Rising levels of carbon dioxide in the atmosphere could drive trees to make more efficient use of water, new research reveals. Investigators studied forests around Europe in order to understand how changing conditions over the 20th century affected woodlands.

University of Exeter investigators, along with colleagues from other institutions, found the water efficiency of forests in Europe increased by around 20 percent as atmospheric carbon concentrations increased.

Tree rings were examined in an effort to measure the content of atmospheric carbon over time, as well as the response of the organisms to changing climatic conditions. Samples taken from 23 tree ring sites located between Norway and Morocco were studied as part of the investigation.

Carbon dioxide is absorbed by trees through tiny pores in leaves known as stomata, which also serve to release excess water. Higher-than-normal levels of carbon dioxide cause these tiny holes to shrink in an effort to reduce the amount of gas passing into the organism. These smaller apertures also result in a reduction of water released from the leaves. 

For every pound of tree biomass created, the organism releases roughly 100 pounds of water through stomata. The exact ratio of this water to biological mass is a standard sign of the economy of such an ecosystem. Decreased diameters of stomata were found to increase water efficiency by 22 percent in needleleaf species of trees and 14 percent in broadleaf varieties.

"Tree-ring data provide one of the unique opportunities to obtain long-term records of ecosystem responses to climate change," said David Frank from the Oeschger Centre for Climate Change Research at the University of Bern and the Swiss Federal Research Institute WSL.

The team was able to reproduce the observed effects of increased carbon levels in the atmosphere in computer models, adding evidence to the idea that rising levels of the gas were driving increased water efficiency. Ratios of light carbon to heavy forms of the element were also examined in an effort to better determine how the climate changed over time.

This new study should not be interpreted as meaning that global warming, which accompanies increased levels of atmospheric carbon, is a good thing for the health of forests. Investigators showed that, when the effects of a warming climate are all taken into account, the amount of water running through trees increases by 5 percent, eliminating any potential benefit from shrunken stomata.

Examination of how increasing levels of carbon dioxide in the atmosphere affects the water efficiencies of European forests was profiled in the journal Nature Climate Change.

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