The Earth’s carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata. However, uncertainties in the magnitude and consequences of the physiological responses of plants to elevated CO2 in natural environments hinders modelling of terrestrial water cycling and carbon storage. Here we use annually resolved long-term 13C tree-ring measurements across a European forest network to reconstruct the physiologically driven response of intercellular CO2 (Ci) caused by atmospheric CO2 (Ca) trends. When removing meteorological signals from the 13C measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO2 increase, Ci increased by 0.76 ppmv, most consistent with moderate control towards a constant Ci=Ca ratio. This response corresponds to twentieth-century intrinsic water-use efficiency (iWUE) increases of 14 ±10 and 22 ± 6% at broadleaf and coniferous sites, respectively. An ensemble of process-based global vegetation models shows similar CO2 effects on iWUE trends. Yet, when operating these models with climate drivers reintroduced, despite decreased stomatal opening, 5%increases in European forest transpiration are calculated over the twentieth century.This counterintuitive result arises from lengthened growing seasons, enhanced evaporative demand in a warming climate, and increased leaf area, which together oppose effects of CO2-induced stomatal closure. Our study questions changes to the hydrological cycle, such as reductions in transpiration and air humidity, hypothesized to result from plant responses to anthropogenic emissions

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ABSTRACT. New records of δ 13 C and ∆ 14 C values in annual rings of pine and oak from different sites around the world were obtained with a time resolution of 1 yr. The results obtained for Europe (Poland), east Asia (Japan), and South America (Peru) are presented in this paper. The δ 13 C and radiocarbon concentration of α-cellulose from annual tree rings of pine and of the latewood of oak were measured by both accelerator mass spectrometry (AMS) and liquid scintillation spectrometry (LSC). The values of 14 S, which represent decreasing 14 C concentrations caused by the emission of CO 2 from fossil fuel use (Suess effect; Suess 1955), were calculated for each site. Low average 14 S (about -0.4 to 0.8%) values for clean areas and high values (about 3.4-3.6%) for industrial and/or urbanized areas were noted. Records of the δ 13 C values obtained for pine and oak from Poland were used to reconstruct climate changes during the last 400 yr. The results clearly indicate the climate cooling during the periods of the Maunder minimum (1645-1715) and the Dalton minimum (1790-1820). The anti-correlation between the δ 13 C and ∆ 14 C records during those 2 periods is clear if the 14 C record is shifted toward older ages by 24 yr.