Trees don't suck up carbon dioxide as hoped
Forests do not get a growth spurt from greenhouse gas.
Trees don't seem to grow any faster when given an exrtra dose of carbon dioxide, Swiss scientists have found. Their study could shatter the widespread belief that rising concentrations of carbon dioxide may be kept partly in check by blossoming plant growth.
Some researchers have suggested that as carbon dioxide levels rise, plants will thrive on the gas, which they use to photosynthesize; trees may be prompted to grow faster and grasses to spread, for example, which would help to suck up some of the excess carbon dioxide.
But a study of a large patch of deciduous forest near Basel in Switzerland, which has been artificially sprayed with excess carbon dioxide for years, has shown no such increase in growth.
"Some scientists and politicians cling to the idea that a carbon-dioxide-rich future might favour the greening of planet Earth. It's time to disillusion them," says Christian Krner, a plant ecologist at the University of Basel who led the study. "What remains is the greenhouse gas effect," he adds.
Simulating the future
The team artificially created sustained carbon-dioxide-rich conditions in the patch measuring 500 square metres by spraying pure carbon dioxide into the canopy of about a dozen mature deciduous trees. Each day during the six-month annual growth season, the scientists sprayed two tons of extra carbon dioxide, from industrial waste, into the canopy. This simulated an atmosphere loaded with about 530 ppm of carbon dioxide, roughly 1.5 times what exists today.
But after four years the researchers found no signs of enhanced biomass growth in stems or leaves, they report in Science1. The trees had merely pumped the extra carbon through their bodies, quickly re-releasing it through root and soil microbe respiration; there was no lasting effect on growth and photosynthesis.
To rule out confounding factors, the team determined the extent of natural variations in tree growth, with the help of the tree-ring record, during a two-year pre-treatment study. They were also careful to select a multi-species forest in the middle of its life that is widely undisturbed by human interference.
Given the limited duration and extent of the experiment, it is too early to say whether the results can be generalized, says Krner. The team was unable to include conifers in their study, for example.
It also remains to be seen whether a fraction of all this extra carbon might be stored in the soil rather than in the trees. If so, there may still be cause to think that forests will suck up more carbon dioxide in a warming world.
Krner says his carbon-dioxide fertilization technique should be applied in three or four large experiments in different vegetation zones, from boreal forests to tropical rain forests, with an international board of scientists overseeing the studies. "This is the only way to settle the fundamental question of how changes in the air affect the bulk of the Earth's biomass," he says.
The results of some previous small-scale experiments have suggested that carbon-dioxide enrichment does stimulate plant growth. Yadvinder Mahli, for example, a plant ecologist at the University of Oxford, found a small biomass increase in the Amazon rain forest over the past 25 years (ref 2). How much of this can be attributed to increased carbon dioxide is unknown, however.
"The Swiss study is fantastic in terms of methodology and species mix," says Mahli. But it is too early to come to general conclusions, he says. "A similar experiment in tropical forests should definitely become a top priority."
- Krner C.Science, 309. 1360 - 1362 (2005).
- Malhi Y. . The Carbon Balance of Forest Biomes (eds Griffiths, H. & Jarvis, P.G.) (Taylor and Francis, Oxford).2005).
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