Elevated Carbon Dioxide Spurs Shrub Growth
By Rosalie Marion
August 28, 2007
Shrubs far outgrew native grasses in
Colorado rangeland when exposed to elevated levels of carbon dioxide
(CO2), according to a study published by Agricultural Research
Service (ARS) scientists and cooperators
at Colorado State University.
The results suggest that rising CO2 levels in the Earths
atmosphere may be contributing to shifts in plant community dynamics, in which
woody vegetation is favored over perennial forage grasses.
The study will be published in this weeks online edition of the Proceedings of the National Academy of
Sciences. Plant physiologist
Morgan, research leader of the ARS
Resources Research Unit in Fort Collins, Colo., led the study. ARS is the
U.S. Department of Agriculture's chief
scientific research agency.
During the past 200 years, shrubs have expanded their reach into many of the
world's grasslands, reducing the amount and quality of forage available to
livestock. Some scientists theorize that elevated atmospheric CO2
concentrations have spurred that growth. But evidence as to the underlying
reasons behind the problem of woody plant encroachment has been lacking.
The experiment was conducted on native short grass prairie in northern
Colorado. To simulate elevated atmospheric CO2 conditions and make
comparisons, the scientists used six large open-top chambers. Three of the
chambers were infused with air injected with 360 parts per million (ppm) of
CO2to model today's atmosphere. The other three chambers were
infused with air injected with twice the control amountor 720 ppmof
CO2. That's roughly the atmospheric concentration expected by the
end of the century.
Among the 34 plant species exposed to 720 ppm of CO2, the
scientists found a 40-fold increase in aboveground biomass of fringed sage,
Artemisia frigida, a widespread shrub in rangelands of North America and
Asia. Growth of only one other species, Stipa comata, a relatively
low-forage-quality perennial grass, was also significantly affected by the
CO2, as previously reported. S. comata showed only a two-fold
increase in biomass due to CO2.
The results support present theories that rising CO2 may be
altering the botanical structure of world grasslands and contributing to their