Australian Broadcasting Corporation:

The ability of forests to absorb carbon dioxide from the atmosphere has been overrated, according to a new study by US and Australian scientists.
In a paper appearing this week in the Proceedings of the National Academy of Sciences, Dr Richard Norby of the Oak Ridge National Laboratory, Tennessee, and colleagues argue that an essential element, nitrogen, had previously not been adequately factored into the equation.
One of the co-authors of the paper, Dr Belinda Medlyn of the Department of Biological Sciences at Macquarie University in Sydney, says the current study is a continuation of earlier research investigating Liquidambar styraciflua trees that were exposed to carbon dioxide (CO2) concentrations about 25 per cent higher than current levels.
“The CO2 concentration at the moment is about 390 parts per million (ppm) and the concentration in the study at Oak Ridge was about 550 ppm,” said Dr Medlyn.
“It depends on whether we manage to get our CO2 emissions under control, but 550 ppm will probably be reached somewhere in the second half of this century.”
The trees’ response to this higher CO2 environment was measured over a five year period. Initially they grew faster, and Dr Medlyn says modelling of the first five years of the experiment gave some quite optimistic results.
“In the first five years they didn’t find any reduction in the CO2 response,” she said.
“So basically what happened was the trees had a higher productivity level as soon as the CO2 was switched on, and they maintained that for the first five years.”

The rest here.