In terms of curbing climate change, trees are man's best friend. As they grow, they pull carbon dioxide out of the air and convert it into sugars that add bulk to their trunks. This carbon storage ability is so well-known it has spurred an international tree-planting movement to slow global warming.

But drought could compromise the ability of trees to protect us from climate change, according to a new study. Scientists have shown that drought slows tree growth for many years beyond the initial dry spells, creating what researchers call a “drought legacy.” And scientists trying to predict climate change “could be really missing the boat,” if they are not including the effects in their computer models, says Melinda Smith, a community ecologist at Colorado State University, Fort Collins, who was not involved with the work.

To determine the true toll of droughts, Princeton University ecophysiologist William Anderegg and colleagues turned to the International Tree-Ring Data Bank, which stores 100 years or more of tree-ring data from more than 1500 nontropical areas of the world. In these temperate zones, trees lay down new trunk each summer but go dormant in winter, creating a pattern of rings that track the intensity of annual growth. The broader the area between two rings, the more productive the tree and the more carbon it stores.

Examining tree-ring data from Europe, North America, and parts of Asia between 1940 and 2008 (a time with reliable weather data), researchers discovered that, in trees with at least 25 years’ worth of rings, these layers were thinner than usual not just in years of droughts, but on average for 3 or 4 years afterward. Scientists had known droughts could damage tree tissues, leaving a drought legacy, but they didn’t know how long the legacy lasts. As they report online today in Science, trees growing in drier climates, such as the U.S. Southwest, fared the worst.

"The study represents our best understanding of how long the trees need to recover after drought disturbances at a broad range," says Yongguang Zhang, an ecosystems ecologist at the GFZ German Research Centre for Geosciences in Potsdam, who was not involved with the work. "Drought legacy effects will reduce our carbon storage in the future."

Anderegg and his colleagues do not know what leads to the prolonged downturn in growth. It could be that the hydraulic system plants use to pull water into leaves breaks down. "It's a big unknown in the field as to how much plants can repair their hydraulic systems," he notes.

Anderegg's team also checked predictions made by computer programs that look at the big picture: climate warming caused by rising carbon dioxide levels. The most sophisticated models factor in plant growth to calculate carbon storage potential, but these models were inaccurate when it came to tree growth in the regions that Anderegg and his team studied. "There’s a reasonably large disconnect between reality and the models," Anderegg says.

That’s a little worrisome, says Milena Holmgren, a plant ecologist at Wageningen University and Research Centre in the Netherlands, who was not involved in the research. "The paper shows that current climate-vegetation models are not able to reproduce the observed legacy effects and are therefore underestimating the effects of drought on vegetation and carbon cycling."