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The photosynthetic characterization of Populus euphratica and their response to increasing groundwater depth and temperature were analyzed based on net photosynthetic rate (P N ), stomatal conductance (g s ), intercellular CO 2 concentration (C i ), transpiration rate (E), water use efficiency (WUE) and stomatal limitation (L s ) measured by a portable gas-exchange system (LI-6400) in the lower reaches of the Tarim River. Light-response curves were constructed to obtain light-compensation and light-saturation points (LCP and LSP), maximum photosynthetic rates (P max ), quantum yields (AQY), and dark respiration rates (R D ). The growth condition of P. euphratica, soil moisture, and groundwater depth in the plots were analyzed by field investigation. The results showed that the growth condition and photosynthetic characterization of P. euphratica were closely related to groundwater depth. The rational groundwater depth for the normal growth and photosynthesis was 3-5 m, the stress groundwater depth for mild drought was more than 5 m, for moderate drought was more than 6 m, for severe drought was more than 7 m. However, P. euphratica could keep normal growth through a strong drought resistance depended on the stomatal limitation and osmotic adjustment when it faced mild or moderate drought stress, respectively, at a normal temperature (25°C). High temperature (40°C) significantly reduced P N and drought stress exacerbated the damage of high temperature to the photosynthesis. Moreover, P. euphratica would prioritize the resistance of high temperature when it encountered the interaction between heat shock and water deficit through the stomata open unequally to improve the transpiration of leaves to dissipate overheating at the cost of low WUE, and then resist water stress through the osmotic adjustment or the stomatal limitation.

Water physiological integration plays an important role and has profound effects on the population renewal and expansion of clonal plants. This work discussed the spatial distribution architecture, water sources, water physiological integration, and the ecological significance of Populus euphratica young ramets in an extremely drought environment based on field investigation, stable isotope technology, and eco‐physiological experiments. The spatial distribution architecture of P. euphratica young ramets assumes a guerrilla growth form that is adapted to significant heterogeneity and patches of habitat resources. There is obvious water integration that is characterized by an acropetally flow. The water integration process and daily flow showed the bimodal pattern following the physiological rhythm of parent trees and declined with increasing length of spacers connecting parent trees and their young daughter ramets. The young daughter ramets of P. euphratica can use the deeper soil water in a similar manner to the parent trees and obtain an average daily water acquisition of about 1.09 kg by water integration when the spacer length is about 2–3 m. It is about 5 times as much water as seedlings sucking by themselves. This lets young ramets maintain higher leaf water content and midday leaf water potential of 10.27% and 29.73%, respectively, than seedlings in same habitats. Therefore, P. euphratica young ramets can benefit from a higher survival advantage in extreme drought habitats compared to seedlings. This facilitates the establishment and growth of young ramets in those adverse habitats where P. euphratica seedlings are less likely to survive.

<p><strong>Abstract.</strong> Studies of the water use of the desert riparian forest plant community in arid regions and analyses of the response and adaptive strategies of plants to environmental stress are of great significance to the formulation of effective ecological conservation and restoration strategies. Taking two inland rivers in the arid regions of northwestern China, downstream of the Tarim River and Heihe River Basin as the research target regions, this paper explored the stem water potential, sap flow, root hydraulic lift, and characteristics of plant water sources of the major constructive species in the desert riparian forest, <i>Populus euphratica</i> and <i>Tamarix ramosissima</i>. Specifically, this was accomplished by combining the monitoring of field physiological and ecological indicators, and the analysis of laboratory tests. Then, the water use differences of species in different ecological environments and their ecological significance were analyzed. This study indicated that: (1) in terms of water sources, <i>Populus euphratica</i> and <i>Tamarix ramosissima</i> mainly used deep subsoil water and underground water, but the plant root system in the downstream of the Tarim River was more diversified than that in the downstream of the Heihe River in water absorption, (2) in terms of water distribution, <i>Populus euphratica</i> root possessed hydraulic lift capacity, but <i>Populus euphratica</i> root in the downstream of the Tarim River presented stronger hydraulic lift capacity and more significant ecological effect of water redistribution, (3) in terms of water transport, the plants in the downstream of the Heihe River can adapt to the environment through the current limiting of branch xylem, while plants in the downstream of the Tarim River substantially increased the survival probability of the whole plant by sacrificing weak branches and improving the water acquisition capacity of dominant branches; and (4) in terms of water dissipation, the water use and consumption of <i>Populus euphratica</i> at night exhibited no significant difference, but the water use and consumption of <i>Populus euphratica</i> in the downstream of the Tarim River in the day was significantly higher than that in the downstream of the Heihe River, and the essential reason for this is the groundwater depth. The ecology in the downstream of the Heihe River has been in balance in the maintenance and development stage, while desert riparian forest plants in the downstream of the Tarim River are still in severe arid stress.</p>