Discussion
Predicted droughts and anthropogenic water intensify groundwater limitation, particularly for groundwater-dependent ecosystems arid environments (Ashraf et al., 2017; Challis, Stevens, McGrath, & Miller, 2016). Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Increasing depth of groundwater (DGW) alters the soil water supply by dramatically reducing the water availability (Garrido et al., 2020; Hultine et al., 2020). Water absorption occurs deeper as groundwater depths decline, which increases the water transport distance along with the DGW (Wu, Zheng, Li, et al., 2019). Here, DGW was the most important driver of ecophysiological adjustments for a small, xeric phreatophytic tree. Increasing DGW negatively affected the hydraulic properties and growth rate of H. ammodendron (Fig. 4, Fig. 6 and Fig. 7). Effective osmotic adjustments (accumulating organic compounds to tolerate low assimilating branchΨ and facilitate water absorption from deeper soil layers) decreases the cell turgor, which constrained leaf growth (Fig. 8 and Fig. 9). The decline in the growth rate, in turn, led to a Huber value adjustment to increase the water supply of a unit of sapwood area (Fig. 9). The switching of water sources (Wu, Zheng, Li, et al., 2019), osmotic regulation, and plasticity in Huber value, however, did not sufficiently compensate for the impact of drought stress on the physiological performance as reflected in decreased growth rates in response to DGW during the EGS and even a negative growth rate at most sites during the LGS. Therefore, more attention should be given to the influence of groundwater level decline on plant health in natural ecosystems, especially the most vulnerable arid, desert ecosystems mainly dominated by phreatophytic species.
We divided the growing season into early and late phases to reflect contrasting meteorological conditions in different growth stages. In our research area, snow usually covers the land, with a maximum depth of 20 cm, from late November to late March of the next year (H.-F. Zhou, Zheng, Zhou, Dai, & Li, 2012). Due to the seasonal recharge from snowmelt, the soil water content was higher in spring and early summer but quickly depleted in the dry, hot mid/late summer (Tiemuerbieke et al., 2018; Wu, Zheng, Li, et al., 2019). Therefore, H. ammodendron experiences different upper soil water conditions during the growing season despite the pattern of upper soil water content variation being the same across sites with different DGW values. Thus, it is necessary to distinguish the EGS and LGS, which may help clarify the impact of groundwater on hydraulic traits and growth. Additionally, annual growth rhythms may influence the impact of DGW on plants. Future studies should distinguish dynamic changes in leaf growth caused by soil water supply conditions.