Plant physiologists Jack Morgan (left) and Dan
LeCain measure photosynthesis in prairie grasses to determine how elevated CO2
affects growth. Exposing native vegetation to doubled concentrations of carbon
dioxide in open-top chambers for 5 years reduced forage quality due to a
lowering of plant tissue nitrogen. Click the image for more information
More Forage, But Less Filling
By Rosalie Marion
January 18, 2005
Continued elevated carbon dioxide
concentrations in the atmosphere may reduce forage quality among the world's
grasslands and lead to reduced weight gain among animals, according to
Agricultural Research Service scientists
and cooperators. Their five-year study was published in the journal Ecological
Morgan led the study with ARS colleagues and cooperators at
Colorado State University. Morgan
heads the ARS
Resources Research Unit in Fort Collins, Colo.
Atmospheric concentrations of carbon dioxide have been rising steadily
during the last 150 years. The compound is considered a major greenhouse gas
because of its ability to trap heat near the Earth's surface. Fossil-fuel
burning, forest clearing and industrial manufacturing account for most of the
increased carbon dioxide emissions.
An intergovernmental panel on climate change has estimated that atmospheric
CO2 concentrations will double over today's levels by the end of the 21st
The experiment was conducted on native shortgrass prairie in northern
Colorado. The plant community tested was characteristic of vegetation grown in
semi-arid grassland for thousands of years. To simulate elevated atmospheric
carbon dioxide conditions and make comparisons, the scientists used six
open-top chambers. Each chamber contained more than 25 different plant species,
but was dominated by three perennial native grass species.
Three of the chambers were infused with ambient air containing about 360
parts per million (ppm) carbon dioxide to model present atmospheric conditions.
The other three chambers were infused with ambient air that had been injected
with pure CO2 to double the amount of carbon dioxide to 720 ppm.
The scientists found that forage quality declined in all three dominant
grasses under the elevated carbon dioxide conditions, due largely to lower
tissue nitrogen content. Further, they found that production of the least
desirable of the three dominant grasses, Stipa comata (needle-and-thread
grass), significantly increased under elevated CO2, while production in the two
higher-quality grasses remained unchanged.
ARS is the U.S. Department of
Agriculture's chief scientific agency.