The development of vascular features in stems of Populus x euramericana was studied in two adjacent floodplain forest stands near the Danube in Austria. The construction of a hydropower plant caused one of the stands to loose contact with the groundwater at the age of 12years, whereas the other stand remained uninfluenced. The investigation estimated the extent of vascular acclimation to altered site hydrology. Vessel lumen areas were separately measured for 26tree-rings from six trees. Vessel densities (VD) and the percentages of the total vessel-lumen area per tree-ring area (PLA) were determined. Our results suggest that vessel size and density are correlated with circumferential stem growth, which is in turn governed by site water supply. In relation to the control trees, basal area increment (BAI) of the hydrologically altered trees was 66% lower. BAI was positively correlated to vessel size and negatively to VD and PLA. Vessel diameters of the mature control trees were large (with a mean of 95m and maxima around 160m), while average VD (42no.mm-2) and PLA (31%) were within the normal ranges. In comparison to controls, the hydrologically altered trees reduced their vessel-lumen area by 15% (i.e. 8% in terms of diameter); average VD increased by 43%, and average PLA increased relatively by 19%. Vessel density proved to be the most sensitive indicator of hydrological alterations. Intra-annual development of vessel size indicated a rapid response to the hydrological conditions of the current growing season.