Varying responses of two Haloxylon species to extreme drought and groundwater depth

In the context of global change, frequent extreme droughts and artificial groundwater overexploitation have significantly changed water availability, which will have profound effects on ecosystem water balance and plant survival. However, few studies have explored the changes in physiology and water-use of desert plants caused by these emerging variations in water availability. Therefore, we investigated the photosynthesis, water potential, and water absorption of two dominant woody species (Haloxylon ammodendron and H. persicum) at three sites with different groundwater depths in the Gurbantunggut Desert of Central Asia during an extreme drought period. Our results showed that: (1) H. ammodendron exhibited more negative pre-dawn leaf water potential and lower photosynthetic capacity (-2.85 MPa, 24.19 mu mol CO2 m(-2) s(-1)) than H. persicum (-2.31 MPa, 33.92 mu mol CO2 m(-2) s(-1)) during the extreme drought period; (2) pre-dawn leaf water potential and carbon assimilation of H. ammodendron significantly decreased with declining groundwater depths, whereas those of H. persicum were barely affected; (3) both species converted to deeper but different water sources during the extreme drought period: H. ammodendron obtained 56-100% soil water from the near-groundwater layer, while H. persicum obtained 64-100% soil water from the deep soil layer; (4) the depths of water absorption constantly deepened with declining groundwater depths. Variations in water availability have led to the adjustment in plant water uptake, but this adjustment was not sufficient for the optimal physiological performance of H. ammodendron. Thus, under more intense and frequent drought events, further declines in groundwater depth could significantly inhibit growth of this species and may ultimately threaten its survival. Our findings on water use and physiological responses of dominant plant species to decreasing water availability provide a basis for predicting the future of desert plants under deteriorating water conditions.