The influence of DGW on hydraulic traits
The impact of groundwater depth on the water available to H. ammodendron was evident from leaf Ψ pd values experienced by trees over shallow and deep groundwater (Fig. 2, Fig. 3, Fig. 4). The decrease in the twig Ψ pd suggested reduced water availability, implying that H. ammodendron was under more severe water stress at deeper groundwater sites. The decline inΨ as DGW increased had been previously reported (Wu, Zheng, Li, et al., 2019) and was consistent with the study by Cater (2011).Ψ pd-Ψ md values also increased as the DGW increased (Fig. 4), which we anticipated because the xylem transport distance increases at a greater groundwater depth. Lower leafΨ s and greaterΨ pdΨ md values are required for water uptake (Gries et al., 2003).
Prior analyses of leaf P-V traits in multiple species across gradients of aridity indicate that π 100 andΨ TLPare robust proxies for important components of drought tolerance (Bartlett, Scoffoni, & Sack, 2012; Bartlett et al., 2014).Ψ TLP indicates the capacity of a plant to maintain cell turgor pressure during dehydration, which is strongly predictive of the plant’s response to drought (Zhu et al., 2018). In our currently study, Ψ TLP decreased as DGW increased (Fig. 5 and 6), which indicates an increasing drought resistance as DGW increases. These results are consistent with previous studies in which water stress was induced by groundwater level decline (Pan et al., 2016; Zolfaghar, Villalobos-Vega, Cleverly, et al., 2015). However, theπ 100 was lower than the averageΨ pd during the EGS and LGS in our current study, which is inconsistent with previous results (Zheng et al., 2019). This inconsistency may be attributed to the long-term average ofΨ pd during the EGS lowering theΨ pd values corresponding to the period when the P-V curve measurements were taken or to the absorption of atmospheric water or dew water in the early morning, which lowersΨ pd values (Gong, Lu, He, Sarkar, & Yang, 2019).