Dawson Todd E
Section of Ecology and Systematics, Cornell University, Corson Hall, 14853, Ithaca, NY, USA.
Oecologia. 1993 Oct;95(4):565-574. doi: 10.1007/BF00317442.
During drought periods, sugar maple (Acer saccharum) demonstrates "hydraulic lift"; nocturnal uptake of water by roots from deep soil layers that is released from shallow roots into upper soil layers. Using standard water relations methods and stable hydrogen isotope analysis of both source-water and plant-water, I investigated (1) the magnitude and radial extent of hydraulic lift by mature, relatively open-grown trees, of A. saccharum, (2) the proportion of hydraulically-lifted water (HLW) used by shallow-rooted neighbors growing at different distances from target trees, and (3) the influence that this water source had on stomatal conductance to water vapor (g), water balance and growth of these neighbors. Soil water potentials (ψ) at -20 and -35 cm showed a distinct diel fluctuation. Soil pits dug beneath three mature trees revealed a distinct hard-pan (e.g. fragipan) layer at a depth of approximately 50 cm. Examination of root distributions obtained from soil cores and soil pits revealed that some larger diameter roots (1.9-3.7 cm) did penetrate the fragipan and were established in the ground water table. The presence of the fragipan indicated that the "rewetting" of the upper soil layer during the night could not be explained by capillary rise from the shallow water table; it was the trees that were taking up ground water and then redepositing it at night into the upper 35 cm of soil, above the fragipan. The greatest fluctuations in ψ occurred within 2.5 m of trees and only extended out to approximately 5 m. Application of a two-end-member linear mixing model which used stable hydrogen isotopic data obtained from environmental water sources and xylem-sap demonstrated that all neighbors used some fraction (3-60%) of HLW supplied by sugar maple trees. Plants that used a high proportion of HLW (e.g. rhizomatous or stoloniferous perennials) maintained significantly higher leaf water potentials and g, and showed greater aboveground growth when compared with (i) neighbors that used little or no HLW or (ii) conspecifics found growing at distances greater than about 3 m away from maple trees. Three important conclusions can be drawn from the results of this investigation that have not been demonstrated before: (1) hydraulic lift need not only occur in arid or semi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, (2) that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and (3) that the HLW source can effectively ameliorate the influence of drought on the performance and growth of neighboring vegetation. The results are also discussed in terms of their influence on plant nutrient relations (including plant-mycorrhizal associations), the nature of plant-plant interactions and the water balance of individuals, communities and floristic regions.
在干旱时期，糖枫（糖槭）表现出“水力提升”现象；即根系在夜间从深层土壤吸收水分，并从浅层根系释放到上层土壤中。我运用标准水分关系方法以及对水源和植物水分进行稳定氢同位素分析，研究了：（1）成熟、相对开阔生长的糖枫树木水力提升的幅度和径向范围；（2）生长在距目标树不同距离处的浅根邻居所使用的水力提升水（HLW）的比例；（3）这种水源对这些邻居的气孔导水率（g）、水分平衡和生长的影响。-20厘米和-35厘米深度处的土壤水势（ψ）呈现出明显的昼夜波动。在三棵成熟树木下方挖掘的土坑显示，在约50厘米深处有一层明显的硬磐（如脆磐）层。对从土壤芯样和土坑中获取的根系分布进行检查发现，一些直径较大的根（1.9 - 3.7厘米）确实穿透了脆磐并扎根于地下水位。脆磐的存在表明，夜间上层土壤层的“再湿润”不能用浅层地下水位的毛管上升来解释；是树木吸收了地下水，然后在夜间将其重新沉积到脆磐上方的上层35厘米土壤中。ψ的最大波动发生在距树木2.5米范围内，仅延伸至约5米处。应用一种两端元线性混合模型，该模型使用从环境水源和木质部汁液获得的稳定氢同位素数据，结果表明所有邻居都使用了糖枫提供的一定比例（3% - 60%）的HLW。与（i）几乎不使用或不使用HLW的邻居或（ii）生长在距枫树大于约3米处的同种植物相比，使用高比例HLW的植物（如根茎或匍匐多年生植物）保持着显著更高的叶片水势和g，并且地上部分生长得更好。从这项研究结果可以得出三个以前未被证实的重要结论：（1）水力提升不仅可能发生在长期缺水的干旱或半干旱环境中，在相对湿润的环境中，当遭遇周期性土壤水分亏缺时也可能很重要；（2）进行水力提升的树木附近的植物能够利用这一水源的很大一部分；（3）HLW水源可以有效减轻干旱对邻近植被性能和生长的影响。研究结果还从其对植物养分关系（包括植物与菌根的关联）、植物间相互作用的性质以及个体、群落和植物区系水分平衡的影响方面进行了讨论。
