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THE hope that forests will soak up the extra carbon dioxide humans emit into the atmosphere has been dealt another blow. Instead, pollutants such as sulphur dioxide could greatly reduce the amount of CO₂ trees can consume.

Martine Savard and her colleagues from the Geological Survey of Canada in Quebec came to this conclusion after analysing the ratio of carbon isotopes in the rings of spruce trees near a copper smelter. Trees preferentially use CO₂ that contains the common carbon-12 isotope, but if the leaves can鈥檛 absorb enough fresh CO₂ they are forced to use more molecules containing the rarer carbon-13.

Even though levels of CO₂ in the atmosphere are rising, Savard found that in trees near the smelter the ratio of carbon-13 to the more common carbon-12 has been rising too. This, she says, suggests that they are having difficulty getting enough of the gas. Trees farther from the smelter had a much lower ratio of carbon-13 to carbon-12.

Savard calculated that trees near the smelter were taking in 25 per cent less CO₂ than normal. The difference is the pollution, she says. Sulphur dioxide enters the leaves through tiny apertures called stomata the same way CO₂ does. 鈥淪O₂ creates a reaction within the leaves that changes the pH and causes the stomata to close,鈥� she says. The damaged leaves can鈥檛 capture as much CO₂.

And the damage is long-lasting. Even though SO₂ emissions at the smelter were cut by more than 90 per cent in 1980, the trees鈥� CO₂ consumption is still hampered. Previous research has demonstrated the negative effects of SO₂ on CO₂ consumption in greenhouses, but Savard is the first to report the phenomenon in the field.