New findings throw cold water on the view that the world's forests will dampen global warming by soaking up rising levels of carbon dioxide (CO2) in the atmosphere. According to a report this week at the Global Change and Terrestrial Ecosystems Conference in Barcelona, Spain, trees quickly lose their thirst for CO2 as they mature. If the results hold up in larger studies, they could force climate modelers to reassess how fast atmospheric CO2 will accumulate, and how high global temperatures might rise.
In one study, Stephan Hattenschwiler of the University of Basel, Switzerland, presented the first data on how trees grown to maturity respond to elevated CO2. His group studied tree rings in 30-year-old Mediterranean oaks growing near geothermal vents that exposed the trees to nearly double the ambient CO2 concentration. Exposed saplings showed 25% more growth in their base than controls from nearby plots. While the inner annual rings of exposed mature trees showed excess growth, their outer rings showed that after 25 to 30 years there was no difference between their growth and that of the controls. "Fertilization mostly occurs during the early phases of forest re-growth, and it disappears later when the canopy closes and factors other than carbon become limiting," says study co-author Franco Miglietta. Adds Stanford University ecologist Josep Canadell, "this is the first time somebody has shown such a dampening effect through time."
In a second talk, William Schlesinger of Duke University reported results from the first year of the Free Air CO2 Enrichment (FACE) experiment in a loblolly pine forest in North Carolina. At this site, researchers continually pump 50% more CO2 into three 25-meter-wide circular plots, compared to CO2 levels at control plots. Trees breathing in extra CO2 had only 12% more above-ground growth than untreated controls, while all the trees had roughly the same root mass, the researchers found. "This growth response was lower than what a lot of people have been crediting terrestrial ecosystems as being capable of," Schlesinger says.
The two studies paint a consistent picture that less CO2 is likely to be absorbed by forests than originally thought, Canadell says. For climate models, that's likely to spell a faster atmospheric CO2 buildup and more rapid warming.