干旱区河岸柽柳水分利用效率(WUE)对地下水位年内波动的响应

Responses of Tamarixin Water Use Efficiency to IntraAnnual Water Table Fluctuation in an Arid Riparian Zone

柽柳是中亚干旱区河岸林的重要建群种,同时也是美国西南半干旱区河岸林的主要入侵物种。随着国内外生态输水和流量自然化等旨在恢复受损河岸生态系统措施的实施,许多干旱、半干旱区河流沿岸的地下水呈现出显著的年内波动。在此背景下,作为兼性地下水湿生植物的河岸柽柳,其水分利用效率是保持相对稳定还是随水位波动而变化?该问题目前仍未得到解答。基于叶片δ13 C、河水位、河岸带地下水位及土壤水分的动态观测,分析了黑河中游典型河岸林内柽柳水分利用效率对地下水位波动的响应。研究表明:柽柳叶片的碳同位素分馏值具有显著的波动性变化特征,且与地下水涨落呈较好的正相关,通常随水位抬升而增大,在地下水回落一段时间后下降,表明河岸柽柳的水分利用效率对潜水埋深的变化有着显著响应,随地下水位的下降而增高。这可能是因为在高水位时段及低水位时段的初期,柽柳主要吸收上升到或滞留在根系吸水层内的地下水,水分来源充足,故水分利用效率总体上较低;在低水位时段的中后期,因重力释水和植物蒸腾,根系吸水层变得越来越干燥,为了从中吸收到足够的水分,柽柳叶片水势降低,导致部分叶片气孔闭合,水分利用效率增高。

Tamarixis not only a native dominant species of riparian forests in central Asian arid region,but also an invasive species that now dominates southwestern United States riparian forests.Recent management efforts in regulated dry land rivers that aim at restoring degraded riparian ecosystems,such as naturalizing flow regime and ecological water conveyance,have resulted in significant intra-annual water table fluctuations in riparian zone.However,few researchers have addressed the question how Tamarix responds in water use efficiency（WUE）to this kind of groundwater fluctuation.This study investigates Tamarixat a riparian zone in the middle reaches of the Heihe River,northwest China responding in WUE to intra-annual water table fluctuation.Stable carbon isotope ratios of Tamarixleaf,river stage,groundwater table and soil moisture were measured regularly during the growing season from June to September in 2011.We found that leaf carbon isotope discrimination（Δ）fluctuated significantly and positively related to change in water table.It generally increased with rising water table and decreased after water table has fallen for some time,suggesting significant response of riparian Tamarixin WUE to change in depth of groundwater.As a facultative phreatophyte,Tamarix was supposed to derive its water from groundwater when water table was relatively shallow,but switch water source to unsaturated soil moisture when water table declined.It means that saltcedar could easily obtain enough water and thus show a relatively low WUE during high water table period and the early stage of low water table period.Conversely,Tamarix might experience drought stress during the following prolonged low water table period because the stored soil moisture in its water extraction depth was becoming progressively depleted due to gravity drainage and evapotranspiration.To extract water from this gradually drying soil,Tamarixhad to maintain a considerably low xylem water potential,which was probably low enough to cause stomatal closure and thus relatively high WUE in some individuals.