Caldwell Martyn M, Dawson Todd E, Richards James H
Department of Rangeland Resources and the Ecology Center, Utah State University, Logan, UT 84322, USA Fax: 435 797 3796; e-mail:
Section of Ecology and Systematics and the Laboratory for Isotope Research and Analysis at Cornell University, Ithaca, NY 14853, USA, , , , , , US.
Oecologia. 1998 Jan;113(2):151-161. doi: 10.1007/s004420050363.
Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. Here, we review the brief history of laboratory and field evidence supporting this phenomenon and discuss some of the consequences of this below-ground behavior for the ecology of plants. Hydraulic lift has been shown in a relatively small number of species (27 species of herbs, grasses, shrubs, and trees), but there is no fundamental reason why it should not be more common as long as active root systems are spanning a gradient in soil water potential (Ψ) and that the resistance to water loss from roots is low. While the majority of documented cases of hydraulic lift in the field are for semiarid and arid land species inhabiting desert and steppe environments, recent studies indicate that hydraulic lift is not restricted to these species or regions. Large quantities of water, amounting to an appreciable fraction of daily transpiration, are lifted at night. This temporary partial rehydration of upper soil layers provides a source of water, along with soil moisture deeper in the profile, for transpiration the following day and, under conditions of high atmospheric demand, can substantially facilitate water movement through the soil-plant-atmosphere system. Release of water into the upper soil layers has been shown to afford the opportunity for neighboring plants to utilize this source of water. Also, because soils tend to dry from the surface downward and nutrients are usually most plentiful in the upper soil layers, lifted water may provide moisture that facilitates favorable biogeochemical conditions for enhancing mineral nutrient availability, microbial processes, and the acquisition of nutrients by roots. Hydraulic lift may also prolong or enhance fine-root activity by keeping them hydrated. Such indirect benefits of hydraulic lift may have been the primary selective force in the evolution of this process. Alternatively, hydraulic lift may simply be the consequence of roots not possessing true rectifying properties (i.e., roots are leaky to water). Finally, the direction of water movement may also be downward or horizontal if the prevailing Ψ gradient so dictates, i.e., inverse, or lateral, hydraulic lift. Such downward movement through the root system may allow growth of roots in otherwise dry soil at depth, permitting the establishment of many phreatophytic species.
水力提升是指水分从根系被动地向水势较低的土壤层移动，而根系系统的其他部分则位于湿度较大的土壤层（通常在较深位置）吸收水分。在此，我们回顾支持这一现象的实验室和田间证据的简要历史，并讨论这种地下行为对植物生态学的一些影响。水力提升已在相对较少的物种（27种草本植物、禾本科植物、灌木和树木）中得到证实，但只要活跃的根系跨越土壤水势（Ψ）梯度且根系水分损失阻力较低，就没有根本理由认为它不应该更普遍。虽然田间记录的大多数水力提升案例是针对栖息在沙漠和草原环境中的半干旱和干旱土地物种，但最近的研究表明，水力提升并不局限于这些物种或地区。夜间会提升大量水分，其数量占每日蒸腾量的相当一部分。上层土壤层的这种暂时部分再水化提供了一个水源，与土壤剖面中更深层的土壤水分一起，用于次日的蒸腾作用，并且在大气需求较高的条件下，可以极大地促进水分在土壤 - 植物 - 大气系统中的移动。已证明向土壤上层释放水分使邻近植物有机会利用这一水源。此外，由于土壤往往从表面向下干燥，且养分通常在上层土壤层中最为丰富，提升的水分可能提供湿度，有利于形成良好的生物地球化学条件，从而提高矿质养分的有效性、微生物过程以及根系对养分的获取。水力提升还可能通过保持细根湿润来延长或增强细根活性。水力提升的这种间接益处可能是这一过程进化中的主要选择力量。或者，水力提升可能仅仅是根系不具备真正整流特性（即根系对水有渗漏）的结果。最后，如果主导的Ψ梯度如此决定，水分移动方向也可能是向下或水平的，即逆向或侧向水力提升。通过根系的这种向下移动可以使根系在其他情况下干燥的深层土壤中生长，从而允许许多深根性植物的建立。
