Shrubs have been steadily encroaching on traditional rangelands from the Great Plains in the United States to Mongolia and Kazakhstan for at least two centuries, and in the past 10 to 15 years, ecologists have linked the trend to increasing levels of carbon dioxide in the atmosphere.

The study provides the first piece of evidence to support that theory and suggests that these kinds of environmental conditions favour shrubs over native grasses, presumably because the plants are better able to use CO2 for photosynthesis.

With carbon dioxide levels projected to increase sharply by the end of the century, the findings have serious implications for the management of these open rangelands which cover about 40 percent of the Earth's surface and are typically used to support livestock, the authors of the paper said.

"As the shrubs displace the grasses, the forage quality of the land decreases and the land becomes less valuable as a place where livestock can graze," said Jack Morgan, a research scientist at the US Department of Agriculture's facility in Fort Collins, Colorado.

"People may have to consider a change in land-use whether it's tourism or carbon storage."

To probe the influence of CO2 levels on the semiarid cattle ranch country of northeastern Colorado, Morgan and colleagues with the USDA's Agricultural Research Service set up an experimental field station northeast of Fort Collins with glass enclosures.

They pumped carbon dioxide into the enclosures, boosting CO2 concentration to double current levels in order to simulate the conditions that climatologists expect will prevail by the end of the century.

At the end of five years, there had been an explosion in the growth of a small woody shrub called Artemisia frigida. The plant's weight or biomass had increased 40-fold, and its coverage had increased 20-fold.

The results support the notion that rising atmospheric CO2, which is partly a function of burning fossil fuels, have contributed to shrubland expansions of the past 100 -200 years, the authors wrote.

"The increased proportion of woody plants has reduced significantly the available forage in many world grasslands and, without proactive management measures like burning, has rendered these lands less suitable for livestock grazing," they pointed out.

The paper appears in the journal of the Proceedings of the National Academy of Sciences.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.