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Forests are not about to save the planet from greenhouse effect by soaking up carbon dioxide.
Under experimental conditions, extra doses of carbon dioxide encourage trees to grow faster. This has led to the idea that, as atmospheric levels of the gas rise due to industrial pollution, fast-growing forests could absorb some of the gas and slow down global warming.
Researchers have dubbed this the "CO2 fertilisation effect", and the forests "carbon sinks". But new research published this week by two teams of researchers suggests there is a big difference between what happens in greenhouse experiments and the real world.
The first team bathed trees in extra CO2 on research plots at Duke University in Durham, North Carolina. The trees grew faster for three years, but then reverted to their former growth rates. They only resumed faster growth when they were dosed with nitrogen fertiliser.
Supply and demand
In the real world, Ram Oren and colleagues conclude, "forest growth is limited by nutrient supply, in particular nitrogen", as much as CO2. Most forests are unlikely to be able to turn the extra CO2 in the air into plant growth.
And even where trees do grow faster and take up more CO2, most of it returns to the air again before long, warns William Schlesinger, also of Duke University.
In the second study, Schlesinger found that almost half of the CO2 absorbed by trees goes to form foliage rather than wood. Once the leaves fall to the ground, most of their carbon decays and returns to the air within three years - very little is taken up in soils.
"Carbon going into the soil has a rapid turnover time, so it seems we should not expect carbon in the soil to increase significantly," said Eric Davidson of the Woods Hole Research Center in Massachusetts, who reviewed the new studies for Nature.
Abandoned land
Climate analysts have estimated that around a fifth of industrial emissions of CO2 are reabsorbed by vegetation on land. The question is: where and how?
Davidson says the studies suggest that CO2-fertilisation cannot be responsible. It is also unlikely that acid rain is supplying extra nitrogen to allow trees to grow taller, he says. "Soils immobilise most of the nitrogen inputs".
More likely, he concludes, the CO2 is being absorbed by "the regrowth of forests on abandoned farm land and forest land harvested a few decades ago" in Europe and North America.
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