Growth and water use of Populus euphratica trees and stands with different water supply along the Tarim River, NW China

For the last four decades of investigation, the average tree-ring width and basal area increment (BAI), the cumulative Etree (May-August) and Estand were significantly higher on the plots with the largest water supply than on the plots with the largest distance to the groundwater. However, the maximum values of xylem sap flow density and Etree values averaged for the vegetation period did not differ significantly among plots and between sites. These transpiration values and the relatively large BAI even at a large distance to the groundwater indicate that-in contrast to previous studies-Euphrates poplars can grow vigorously even at a groundwater depth of up to 12 m. Supply with additional water resulted in a marked increase in BAI. Lower Estand at lower water supply can be explained with a sparser stand density. Interannual fluctuations in the water supply resulted in distinct alterations in BAI in trees that grow at a short distance to the water table or are supplied with additional water. Against the background of limited water availability in the region, we recommend concentrating the conveyance of additional water to forest stands with a short distance to groundwater to keep those forests fully functional and to maintain their ability to regenerate.