Physiological responses of Populus euphratica Oliv. to groundwater table variations in the lower reaches of Heihe River, Northwest China

Riparian vegetation in the lower reaches of Heihe River serves important ecological functions. However, the riparian ecosystems have been constantly deteriorating in the past 30 years simply due to water interception for oasis agricultural irrigation in the middle reaches of the river. This study pays a particular attention to Populus euphratica Oliv. forest because it is a dominant component of the riparian ecosystem in the lower reaches of Heihe River where the depth of groundwater table is the controlling factor in sustaining riparian ecosystems. To reveal leaf-related physiological responses of Populus euphratica Oliv. forest to groundwater table variations, we analyzed the relationships between the depth of groundwater table (DG) and three leaf-related parameters, i.e. leaf stomatal density (SD), specific leaf area (SLA), and stable carbon isotopic composition (δ 13 C). Our results show that the relationship between DG and leaf SD is a bi-mode one shaped by both salt stress and water stress. That is, salt stress appeared in shallow groundwater conditions and water stress happened in deep groundwater conditions, and the thin layer around 2.7 m of DG is a stress-free layer. Leaf SD fluctuated according to the DG variation, first decreased with increasing DG, then increased at depths ranging 2.7–3.7 m, and after a relatively stable plateau of SD at depths ranging 3.7–5.2 m, decreased again with increasing DG. Our results also show that SLA decreased exponentially with increasing DG and foliar δ 13 C values are also strongly dependent on DG, further demonstrating that these two parameters are sensitive indicators of water stress. The exponential curve suggests that SLA is more sensitive to DG when groundwater table is shallow and 3 m seems to be a threshold beyond which SLA becomes less sensitive to DG. Foliar δ 13 C becomes more sensitive when the groundwater table is deep and 7 m seems to be a threshold below which the δ 13 C signature becomes more sensitive to DG. These findings should be helpful in monitoring the growth and development of Populus euphratica Oliv. forests and also in providing protection measures (i.e. DG related) for Heihe River riparian forests.