Annual biomass production of the tree layer on poplar sites in the floodplain forests along the river Danube seems to be dependent upon the onsite photosynthetic active leaf mass, which in turn is related to the water supply of the site. To investigate the validity of this working hypothesis a study was conducted in 33 sample plots in old growth hybrid poplar stands of different site classes. Assuming the validity of the above hypothesis such variables, which relate to site water supply like plot elevation, fine sediment depth and water storage capacity of the fine sediment pack were evaluated. From the first two variables together with groundwater observations the duration of groundwater contact could be derived. In each sample plot LAI was measured with a LICOR Canopy Analyzer. In a subset of plots these measurements were checked against LAI as measured with litter traps. For each sample plot yield class of the stocking stand was derived from age, DBH and height measurements. The investigated sample plots could be grouped into three categories: i.e. poor growth sites, good growth sites and excellent growth sites. Individual correlation of LAI with the independent variables water storage capacity of the soil (r2=0.47), depth of the fine sediments (r2=0.51) and average duration of groundwater contact (r2=0.38) was highly significant. These independent variables, which characterize site water supply, arc well suited to predict LAI, as well as site yield class. LAI is predicted by the above variables with a r2=0.68. When fine sediment depth is more than 200 cm and water storage capacity of the top 100 cm soil is more than 300 mm a LAI of 5.8 and 6.1 m2*m-2 and a yield class, expressed as average annual increment, of 27.2 and 27.4 m3*ha-1 can be expected. Duration of groundwater contact with the fine sediment seems to play an even more pronounced role. When duration of groundwater contact drops below 2% of all days during the growing season, average LAI decreases to 3.2 m2*m-2 and yield class to 16.5 m3*ha-1*a-1. From these relationships it seems that LAI may be a useful and good bioindicator for site water supply in the poplar floodplain forests.