Aim The aim was to decipher Europe‐wide spatio‐temporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location Europe and North Africa (30‒70° N, 10° W‒35° E). Time period 1901‒2003. Major taxa studied Temperate and Euro‐Siberian trees. Methods We characterize changes in the relationship between tree growth and carbon isotope fractionation over the 20th century using a European network consisting of 20 site chronologies. Using indexed tree‐ring widths (TRWi), we assess shifts in the temporal coherence of radial growth across sites (synchrony) for five forest ecosystems (Atlantic, boreal, cold continental, Mediterranean and temperate). We also examine whether TRWi shows variable coupling with leaf‐level gas exchange, inferred from indexed carbon isotope discrimination of tree‐ring cellulose (Δ13Ci). Results We find spatial autocorrelation for TRWi and Δ13Ci extending over a maximum of 1,000 km among forest stands. However, growth synchrony is not uniform across Europe, but increases along a latitudinal gradient concurrent with decreasing temperature and evapotranspiration. Latitudinal relationships between TRWi and Δ13Ci (changing from negative to positive southwards) point to drought impairing carbon uptake via stomatal regulation for water saving occurring at forests below 60° N in continental Europe. An increase in forest growth synchrony over the 20th century together with increasingly positive relationships between TRWi and Δ13Ci indicate intensifying impacts of drought on tree performance. These effects are noticeable in drought‐prone biomes (Mediterranean, temperate and cold continental). Main conclusions At the turn of this century, convergence in growth synchrony across European forest ecosystems is coupled with coordinated warming‐induced effects of drought on leaf physiology and tree growth spreading northwards. Such a tendency towards exacerbated moisture‐sensitive growth and physiology could override positive effects of enhanced leaf intercellular CO2 concentrations, possibly resulting in Europe‐wide declines of forest carbon gain in the coming decades.

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Globally 40–70 Pg of carbon (C) are stored in coarse woody debris on the forest floor. Climate change may reduce the function of this stock as a C sink in the future due to increasing temperature. However, current knowledge on the drivers of wood decomposition is inadequate for detailed predictions. To define the factors that control wood respiration rate of Norway spruce and to produce a model that adequately describes the decomposition process of this species as a function of time, we used an unprecedentedly diverse analytical approach, which included measurements of respiration, fungal community sequencing, N2 fixation rate, nifH copy number, 14C‐dating as well as N%, δ13C and C% values of wood. Our results suggest that climate change will accelerate C flux from deadwood in boreal conditions, due to the observed strong temperature dependency of deadwood respiration. At the research site, the annual C flux from deadwood would increase by 27% from the current 117 g C/kg wood with the projected climate warming (RCP4.5). The second most important control on respiration rate was the stage of wood decomposition; at early stages of decomposition low nitrogen content and low wood moisture limited fungal activity while reduced wood resource quality decreased the respiration rate at the final stages of decomposition. Wood decomposition process was best described by a Sigmoidal model, where after 116 years of wood decomposition mass loss of 95% was reached. Our results on deadwood decomposition are important for C budget calculations in ecosystem and climate change models. We observed for the first time that the temperature dependency of N2 fixation, which has a major role at providing N for wood‐inhabiting fungi, was not constant but varied between wood density classes due to source supply and wood quality. This has significant consequences on projecting N2 fixation rates for deadwood in changing climate.

We explore needle sugar isotopic compositions (δ 18 O and δ 13 C) in boreal Scots pine (Pinus sylvestris) over two growing seasons.A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ 18 O and δ 13 C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool.Model results agreed well with observed needle water δ 18 O, δ 18 O and δ 13 C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ 13 C (R 2 = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO 2 concentration ratio (C i /C a ) were the dominant drivers of δ 18 O and δ 13 C variability, respectively. However, the variability of needle sugar δ 18 O and δ 13 C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C i /C a , due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical 18 Ofractionation factor and mesophyll resistance in 13 C-discrimination were critical.Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.

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