The ability of forests to absorb carbon dioxide from the atmosphere has been overrated, according to a new study by U.S. 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 in 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, Australia, 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 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 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."

Deep roots

After extending the study for a further five years, the latest modelling tempers the optimistic predictions from the 2005 analysis.

Medlyn says the study into how the trees would respond to increased concentrations of CO2 confirmed their hypothesis that there would be a point at which nutrient availability, specifically nitrogen availability, would limit the CO2 uptake of plants.

"The exciting thing about these results is that after another five years, we found that the CO2 response is not actually maintained," she said. "What that's telling us is that plants have the capacity to go looking for extra nutrition in the soil and that's what's happened here.

"They put down deeper roots, they found extra nutrition in the soil, and that's kept them going for a while, and certainly longer than we thought they would be able to. But after 10 years they've finally run out of nitrogen.

"The implication of that for the broader landscapes is that particularly in nutrient poor soils, the rising CO2 concentration in the atmosphere is probably not going to be as beneficial to plants as we've been hoping."

Guelph study agrees

The joint American-Australian study dovetails with recent findings from the University of Guelph in Ontario, which warn against relying on forests to soak up rising carbon dioxide emissions.

Geography professors Ze'ev Gedalof and Aaron Berg looked at archived tree-ring measurements from 2,300 forest sites on six continents held by the National Oceanic and Atmospheric Administration in the United States.

They found carbon dioxide fertilization spurred faster tree growth at only about 20 per cent of sites worldwide, despite the fact that the gas typically accelerates growth in plants.

“Eighty per cent of the world’s forests don’t care,” said Gedalof.

Medlyn argues that the models that were used in the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report to predict land-based CO2 sequestration did not include nitrogen feedbacks on the CO2 response, and therefore these predictions are too optimistic.

She says currently plants soak up about 25 per cent of human CO2 emissions. Previously, models had assumed that this was because plants are responding to rising CO2, and that they would continue to respond, taking up ever more CO2 out of the atmosphere.

"So the models are being revised at the moment to try and incorporate these nitrogen feedbacks," she said.

"Where they are incorporated, we generally find that the predicted carbon sink in the land over the next century really goes down a fair bit."