Tree species growth of Quercus robur/Quercus petraea and Pinus sylvestris across Europe - dynamic sensitive equilibrium

Quercus robur/Quercus petraea and Pinus sylvestris are light demanding, drought resistant wide spread European tree species occupying a similar ecological niche on mesotrophic xeric and mesic sites. Increasing drought and heat-spells will decrease their growth, but growth reductions depend on the timing of drought within a year, site characteristics, and inter- and intraspecific competition. In this study we analysed tree ring series of 1589 trees growing on 36 triplets installed across Europe, spanning a gradient of mean annual temperatures from 5.5°C to 11.5°C. Tree ring data were analysed using a generalized additive model with random effects for triplet, year and tree level and a random slope for age, explaining 87 % of the variation of diameter increment. High values for potential evapotranspiration in spring and autumn, indicating warm temperatures, positively and non-linearly influenced diameter increment of both species, while the effect in June was linear and negative. Across Europe, June was the most influential month for tree growth. Further, tree growth declined with age and local density and increased with social position. Modelled random patterns showed no large scale trend, when plotted per site and year indicating highly site specific shifts in competitive advantages.