因子进行同步测定。结果显示:毛白杨存在两种水力再分配模式,分别为干旱驱动的水力提升和降雨驱动的水力下降,水力再分配的发生模式与特征受侧根分布深度与直径大小的影响。在整个生长季尺度上,毛白杨根系再分配的水量较低;但在极端干旱条件下,部分侧根再分配的水量可达其日总液流量的64.6%,表明水力再分配会为干旱侧根提供大量水分。根系吸水与气象-土壤的耦合因子(太阳辐射(Rs)×土壤含水率(SWC)、水汽压亏缺(VPD)×SWC、参考蒸散发(ET_(o))×SWC)间存在显著相关关系,但水力再分配与所选因子基本不相关。此外,毛白杨浅层根中存在特殊的日间逆向液流现象,其液流量最高可占日液流总量的79.2%(R1)到90.7%(R2),该现象可能对浅层根系抗旱起到重要作用。

Aims To determine whether there is hydraulic redistribution in the root system of Populus tomentosa,and to explore its characteristics and influencing factors.Methods The heat ratio method was used to monitor the long-term sap flow of 7 lateral roots(R1–R7)of four-year P.tomentosa trees,and the soil moisture and meteorological factors were measured simultaneously.Important findings This study showed two patterns of hydraulic redistribution of P.tomentosa,namely,drought-induced hydraulic lift and rainfall-induced hydraulic descent.The occurrence and characteristic of hydraulic redistribution were affected by the distribution depth and diameter size of the lateral roots.In general,the magnitude of hydraulic redistribution was relatively low.In the growing season,the amount of water redistributed by P.tomentosa roots was low;however,under extreme drought conditions,the amount of water redistributed by the lateral roots could reach 64.6%of its total daily sap flow,indicating that hydraulic redistribution would provide plenty of water for dry lateral roots.This study showed that the root water uptake was significantly related to the meteorology-soil coupling factors(solar radiation(Rs)×soil water content(SWC),vapor pressure deficit(VPD)×SWC,reference evapotranspiration(ET_(o))×SWC),but not to the hydraulic redistribution.In addition,this study found a unique daytime reverse sap flow occurred in shallow lateral roots.The reverse sap flow could account for up to 79.2%(R1)and 90.7%(R2)of the total daily sap flow,which could play an essential role in the drought resistance of shallow roots.