Productivity changes in four Sessile Oak forests since 1930: a stand-levelapproach.

We addressed the topic of longterm growth trends by using stand-level data gained in a network of permanent plots measured since 1930 in 4 Oak forests of northern France. These forests spread along a climatic gradient, from t he atlantic sites (Normandie) to the more continental sites (Lorraine). In each forest, 10 plots have been observed, in stands of various ages, and su bmitted to different silvicultures. We analysed dominant height and stand b asal area increments. The modeling of height-age curves revealed different curve shapes between forests. In 2 forests out of 4, the residuals exhibit a very strong structure with date: growth rate has steadily increased since the 1930s. Basal area increment was modelled with much more details, takin g into account the effects of age, stand density, periodic fluctuations and different growth levels between plots. These effects are combined with a s mooth trend function of date, the amplitude of which was fitted on a per fo rest basis. Several models were compared and we discussed the sensitivity o f trends with regard to the underlying model. Whatever the model, we could estimate trends of considerable amplitude: most of them are increases of ba sal area growth of +25 to +50% between 1930 and 1990. Our results confirm v ery closely those gained by dendrochronological methods. Curiously, Oak sta nds from Normandie have a more complex trend: decreasing productivity betwe en 1930 and 1960, then increase until now.