Intrinsic water use efficiency (WUE_i) in trees is an indication of the ratio of carbon assimilation to the rate of transpiration. It is generally assumed that it is a response to water availability. In agricultural research, the question of drought tolerance by increased WUE_i has been well studied. In general, the increase is a trade-off for productivity and is therefore not desired. For forest trees, this question is less clearly understood. Using stable carbon isotopes derived from tree rings combined with productivity as the product of the annual growth increment and annual density measurements, we compared the change in WUE_i over a 15 year period. While WUE_i increased over this period, the productivity decreased, causing an opposing trend. The gradient of the correlation between WUE_i and productivity varies between provenances and sites. Counterintuitively, the populations at the drier site showed low WUE_i values at the beginning of the investigation. Slopes vary with the provenance from Poland showing the least decline in productivity. In general, we found that a decline in productivity aligned with an increase in WUE_i.