Transpiration of mature floodplain forest stands in the alluvium of the Dyje river in South Moravia has been monitored for more than three decades. The monitoring was carried out both under original natural conditions and after the water management measures. The transpiration (actually sap flow measured by the trunk heat balance method) was monitored in pedunculate oak (Quercus robur L.), European ash (Fraxinus excelsior L.) and small-leaved lime (Tilia cordata Mill.) during growing seasons of ten years. Tree transpiration represented over 80% of potential evapotraspiration under conditions of non-limited soil water. Actual transpiration decreased significantly under limiting soil water conditions following the major groundwater level decline after the canalization of the Dyje river.