SCIENCE

Levels of atmospheric carbon dioxide, a potent greenhouse gas, have been on the upswing over the last century. How the earth's plant life, particularly trees, will react to the change remains unclear. Some researchers have proposed, however, that the rising concentrations will spur plant growth and thus allow them to store additional amounts of carbon dioxide, thereby mitigating the atmospheric increase to some degree. Now a report published in the journal Science disputes this claim. A four-year study of a forest in Switzerland indicates that additional carbon dioxide does not boost tree growth.

A team of researchers led by Christian Korner of the University of Basel sprayed a 500-square-meter patch of deciduous forest with excess carbon dioxide for four years. The mature trees within it were exposed to two tons of extra carbon dioxide each day during the six-month-long growing season, giving them access to almost 50 percent more of the gas than is currently in the atmosphere. Overall, there was no increase in stem growth or leaf production during the study period, although the trees did cycle carbon dioxide and rerelease in to the air more quickly. It turns out not all species reacted in the same way to the excess carbon dioxide, revealing a flaw in earlier studies that focused on a single species.

The new experiment still has drawbacks, its planners note, making it a "compromise between realism and precision." For one, the timescale may still be too short to fully understand the effects of increased carbon dioxide. It is also possible that belowground growth of root systems could potentially store additional carbon in the soil. Future work will need to ascertain whether the new results apply to conifers and tropical trees, among other varieties.