Shifts in the height‐related competitiveness of tree species following recent climate warming and implications for tree community composition: the case of common beech and sessile oak as predominant broadleaved species in Europe

Height growth is a trait that contributes to tree species fitness. How height growth responds to environmental changes may therefore provide indications on species ability to compete and maintain, and on changes in tree community composition. Common beech Fagus sylvatica and sessile oak Quercus petraea are the predominant late‐successional broadleaved species in Europe, and they differ in their shade‐tolerance. On common beech (a shade tolerant species), recent observations across Europe have shown a growth decline during recent climate warming. Because sessile oak is a warmth‐ and light‐demanding species, we therefore hypothesised that it may gain in competitiveness relative to common beech. We conducted analyses of historical height growth in several regions spanning the distributional range of the two species across a temperate‐continental gradient in France. Common beech and sessile oak were sampled in two and four regions, respectively, and were compared in two neighbouring regions. We documented the climatic and nutritional conditions of regional samples. Height growth of 408 trees of various ages was reconstituted from stem analyses. We estimated 20th‐century regional chronologies of height growth using a statistical modelling approach that filtered out the effects of ontogeny and site fertility. In regions where both species were sampled, modelled height trajectories were compared at different periods over the 20th century. Growth chronologies revealed 1) long‐term growth rate increases of a magnitude of 50–100% over 100 years in both species, more acute in the continental domain, 2) recurrent historical inversions in growth fluctuations between species, 3) a recent divergence, with growth decline in common beech versus a dramatic growth increase in sessile oak, more acute in colder regions. The analysis of height trajectories indicated a recent reduction in common beech competitiveness relative to sessile oak. In the face of future climate warming, we conclude that increased prevalence of beech–oak mixtures may arise.