Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise

Present-day forests use water more efficiently, probably owing to the effect of increased atmospheric carbon dioxide on leaf stomata, which partially close to maintain a near-constant level of carbon dioxide inside the leaves despite increasing atmospheric levels. Large increase in forest water-use efficiency Theory suggests that rising atmospheric CO 2 concentrations should increase the efficiency with which plants use water, but the actual magnitude of this effect in natural forest ecosystems remains unknown. An analysis of long-term measurements of carbon and water fluxes from forest research sites across the Northern Hemisphere has identified an unexpectedly large increase in water-use efficiency during the past two decades, coinciding with an increase of atmospheric CO 2 from 350 to 400 parts per million. This trend is often accompanied by concurrent increases in rates of photosynthetic uptake and carbon sequestration. The authors suggest partial closure of stomata — to maintain constant CO 2 concentrations in the plant leaves — as the most likely explanation for the observed trend in water-use efficiency. The results are inconsistent with current theory and terrestrial biosphere models. Terrestrial plants remove CO 2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves 1 . The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon 2 . Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange 3 . We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO 2 fertilization effect. The results suggest a partial closure of stomata 1 —small pores on the leaf surface that regulate gas exchange—to maintain a near-constant concentration of CO 2 inside the leaf even under continually increasing atmospheric CO 2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in