The results should hammer home the message that the world can't rely on trees to solve the problem of CO2 emissions, according to William Schlesinger at Duke University in North Carolina, whose team carried out the work. "It throws doubt on nations such as the US who have carbon sequestration as their only strategy for dealing with the problem," he says.
Global CO2 emissions from sources such as car exhausts and industry are predicted to double between now and 2050. More CO2 means that trees will grow faster and lock up more carbon. This led some to hope that plants might mop up all the extra gas, says Schlesinger. But earlier experiments to find out how much CO2 plants can absorb have been inconclusive because they took place in sealed environments such as greenhouses. These can't maintain realistic outdoor climate conditions of temperature, humidity and rain.
To find out more, Schlesinger and his colleagues have monitored growth of mature trees in Duke Forest, North Carolina. They staked out six plots of trees with rings of 32 vertical pipes. Each plot is 30 metres in diameter. At three sites, the pipes pump out air enriched with CO2 to mimic conditions predicted for 2050; at the other three sites, they pump out normal turn-of-the-century air. The system monitors CO2 levels within the ring and adjusts delivery to maintain the right mix.
The team found that the trees in the 2050 atmosphere converted more carbon dioxide into plant matter, locking up 27 per cent more carbon than at control sites. However, even if this extra growth occurs in existing temperate forests all over the world in 2050, the trees will only absorb 10 per cent of human-generated CO2. "They will soak up carbon, but the study contradicts those who say they will soak up large amounts," says Schlesinger.
"The effect is not as large as people had expected," agrees Peter Cox at Britain's Meteorological Office in Bracknell, Berkshire. He says Schlesinger's results warn us that forests cannot solve the problem of global warming, and emissions need to be reduced. "Eventually you have to deal with the root cause," he says.
Will Steffen at the Royal Swedish Academy of Sciences adds that there are still uncertainties as to how much carbon forests can mop up. As CO2 levels rise, the atmosphere will warm, causing more fires in forests and releasing more carbon. The warmer climate could also speed up breakdown of leaf litter by microbes, releasing yet more CO2 into the atmosphere.
In this week's Nature (vol 416, p 617), Jeffery Richey of the University of Washington in Seattle and his colleagues say the breakdown of leaf litter is already releasing large amounts of carbon in the rivers and wetlands of the Amazon rainforest. This means the forest may be releasing as much CO2 as it absorbs, if not more.
On the other hand, woody species might colonise semi-arid areas of land in future because plants can retain water more efficiently in high-CO2 atmospheres. They would then help take CO2 out of the atmosphere. "We really must look at the whole ecosystem," says Steffen.
Author: James Randerson
More at: Oecologia (DOI 10.1007/s00442-002-0884-x)
New Scientist issue: 13th April 2002
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