Abstract:

Populus euphratica is an important forest genetic resource in the world and has important ecological functions. Taking the lower reaches of Tarim River, Xinjiang, China as the research target, combined with field monitoring data and literature reading, we review and analyze the water transport processes, including water uptake, conduction, and dissipation, of P. euphratica. Under different habitats, the groundwater depth was closely related to the water source of P. euphratica in the lower Tarim River and determined the water use of P. euphratica. The water source of P. euphratica of different ages not only depends on its root system characteristics and types but also is controlled by the groundwater depth. Regardless of the water conditions, mature P. euphratica always mainly used groundwater. The water transport strategies of P. euphratica leaves differed under different groundwater depth conditions. When the groundwater depth increases, P. euphratica leaves not only increase their water acquisition by improving water transfer efficiency but also increase the risk of hydraulic dysfunction. Mature P. euphratica reduces water transpiration by downregulating xylem hydraulic conductivity to adapt to drought, whereas seedlings obtain more water by increasing xylem hydraulic conductivity. The root system of P. euphratica functions as a hydraulic lift, and the elevated water volume generally provides 10%-39% of its transpiration. The annual transpiration of P. euphratica stands ranged from 296.7 to 750.0 mm, with a flux density of 0.005- 0.040 L·cm^-2·h^-1 and decreasing with increasing groundwater depth. Furthermore, we propose the following research prospects: (1) linking the isotope technique, root morphology, groundwater depth, and water uptake to construct a quantitative analysis model with a process mechanism and explicitly quantify the water source of P. euphratica; (2) understanding the regulation mechanism of water channel proteins, osmosis, water storage, and embolism on the water transport of P. euphratica under arid environments and the adaptation mechanism of P. euphratica in water transport to the environment; (3) strengthening the integrated study of water transport processes to deeply reveal that the complex physiological processes and regulatory mechanisms of P. euphratica as the problem of the interrelationship between water acquisition, water conduction, and water dissipation in P. euphratica have not been solved; and (4) combining remote sensing, meteorological and soil data, and high-precision observation to extend the simulated scale of evapotranspiration to forest ecosystems to provide a scientific basis for regional ecological water consumption and ecological water demand.

Key words: Populus euphratica, water transport, drought, the lower reaches of Tarim River