Long-term course of transpiration in a floodplain forest in southern Moravia associated with changes of underground water table

Transpiration of fully grown floodplain forests growing on heavy alluvial s oils in southern Moravia was studied during 10 years over the period of 27 years under changing environmental conditions;Ind impacts of water manageme nt measures. The study period characterized the situation under last regula r floods in early seventies, about 20 year long transition period with no f loods and decreased underground water table and several years long recent p eriod, when water table increased again due to controlled watering. Transpi ration was estimated through continuous sap now 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). Flood plain forests growing under nonlimiting soil water supply and high evaporat ion demands transpired high amounts of water approaching up to about 80% of potential evapotranspiration. This amount was supplied up to 70% from unde rground water sources and 30% from precipitation. Lack of soil water as wel l as soil air supply limited transpiration, absorption of water decreased i n locally long-term flooded trees. Water consumption of forests significant ly decreased after water management measures, which led to decreasing under ground weller 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 develo ped (during tile flood period) small root systems as shown before. Such a s ituation can endanger functional stability of forests if dry year; would oc cur. 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 (2 4 years after cessation of floods) functionally adapted to changes of water supply and after period of increased danger of tree decline by drought hav e,low a better chance to survive. It can be expected that controlled period ical soil water anti air supply will further improve tile situation, if alr eady occurred adaptation of forests will be considered.