Woody Weed Replacing Grasses in Eastern Colorado

Rising carbon dioxide in the atmosphere may be changing the makeup of Colorado’s grasslands from grasses that are hospitable to livestock grazing to woody plants that cow don’t eat but which some wildlife species do.

Research  by Colorado State University scientists indicates that rising CO2 levels may be responsible for the encroachment of fringed sage, Artemisia frigida, which is generally considered a weed.

Jack Morgan, a plant physiologist with the U.S. Department of Agriculture’s Agricultural Research Service in Ft. Collins, says:

“Native grasslands that are pure perennial forage grasses are the forage of choice for domestic livestock. When shrubby plants move in, they don’t prefer that vegetation. Once the ecosystem moves from grasslands to shrublands, it becomes less and less useful for livestock grazing.”

Morgan and colleagues are the authors of a paper that is being published today in the Proceedings of the National Academy of Sciences. Carbon dioxide is an important nutrient for plants, and it has been hypothesized that the increase in CO2 that is causing a warmer world would spur plant growth, perhaps providing some benefit in the form of increasing food supplies.

The CSU group found that there was a substantial increase in biomass production —  that is, more plant growth — but that the largest increases were the growth of fringed sage, which pushed out more desirable grasses.

They induced a doubling of carbon dioxide in test plots and followed the plant growth for five years, comparing it a plot maintained at current CO2 levels. They found:

“Although doubling CO2 over the COlorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day CO2. Growth at the doubled CO2 resulted in an approximately 40-fold increase in aboveground biomass and a 20-fold increase in plant cover of Artemisia frigida.”

Woody plants have slowly been encroaching on grassy rangelands for two hundred years, and several hypotheses have been offered for the situation, including overgrazing and fire suppression, in addition to the increase in atmospheric CO2. When the Industrial Revolution got under way in the late 18th century, atmospheric CO2 was about 280 parts per million. It is now above 370 ppm and is projected to reach 600 ppm by the end of the 21st century.

Morgan says, “You can extrapolate these results to grasslands around the world. Our research supports the notion that CO2 is part of the problem.”

The PNAS paper focused on eastern Colorado grasslands, but it has implications for rangelands around the world. About 40 percent of the earth’s land surface is rangeland, water-poor lands that are unsuited for intensive agriculture. They support a wide variety of pastoral cultures and societies from Siberia to sub-Saharan Africa.

Fringed sage is short — only from 10 to 60 centimeters tall (four inches to two feet) — but its potential impact is large. The paper says:

“Although small in stature, it is the most widely distributed and abundant Artemisia species in the world, occurring from Mexico north through primarily the western United States and Canada to Alaska. Although considered a native of the United States, it can also be found in Siberia, Mongolia and Kazakhstan.”

The intrusion of fringed sage may be bad for livestock, but it could have some beneficial effects. Woody plants store more carbon that grasses do. And some wildlife species eat it, notably pronghorn, elk and sage grouse.

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Dan Whipple

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