Water-use efficiency and transpiration across European forests during the Anthropocene

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 CO 2 in natural environments hinders modelling of terrestrial water cycling and carbon storage. Here we use annually resolved long-term δ 13 C tree-ring measurements across a European forest network to reconstruct the physiologically driven response of intercellular CO 2 (C i) caused by atmospheric CO 2 (C a) trends. When removing meteorological signals from the δ 13 C measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO 2 increase, C i increased by ∼0.76 ppmv, most consistent with moderate control towards a constant C i /C a 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 CO 2 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 CO 2 -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.