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

摘要

Terrestrial plants remove CO_2 from the atmosphere through photo-synthesis, a process that is accompanied by the loss of water vapour from leaves. 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. Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange. We find a substantial increase in water-use effi-ciency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various compet-ing 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-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.