Spatiotemporal patterns of tree growth as related to carbon isotope fractionation in 1 European forests under changing climate 2 of forest growth in Europe

spatiotemporal patterns of forest growth dynamics associations with carbon isotope fractionation processes inferred from tree rings as modulated We characterize changes in the relationship between tree productivity and carbon isotope fractionation over the 20 th century using a European network. Using indexed tree-ring widths 93 (TRW i ), we assess shifts in the temporal coherence of radial growth (synchrony) for five forest ecosystems (Atlantic, Boreal, cold continental, Mediterranean and temperate). We also examine whether TRW i shows increased coupling with leaf-level gas exchange, inferred from indexed carbon isotope discrimination in tree-ring cellulose ( Δ 13 C i ), through enhanced stomatal regulation in response to amplified drought stress spreading northwards. Increased growth synchrony from the first to the second half of the 20 th century in Mediterranean, temperate and cold continental forests, together with a tighter relationship between TRW i and Δ 13 C i , indicate increasing drought effects on productivity across Europe. Such recent tendency towards exacerbated moisture-sensitive forest growth could override a positive effect of enhanced leaf intercellular CO 2 concentration, resulting in forthcoming declines in forest carbon gain continent-wide.