Aug. 28, 2007 — Rising carbon dioxide levels are almost certainly fueling the encroachment of shrubs on global grasslands, a trend that could eventually jeopardize the use of these lands for cattle grazing, according to a new study.
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 favor 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.
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"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.
