Higher CO{-2} May Imperil Grasslands
Scientists warn of dire consequences for grazing areas
http://www.commondreams.org/archive/2007/08/28/3461/
by Alan Zarembo
Rising levels of carbon dioxide in the atmosphere might be contributing to the conversion of the world’s grasslands - critical for livestock grazing - into a landscape of useless woody shrubs, according to a study released Monday.
By artificially doubling carbon dioxide levels over enclosed sections of the Colorado prairie, researchers created a dramatic rise in Artemisia frigida, commonly known as fringed sage. 0828 03
The study paints a harsh picture of what grazing lands could look like in 2100, when some estimates project carbon dioxide levels will be double today’s.
“To the extent that CO{-2} is driving this conversion, this suggests the problem is going to become more intractable in the future,” said Jack Morgan, a plant physiologist at the U.S. Department of Agriculture and lead author of the study, published in the Proceedings of the National Academy of Sciences.
Scientists say they believe the degradation of range lands, which cover about 40 percent of Earth’s land surface, is mostly the result of overgrazing and the modern practice of putting out fires rather than letting them burn, which destroys woody vegetation.
But researchers have long suspected that rising carbon dioxide levels also play a role.
Since at least the dawn of the Industrial Revolution, when carbon dioxide levels began to rise with the burning of fossil fuels, large swaths of the world’s seasonal grasses favored by livestock have been replaced by woody shrubs.
The concentration of carbon dioxide has risen from 280 parts per million at the end of the 18th century to 385 parts per million today.
To test the effects of the greenhouse gas, scientists set up open-topped cylinders of clear plastic, 15 feet in diameter, on a prairie 40 miles northeast of Fort Collins, Colo.
They pumped pure carbon dioxide into one group of cylinders, maintaining a concentration of 720 parts per million. The level in another group of cylinders was left at atmospheric concentrations and used as a control.
Each plot was analyzed every July, the end of the peak growing season. Then half the vegetation was removed to simulate grazing.
After five years, the researchers found a fortyfold increase in the biomass of fringed sage, from 0.72 of a gram per square meter in the first year to 28.7 grams per square meter in the fifth year. The area it covered increased from 0.2 percent to 4.1 percent.
The trend suggested that, given time, the sage eventually could squeeze out the grasses, Morgan said.
Woody shrubs have the ability to use carbon dioxide more efficiently than many grass species and have deeper roots than grasses, allowing them to tap into deeper water supplies.
Still, some scientists said carbon dioxide concentrations were likely to play a smaller role compared to overgrazing and fire suppression.
“The ranchers, through their own practices, have a stronger influence on the landscape than CO{-2} is likely to have, at least in the foreseeable future,” said Jeffrey Dukes, a biologist at the University of Massachusetts Boston who was not involved in the study.
Copyright 2007 Los Angeles Times
