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Increasing atmospheric carbon dioxide levels could change the nature of grasslands and decrease their usefulness as grazing pastures, say researchers.

The study was published in the journal Proceedings of the National Academy of Sciences this week (27 August).

If carbon dioxide levels in the atmosphere continue to rise, important grazing areas in parts of Africa, Kazakhstan, Mexico, Mongolia, and southern and South East Asia could be under threat, according to lead author Jack Morgan, a plant physiologist from the US Department of Agriculture's Agricultural Research Service.

Morgan and colleagues constructed six clear chambers on semi-arid grasslands — used for grazing livestock — in Colorado, United States. The grasslands have a similar climate and plant communities to many grasslands in the developing world.

Half of the chambers contained carbon dioxide at today's levels, while the other half were fed levels double that of today.

Over the course of five years, the researchers found that the plant community in the chambers with elevated carbon dioxide levels changed dramatically. Woody shrubs — in particular, a shrub called fringed sage or Artemisia frigida — thrived.

Morgan told SciDev.Net that the shrub comprised less than one per cent of vegetation at the beginning of the study, but became ten per cent of the plant cover by the end.

The main reason why these woody shrubs out-compete grasses in conditions of high carbon dioxide, says Morgan, is because their method of photosynthesis is better suited to high levels of the gas.

The major concern, he says, is that woody shrubs like fringed sage are unpalatable to most domestic livestock, so domination by these types of plants would render land poor for grazing.

Guy Midgley, chief specialist scientist at the South African National Biodiversity Institute told SciDev.Net that there is already evidence of shrub encroachment in many grasslands of the world.

He said that it is still not clear whether carbon dioxide is the main driver of this change. But nevertheless, Morgan's experiment "suggests that we really need to take carbon dioxide more seriously".

Midgley said that in parts of South Africa high levels of bush encroachment may make cattle ranching impossible without expensive mechanical intervention.

Both Midgley and Morgan agree that a possible way to lessen the transformation of grasslands is to use controlled burning, which kills shrubs but not grasses, and to prevent overgrazing, which weakens grasses and allows woody plants to move in.

Reference: Proceedings of the National Academy of Sciences doi:10.1073/pnas.0703427104 (2007)