The quantification of radial growth losses and an analysis of the causes and mechanisms of oak decline in the Haguenau forest (Alsace, northeastern France) were made based on a dendroecological analysis of 450 trees. Comparison between defoliated and non-defoliated trees proved the substantial negative impact of defoliation by Lymantria dispar L. on radial growth (39 % loss on average). These losses were entirely due to a decrease in latewood width while earlywood remained constant, suggesting the triggering role of winter carbon storage in tree mortality. The trees which died during this decline showed significantly lower levels of growth since the beginning of the XXth century than survivors. Among the factors explaining these susceptibility differences, soil variation appeared to play an important role in Quercus robur decline.