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

Abstract

The Earth’s carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata^1,2,3. However, uncertainties in the magnitude^4,5,6 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 storage¹¹. Here we use annually resolved long-term δ¹³C tree-ring measurements across a European forest network to reconstruct the physiologically driven response of intercellular CO₂ (C_i) caused by atmospheric CO₂ (C_a) trends. When removing meteorological signals from the δ¹³C measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO₂ 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 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.