Forests cannot be relied on to remove carbon dioxide from the atmosphere, according to a new study by Swiss researchers. The results call into question claims that forests could be used as carbon sinks to help control global warming.
The results of the four-year study, conducted by the Federal Research Institute for Forest, Snow and Landscape Research, show that trees reach "saturation point" in terms of the quantity of CO2 they can absorb.
Jürg Bucher, head of the institute's department of forest protection, said the study showed that even if more forests were planted, they would be unlikely to have much impact on the increased CO2 levels that could be expected in the future.
He said trees planted as part of the study had increased their growth rate when exposed to higher levels of CO2. But he said the effect had been only temporary, and that the growth rate had slackened by the end of the experiment, which lasted four years.
"If the growth had continued, it could have had a significant effect on CO2 levels," Bucher told swissinfo. "However, the fact that it stopped, suggests that the systems were more or less saturated.
"This means we cannot expect that forests will help us, under elevated levels of CO2, to absorb much more CO2 than they do now."
The planting of new forests to absorb CO2 has been widely touted by the United States and other countries as one way to combat global warming. Indeed, before the US president, George Bush, withdrew America's support for the treaty, Washington had suggested more forests be planted to meet quotas to reduce CO2.
Bucher said the study also showed that in a world where levels of carbon dioxide were higher than today, certain species of trees would proliferate while others would decline.
"The stability of the forests would be threatened because more CO2 could significantly alter the species composition of the world's forests. We can't say exactly what would happen but there could be more pests, for example."
Bucher cautioned that the study had limitations, but added it was clear that "forests would not solve the CO2 problem".
The researchers created models of ecosystems and exposed them to higher levels of CO2 to mimic climatic conditions that may emerge in the future.