Transpiration of fully grown floodplain forests growing on heavy alluvial soils in southern Moravia was studied during 10 years over the period of 27 years under changing environmental conditions and impacts of water management measures. The study period characterized the situation under last regular floods in early seventies, about 20 year long transition period with no floods and decreased underground water table and several years long recent period, when water table increased again due to controlled watering. Transpiration was estimated through continuous sap flow measurement over the whole growing seasons using stem segment heat balance method applied on a series of sample trees (mostly oaks, to much less extent also ash and lime). Floodplain forests growing under non-limiting soil water supply and high evaporation demands transpired high amounts of water approaching up to about 80% of potential evapotranspiration. This amount was supplied up to 70% from underground water sources and 30% from precipitation. Lack of soil water as well as soil air supply limited transpiration, absorption of water decreased in locally long-term flooded trees. Water consumption of forests significantly decreased after water management measures, which led to decreasing underground water table. Then only about 10% of transpired water were supplied from underground, the rest was dependent on local precipitation. Trees with unfavorable root/shoot ratio suffered relative drought due to insufficient hydraulic conductivity of slightly desiccated soil around orginally developed (during the flood period) small root systems as shown before. Such a situation can endanger functional stability of forests if dry years would occur. Trees already adapted to changed water supply conditions during about 16 and more years, were able to absorb about 30% of water from underground sources for their transpiration. Studied floodplain forests are recently (24 years after cessation of floods) functionally adapted to changes of water supply and after a period of increased danger of tree decline by drought have now a better chance to survive. It can be expected that controlled periodical soil water and air supply will further improve the situation, if already occurred adaptation of forests will be considered.