Gas exchange of Populus euphratica leaves in a riparian zone

Riparian vegetation belts in arid regions of Central Asia are endangered to lose their ecosystem services due to intensified land use. For the development of sustained land use, management knowledge of plant performance in relation to resource supply is needed. We estimated productivity related functional traits at the edges of the habitat of Populus euphratica Oliv. Specific leaf area (SLA) and carbon/nitrogen (C/N) ratio of P. euphratica leaves growing near a former river bank and close to moving sand dunes in the Ebinur Lake National Nature Reserve in Xinjiang, Northwest China (near Kazakhstan) were determined and daily courses of CO2 net assimilation (PN), transpiration (E), and stomatal conductance (gs) of two consecutive seasons were measured during July–August 2007 and June–July 2008. Groundwater level was high (1.5–2.5 m below ground) throughout the years and no flooding occurred at the two tree stands. SLA was slightly lower near the desert than at the former river bank and leaves contained less N in relation to C. Highest E and gs of P. euphratica were reached in the morning before noon on both stands and a second low maximum occurred in the afternoon despite of the unchanged high levels of air to leaf water vapor pressure deficit (ALVPD). Decline of gs in P. euphratica was followed by decrease of E. Water use efficiency (WUE) of leaves near the desert were higher in the morning and the evening, in contrast to leaves from the former river bank that maintained an almost stable level throughout the day. High light compensation points and high light saturation levels of PN indicated the characteristics of leaves well-adapted to intensive irradiation at both stands. In general, leaves of P. euphratica decreased their gs beyond 20 Pa/kPa ALVPD in order to limit water losses. Decrease of E did not occur in both stands until 40 Pa/kPa ALVPD was reached. Full stomatal closure of P. euphratica was achieved at 60 Pa/kPa ALVPD in both stands. E through the leaf surface amounted up to 30% of the highest E rates, indicating dependence on water recharge from the ground despite of obviously closed stomata. A distinct leaf surface temperature (Tleaf) threshold of around 30°C also existed before stomata started to close. Generally, the differences in gas exchange between both stands were small, which led to the conclusion that micro-climatic constraints to E and photosynthesis were not the major factors for declining tree density with increasing distance from the river.