European beech grows better and is less drought sensitive in mixed than in pure stands: tree neighbourhood effects on radial increment

Tree neighbourhood can affect the radial increment of Fagus sylvatica and its sensitivity to environmental fluctuation, which partly depends on soil clay content and species identity of the neighbours. In a temperate deciduous forest, we analysed the tree ring chronologies of 152 Fagus sylvatica L. target trees from tree neighbourhoods varying in species composition and tree diversity. We hypothesised that the species identity of the neighbour trees influences radial stem increment and environmental sensitivity of growth of the target trees. Further, we postulated that the effect was stronger under low abiotic stress as expressed by soil clay content and that beech individuals could have a higher wood production in mixed than in monospecific stands. We measured radial increment and analysed the growth response to, and recovery from, selected stress events. Fagus trees in a neighbourhood with more than 30 % of the canopy's 'influence sphere' occupied by allospecific trees had a higher mean stem increment, a lower increment sensitivity to environmental fluctuation and a smaller growth depression after the 1976 drought than beech trees surrounded by conspecific trees. We found that the neighbours' identity can influence beech growth: positive effects on mean increment and a reduced sensitivity were found for Tilia, Fraxinus and Acer neighbourhoods, but not for Quercus neighbourhoods. The growth-promoting effect was significant on clay-poorer soil, but not on clay-richer soil. Tree species diversity in the neighbourhood tended to correlate positively with mean stem growth and negatively with the sensitivity to environmental fluctuation. We conclude that the neighbourhood of a tree can influence its mean stem increment and growth sensitivity to environmental fluctuation in temperate mixed forests with the effect partly depending on the neighbours' species identity.