Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years

We investigated the response of conifer trees in northern Eurasia to climate change and increasing CO₂ over the last century by measuring the carbon isotope ratio in tree rings. Samples from Larix, Pinus and Picea trees growing at 26 high‐latitude sites (59–71°N) from Norway to Eastern Siberia were analysed. When comparing the periods 1861–1890 and 1961–1990, the isotope discrimination and the ratio of the intercellular to ambient CO₂ concentration (c_i/c_a) remained constant for trees growing in mild oceanic climate and under extremely cold and dry continental conditions. This shows a strong coordination of gas‐exchange processes, consisting in a biochemical acclimation and a reduction of the stomatal conductance. The correlation for c_i/c_a between the two investigated periods was particularly strong for Larix (r²=0.90) and Pinus (r²=0.94), but less pronounced for Picea (r²=0.47). Constant c_i/c_a under increasing CO₂ in the atmosphere resulted in improved intrinsic water‐use efficiency (W_i), the amount of water loss at the leaf level per unit carbon gain. We found that 125 out of 126 trees showed increasing W_i from 1861 to 1890 to 1961 to 1990, with an average improvement of 19.2±0.9% (mean±SE). The adaptation in gas exchange and reduced transpiration of trees growing in this region must have had a strong impact on the water and energy budget, resulting in a drier and warmer surface air layer today than would exist without this vegetation–climate feedback.