Recent changes in forest productivity: An analysis of national forest inventory data for common beech (Fagus sylvatica L.) in north-eastern France

Changes in forest growth have been found in European forests and worldwide. However most observations have been derived from samples of restricted size, whose representativeness at a regional forest scale is questionable. National forest inventories provide an interesting perspective for both regional scale assessment of these trends and the investigation of their variations over environmental gradients, but have been little used.The aim of our work was to carry out an exploratory modelling analysis of productivity changes, based on the French national forest inventory data. The objectives were: (i) to assess recent trends in forest productivity and to investigate a possible recent decline as found in previous studies; and (ii) to investigate trend variations relative to site fertility factors. We focused on pure and even-aged stands of common beech (Fagus sylvatica L.) in north-eastern France, already documented from previous studies based on retrospective data.The dataset consisted of 925 temporary plots inventoried between 1979 and 2007. We developed two regression models of stand basal area increment (BAI) against stand developmental stage (dominant height), site fertility (site index in the SI model, environmental indicators in the EI model) and stand density (relative density index). The effect of calendar date was tested in order to investigate possible historical trends. Site fertility–date interactions were also tested to investigate the site-dependence of trends.The fitted models showed a high goodness of fit (adj. R2 over 0.69). We showed an increase in stand BAI of 27.8% between 1977 and 1987, (10.4% between 1979 and 1987). Stand BAI then decreased by approximately 5% between 1987 and 2004. We thus confirmed the hypothesis of a recent decline in common beech vitality in its temperate range. The chronologies clearly depicted the effect of severe drought events (1976 and 2003), pointing out the predominant role of water availability in the changes observed. No significant site-dependence of the trend was identified.