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.