Cattle graze on the TK Ranch, northeast of Hanna. Such grassland could be taken over
by greenhouse gas-boosted woody plants cattle don’t eat, a new study says.
Greenhouse gases threat to prairie pastures
Rising carbon dioxide levels boost growth of woody shrubs cattle won’t eat, study finds
TOM S P EARS
The gases that cause global warming threaten to turn the Prairies from grasslands into an area covered by woody shrubs that cattle can’t eat, a new study says.
But it’s not the warmer temperatures that are to blame.
The main greenhouse gas causing global warming is carbon dioxide, and rising carbon dioxide levels favour the growth of woody plants — but not grasses.
“Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers,” says the study by the U.S. Department of Agriculture.
“This process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.”
The Department of Agriculture scientists in Colorado got their results by blowing extra CO2 gas into greenhouses in Colorado, simulating the future atmosphere as we keep burning coal, oil and gas.
The scientists knew ranchers have been saying for years tough little shrubs are taking over some grasslands. But they needed proof.
They set up six greenhouses on shortgrass prairie typical of large areas in Canada’s Prairie provinces. Three greenhouses had normal air. The other three had increased carbon dioxide levels.
The greenhouses with extra CO2 produced 40 times more than the normal amount of fringed sage, a woody plant that grows from 10 to 60 centimetres high. The change took place over five years.
“A40-fold increase in biomass (amount of the plant material) is huge,” said lead researcher Jack Morgan of the USDA.
Elk and antelope will eat this plant, but cattle won’t. And other experiments suggest other types of sage and small shrubs may also invade grasslands as the atmosphere changes.
This is more than a North American problem, the scientists say. It’s likely to affect grasslands used for grazing by all kinds of livestock in many parts of the world.
These are mostly lands too dry for most farming, but good for grazing.
Morgan says the slow invasion of woody shrubs has been happening for as much as 100 to 200 years, and has been reported in many parts of the world.
“That’s one of the major points in this paper. Most people, when they hear climate change discussed, think it’s always about the future,” he said.
“CO2 has been going up for the past couple of hundred years, so this relates to changes that are already underway.”
Past analysis has blamed these changes on overgrazing and bad land management, he said. The sage is more common in degraded grasslands.
The new study is published today in Proceedings of the National Academy of Sciences, a science journal.
“We are embarking on a period in which plant community dynamics in many of Earth’s ecosystems may be subject to relatively rapid changes due to increased atmospheric CO2 levels and altered climate,” it says.
“These changes may affect not only the traditional goods and services we have come to expect from these lands, but may alter their biodiversity as well. “Our report has implications for other rangelands around the world,” it adds.
“Rangelands are important not only for the plant and animal products they provide, but also as regions in which distinct pastoral cultures and societies have developed.”
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