Discussion on ecological role of hydraulic lift in arid region
干旱区水力提升的生态作用

An effort has been made to review the ecological role of hydraulic lift and discuss the existing questions in the research field. Hydraulic lift is the process by which some deep-rooted plants take in water from deeper soil layers and exclude that into shallower, drier soil layers. According to the published papers, hydraulic lift is of important ecological significance. Hydraulically lifted water (HLW) can benefit the plant that lifts it as well as benefit neighboring plants. Hydraulic lift not only provides new water resource, improves transpiration and postpones water deficit, increases water use efficiency for accompanying plants, but also promotes nutrient absorption of hydraulic lifter and accelerates the process of biochemical cycle, such as catabolism, nitration and mineralization. Especially, water sharing that takes place in inter-ramet of colonal plant, acting as a kind of special hydraulic lift, influences the formation of communities. The volume of HLW can be such that it might influence seasonal water balances of individuals, communities, or even ecosystems. If this phenomenon is widespread, it should be incorporated into models of competitive interactions, as well as ecosystem process models and water budgets. Hydraulic lift, therefore, is a potentially important ecosystem process. Although there are many evidences of discovering hydraulic lift, how many the volume of HLW is there? Although hydraulic lift is actually existent as a physiological phenomenon for some plants, how to value its ecological role correctly? The key to discuss these questions is to distinguish HLW and capillary water in soil accurately, which is an essential condition to study hydraulic lift and is also a basis on estimating its ecological role. In the arid region (including semi-arid region), strong evaporation makes capillary action in soil intensive in day. When air temperature drops quickly and soil temperature drops slowly at night, capillary water accumulates in shallower soil layers and makes water content in soil high there. In an experiment of ours, a phenomenon was observed, i.e., water content in the soil of 20cm deep increased obviously running 10 whole days at night after Calligonum caput-medusae Schrenk was irrigated 5 whole days with basin irrigation in an irrigation period in the hinterland of Taklimakan Dessert. This illustrates the plant may be a hydraulic lifter, but the increased water content contained some capillary water surely. Therefore, choosing adequate method to research hydraulic lift is necessary. After three kinds of methods to study hydraulic lift were reviewed, we found most of them cannot distinguish HLW and capillary water in soil accurately. In order to determine HLW accurately and estimate the ecological role of hydraulic lift correctly, a new method was put forward in the paper. The purpose is promoting the research of hydraulic lift in the future.