Global Warming Will Have Mixed Effects on Eastern Colorado’s Grasslands

New research results from Colorado State University suggest that the effects of rising atmospheric concentrations of carbon dioxide and global warming will lead to an increase in grass production and a decline in forage quality for grasslands of eastern Colorado and Wyoming.

Study results suggest that both elevated CO2 and warming will increase grass production but the quality of the vegetation will decrease due to lower nitrogen concentration in the forage. William Parton, researcher from Colorado State’s Natural Resource Ecology Laboratory, or NREL, and researcher Jack Morgan from the U.S. Department of Agriculture’s Agricultural Research Service, USDA ARS, studied the effects of warming, increased CO2 levels and the combination of both factors on eastern Colorado grasslands to predict how global warming will affect these ecosystems.

Results revealed that elevated atmospheric CO2 levels always increase grass growth; however, warming can have both positive and negative impacts on plant production.

"The potential impact of elevated CO2 levels on Colorado and Wyoming grasslands is mixed since grass production will likely increase while digestibility of forage and cattle weight gains will likely decrease," Parton said. "Increased air temperatures will have a mixed impact with plant production increasing in wet years and decreasing in dry years."

These predictions are based on results from an ecosystem model developed using data from locally observed climatic change experiments that will continue during the next five to 10 years.

In this experiment, Colorado State and ARS scientists were able to use empirical knowledge from relatively short-term experiments combined with ecosystem models to predict long-term ecosystem responses to the effects of global warming. The scientists used results from a five-year-long CO2 enrichment experiment conducted in northern Colorado to test CO2 impacts in the Daycent ecosystem model. They also used field data from the experiment site, Prairie Heating and CO2 Enrichment or PHACE, located in southern Wyoming that will continue for the next five to 10 years.

"One of our biggest challenges is how to interpret relatively short-term experiments and predict the long-term global warming consequences on grasslands," Morgan said. "By taking the results from our field experiments and applying computer models tested using the observed field data, we are able to extrapolate beyond our short-term experiments into the future."

The scientists observed that doubling CO2 levels caused strong and consistent increases in grass growth which was due to improved water-use efficiency. Under the elevated CO2 levels, it was also found that plant nitrogen content was declining in native grasslands. This is a critical matter for livestock and for native animals that have grazed these prairies for thousands of years. Increased CO2 dilutes nitrogen concentration in grazing vegetation. Animals require sufficient forage protein nitrogen to sustain normal weight gains.

The results of this study are published in the April issue of New Phytologist.