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 pd−Ψ 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.