Diminishing CO2-driven gains in water-use efficiency of global forests

There is broad consensus that, via changes in stomatal conductance, plants moderate the exchanges of water and carbon between the biosphere and atmosphere, playing a major role in global hydroclimate. Tree rings record atmospheric CO2 concentration (c(a)) and its isotopic composition (C-13/C-12)-mediated by stomatal and photosynthetic influences-that can be expressed in terms of intrinsic water-use efficiency (W). Here, we compile a global W dataset based on 422 tree-ring isotope series and report that W increased with c(a) over the twentieth century, but the rates of increase (dW/dc(a)) declined by half. Angiosperms contributed more than gymnosperms to the slowdown, and in recent decades, dW/dc(a) for angiosperms was close to zero. dW/dc(a) varies widely across climatic regions and reflects pauses in emissions during the Great Depression and after World War II. There is strong spatial variability in climate forcing via an increasing W, which is weakening globally with time. Under rising CO2, most plants constrict their stomata, lose less water via transpiration and photosynthesize more efficiently. A global dataset of tree-ring isotope measurements reveals a slowdown in water-use efficiency gains over the twentieth century, with marked spatiotemporal variability.