Study links greenhouse gas to changing ecology of global rangelands
Pastor Dale Morgan
Study links greenhouse gas to changing ecology of global rangelands*
CHICAGO, Aug 27 (AFP) Aug 27, 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 study released Monday.
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.