Assessing long-term changes in stand productivity:: A case study of sessile oak high forests

We present a stand-level assessment of long-term productivity changes in four sessile oak (Quercus petraea) high forests, using a mensurational growth and yield model. We estimated productivity trend curves as smooth functions of calendar date, in addition to factors such as age, stand density and site productivity. We used a network of 35 permanent plots observed over 60 years, for which age and date were reasonably independent, and silviculture had been accurately recorded. Stand basal area increment varied markedly over the past 60 years. At two of the four sites studied, the normalised trend in basal area increment increased by between 25 and 55%, depending on the underlying model for the age factor. The westernmost site (Normandie) showed a more complex trend (decreasing productivity until 1960, increasing thereafter). Problems remain concerning the generalization of these results to other species and regions, using dominant height as a summary of productivity. The extrapolation of past growth curves over the next 100 years is also discussed.