Conclusion
For Haloxylon ammodendron , a small xeric phreatophytic tree, we showed that increasing DGW significantly worsened water conditions. We postulate turgor loss traits to be adaptive for osmoregulation due to their decrease with increasing DGW. Consequently, an osmotic adjustment that accompanied Ψ regulation (Ψ pdΨ md) and Huber value plasticity helps phreatophytes absorb water from deeper soil layers and tolerate drought. Soluble sugars, as major components of osmoregulation, drove the decline in osmotic potential at full turgor; thereby, constraining assimilating branch growth during the early growing season (EGS). However, these adaptive adjustments did not fully compensate for worsened water conditions as growth rate continued to decrease during EGS and resulted in negative growth rates during the late growing season at almost all sites. Our results provide insight into how H. ammodendron responds and adapts to changes in DGW in a region experiencing hydrological and climatic drought. Greater depth to groundwater had a significant effect on this small, xeric phreatophytic tree’s survival and growth.