Summary:

The research area is located in Uroczysko Warta, Czeszewo Forest District, Jarocin Forest Division. The Uroczysko Warta is a forest, situated on flood plain terrace on the left bank of the Warta River, between 332 and 337.5 km of its course and the confluence section of the Lutynia River (0 to 3.2 km). Mean annual precipitation for the period of 1951 - 2000 is 546 mm. Variability of annual precipitation is very high: every 10 years precipitation is ca 380 mm and a cyclicity is observed for episodes of wet and dry years. Assessment of seasonal variability of ground water level was performed on the basis of monitoring data for the period from March 2009 to May 2011collected from 26 piezometers. In addition data series of water stages from 7 gauging staffs installed in oxbow lakes and Lutynia River were utilized. Groundwater levels were measured automatically. Flood plain forests are the most complex forest ecosystems. River channelization, construction of embankments, river damming and construction of reservoirs resulted in reduc-tion of flooding events fraquency and degradation of flood plain forests. In Poland only 0.2% of total area of flood plain forest habitats is covered by flood plain forests. Uroczysko Warta forest is one of the most important locations of these sites. The construction of the Jeziorsko reservoir in the middle course of the Warta River has resulted in changes of the river regime. Thus, a system of water engineering constructions with hydrologic monitoring was implemented in Uroczysko Warta to restore hydrological conditions similar to natural to preserve flood plain forest ecosystems. Water storage of soil, strongly related with groundwater level, shows long-term and short-term variability. Short-term changes are related to current weather conditions and use of water by vegetation. Long-term is the effect e.g. vegetation development processes. Due to relatively short monitoring period, the paper presents the results of short-term variability of groundwater level. Groundwater level patterns in the Uroczysko Warta Forest are strongly related to oxbow lakes storage sates (correlation index 0.758, p<0.000001). Variability of groundwater levels is relatively high, maximum weekly changes are higher than 20 cm. Seasonal patterns of groundwater changes are apparent in average values 153.6 for the Winter, 206.5 for the Spring, 239.7 for the Summer and for the Autumn - 235.8 cm b.g.s. Seasonal patterns are also visible for the maximum and minimum groundwater levels. The results of groundwater level monitoring indicate that the system of hydrologic protection of flood plain forests implemented in Uroczysko Warta Forest is effective.