The width of tree rings of Pinus nigra Arn. trees growing near the ecological limits for the species, in the Vienna basin, Austria, showed a strong and positive correlation with spring-summer precipitation, indicating a dependence of growth on water availability during the growing season. During the late 20th century, tree rings grew wider than expected given the predicted relationship between rainfall and growth rate observed in the early 20th century. This resulted in models of the relationship between climate and growth rate systematically overestimating the total spring-summer (April-July) precipitation over the last half of the 20th century. Analysis of the temporal stability of the relationship between tree growth and climate variables shows a decrease in the sensitivity of the growth of tree rings to spring-summer precipitation towards the end of the 20th century. This change in sensitivity suggests that tree growth was no longer primarily dependent on water availability. We propose that there was an improvement in water-use efficiency arising from a stimulation of photosynthesis and declining stomatal conductance as a consequence of the increasing CO₂ concentration in the atmosphere and that this effect was enhanced by a relatively high input of N due to the proximity of N emission sources.