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As a result of their findings, the authors of both studies have called into question the growth models for worldwide vegetation being used in official climate change forecasting for this century. The results of the studies may also have implications for some types of forest carbon projects such as reforestation and improved forest management, depending on the growth models they use. Ex ante, or upfront, crediting under some standards may see too many credits issued on forecast carbon sequestration with issuances having to be revised after later verification events during a project’s lifetime. A study led by Dr Richard Norby of the Oak Ridge National Laboratory in Tennessee, and published in the Proceedings of the National Academy of Sciences, has found that while increased CO2 levels engender higher growth for the first five to six years, after that time the growth rate tails off. The study team, including US and Australian scientists, found the limiting factor was the fixed level of nitrogen in the soil. After five or six years, the extra soil nitrogen being used to fuel the growth of the trees starts to run out, preventing the trees from being continuing to make the most of the elevated CO2 levels. Researchers exposed forest stands to CO2 levels 25 per cent higher than the current global concentration, a level expected to be reached by the second half of this century. “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,” Dr Belinda Medlyn a biologist at Macquarie University, Sydney, said. She said the models used in the IPCC 4th assessment report are likely to overstate CO2 sequestration on land by “a fair bit”. A second study, from the University of Guelph in Ontario, Canada, looked at 86 types of trees at more than 2,300 sites on six continents. It found that 80 per cent failed to respond to higher CO2 levels regardless of their species or geographical location. The researchers drew their results from examining tree rings, the distinctive marks left on trees allowing researchers to see how much growth takes place from year to year. “There might be a very slight increase in the total rate of growth in trees, but they’re not going to be these vacuum cleaners that will magically suck up the CO2 that we’re emitting,” said Ze’ev Gedalof, study co-author and Associate Professor of Geography at the University of Guelph. Other experts examining the study questioned whether growth rates observed in tree rings give an accurate measure of overall carbon uptake. ABC Online, Canadian Press. Source: carbonpositive Some rights for the image is reserved under Creative Commons license
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| Last Updated ( Thursday, 04 November 2010 18:03 ) |
Two separate studies have thrown doubt over forests’ ability to help offset global warming. Because trees rely on carbon dioxide to grow, it has been predicted that as CO2 levels rise in a warming world, trees would thrive on the increase, grow faster, and thus help soak up excess atmospheric carbon. 
