As CO2 Levels Rise, Trees are Using Water More Efficiently

As atmospheric carbon dioxide levels rise, trees are responding by using water more efficiently than anyone suspected, resulting in some short-term gains and longer term climate worries, according to a recent study published in the journal Nature.

The study, conducted by Harvard University, the U.S. Forest Service and several partner institutions, suggests that rising atmospheric carbon dioxide is "having a direct and unexpectedly strong influence on ecosystem processes and biosphere-atmosphere interactions in temperate and boreal forests," said Dave Hollinger, a plant physiologist with the Forest Service.

Researchers gathered their results from data collected form forests in the northeastern United States and other parts of the world. The results were surprising to the researchers, who found the trees use water more effeciently than anything predicted by state-of-the-art computer models.

Trevor Keenan, the study's lead author, said that an increase in water use efficiency could be seen as an unexpected benefit of increasing carbon dioxide levels.

"What's surprising is we didn't expect the effect to be this big," Keenan said in a Harvard news release.

"A large proportion of the ecosystems in the world are limited by water. They don't have enough water during the year to reach their maximum growth. If they become more efficient at using water, they should be able to take more carbon out of the atmosphere due to higher growth rates."

However, while there appears to be a short-term benefit to trees' response to higher carbon dioxide levels, the overall climate picture will remain grim if CO2 levels continue to rise.

"We're still very concerned about what rising levels of atmospheric carbon dioxide mean for the planet," Andrew Richardson, a study co-author, said.

"There is little doubt that as carbon dioxide continues to rise - and last month we just passed a critical milestone, 400 ppm, for the first time in human history - rising global temperatures and changes in rainfall patterns will, in coming decades, have very negative consequences for plant growth in many ecosystems around the world."

The Forest Service also cautioned of the perils of rising CO2 levels, noting that trees' higher water-use efficiency could lead to higher air temperatures, decreased humidity and decreased recycling of continental precipitation. "This could cause increased continental freshwater runoff, along with drought in parts of the world that rely on water transpired in other regions," the agency said in a statement.