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(1-3). However, uncertainties in the magnitude(4-6) and consequences(7,8) of the physiological responses(9,10) of plants to elevated CO2 in natural environments hinders modelling of terrestrial water cycling and carbon storage(11). Here we use annually resolved long-term delta C-13 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 delta C-13 measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO2 increase, C-i increased by similar to 0.76 ppmv, most consistent with moderate control towards a constant C-i/C-a ratio. This response corresponds to twentiethcentury 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.