Haloxylon species were dominant vegetation components of deserts across Central Asia, considering as typical desert plants. Recent studies based on stable isotope showed that they are groundwater-dependent plants, but their tolerance strategies and limits to groundwater variation remain unknown. We adopted the method of spatial-for-temporal, took Haloxylon ammodendron and H. persicum in Gurbantunggut Desert in Central Aisa as study objects, combined field survey with model estimation exploring their quantity, structure, age, and biomass characteristics along a natural groundwater gradient, aiming to reveal responses to groundwater depth changes and predict their future development. The results showed that: (1) Along the groundwater gradient, the stand density, and plant height, canopy width, and basal diameter of two species all decreased significantly. （2）When the groundwater depth descended to lower than 12 m, H. persicum replaced H. ammodendron becoming the more dominant species. (3) As the groundwater depth declined, the dominant diameter class of H. ammodendron increased, its percentage of adult individuals increased, but H. persicum was always dominated by young trees. (4) The above-ground, below-ground and total biomass of two species were all going down with the lowering of groundwater table, especially those of H. ammodendron exhibited a much sharper decline, while the root-to-shoot ratio of H. persicum increased more significantly (p < 0.05). These findings indicated that continuous decline in groundwater depth greatly limited the survival and development of H. ammodendron, the strong allocation regulation of H. persicum helped to cushion the adverse effects at a certain extent. In the long run, decreased accessibility of groundwater would be not conducive to their various ecological roles. The obtained results in this study could provide a scientific basis for the protection and management of these valuable species.
Anthropogenic withdraw of groundwater and climatic drought results in the decline of groundwater depth that, in turn, severely limits the water availability for phreatophytic vegetation in arid regions. In this study, a small xeric, phreatophytic tree Haloxylon ammodendron (C.A. Mey.) was investigated to understand the influence of depth to groundwater (DGW) on hydraulic traits and on the trade-off between drought tolerance and leaf area increment. A suite of traits including leaf water potential, pressure–volume (P–V) curves, Huber value, assimilation branch growth, and osmotic regulation substance were measured across five sites with DGW ranges from 3.45 to 15.91 m. Our results indicate that H. ammodendron was subject to greater water stress with increasing DGW, as indicated by decreased predawn (Ψpd) & midday (Ψmd) branch water potential. We also found that growth rate declined as Huber value increased with increasing DGW in the early growing season (EGS). Solute sugar, as a major osmotic substance, drives decreases in osmotic potential at full turgor, and thus constrains assimilation branch growth with increasing DGW in EGS. Therefore, osmotic adjustment accompanied with water potential regulation (Ψpd－Ψmd) and plasticity of Huber value allows this phreatophyte to absorb water from deeper soil layers and tolerate drought. However, these adaptive adjustments cannot fully compensate for nonoptimal water conditions as growth rate continued to decrease as DGW increased in EGS and even became negative in the late growing season (LGS) at almost all sites. Our results provide an insight into how H. ammodendron responds and adapts to changes DGW in a region experiencing hydrological and climatic drought. Greater depth of groundwater had a significant effect on H. ammodendron and may have similar effects for other non-riparian phreatophytic plants in arid regions.