A long-term study by Colorado State University ecologists suggests that warmer nights are producing a lengthened growing season and changes in prairie vegetation on the shortgrass steppe of eastern Colorado and surrounding states.
Working at Colorado State’s Shortgrass Steppe Long-Term Ecological Research (LTER) station, the ecologists drew on 23 years of climate and vegetation data that suggest a warming trend that could lead to longer growing seasons. Those longer seasons, in turn, seem to favor cool-season grasses and weeds over native warm season plants like blue grama grass. The team’s findings appear in the current (Jan. 8) issue of the journal Science.
Colorado State doctoral student Richard Alward; James Detling, professor of biology; and Daniel Milchunas, research scientist in the university’s Natural Resource Ecology Laboratory and rangeland ecosystems science department, looked at how warmer nights appear to have contributed to a decrease in blue grama, a hardy forage plant favored by livestock and grazing wildlife.
While temperature data are localized, they support global climate change studies that suggest an overall warming. A key factor is an increase in nighttime cloud cover that traps heat near the earth’s surface, Alward said.
"Blue grama is a grass that is very drought resistant and grazing tolerant, and under normal conditions it’ll hang in there," Detling said. "It’s very sensitive to water and is more productive under moist than under dry conditions. But the results of our study indicate it may be sensitive to temperature as well, particularly minimum temperature."
That minimum temperature also is known as the nighttime low temperature. The trio found that average annual minimum temperatures are increasing at about twice the rate of average annual maximum temperatures (the daytime highs) at the Shortgrass Steppe LTER site. In terms of the growing season, that finding implies that the last killing frost of spring comes earlier and the first killing frost of fall later — bad news for blue grama.
"One hypothesis is that cool-season, non-native plants may be able to utilize water and nutrients stored in the soil for a longer time by starting earlier in the season, long before the daytime temperatures are warm enough for blue grama growth," Detling said.
According to Alward, who received a NASA Global Change Fellowship to support his research, "Blue grama makes up about 90 percent of plant cover on the shortgrass steppe, so it is far and away the dominant species out there. It’s the defining species of the shortgrass steppe ecosystem. If it declines dramatically, we have a new system, or at least a different one.
"Some other plant species might be able to take advantage of that different ecosystem, and that could lead to significant changes. These changes are not intrinsically bad. However, our concern is that the species that will take advantage of blue grama’s decline are invaders, species we tend to regard as weeds," Alward said.
Blue grama, found on the plains of eastern Colorado and surrounding states (Wyoming, western Kansas and Nebraska, New Mexico and northwest Texas), declines in a distinct relationship to nighttime lows, Detling said.
"For each one-degree-centigrade increase in average low temperature, blue grama growth decreased by one-third at our site," he said. "A number of cool-season plants, however, can exploit this change (to their advantage) and may eventually out-compete blue grama in the shortgrass steppe ecosystem."
The trio worked at the Central Plains Experimental Range, part of the Shortgrass Steppe LTER site 30 miles northeast of here on the western edge of the Pawnee National Grasslands.
The implications of climate change could affect more than just blue grama and the shortgrass prairie, Detling said. Blue grama has flourished in part because of its ability to survive both long periods of drought and constant grazing, formerly by bison and now by livestock. If cool-season plants supplant blue grama in the short term, Detling said, those plants may not be able to survive these same conditions, leaving the steppe bare following drought or heavy grazing.
Blue grama is also an important member of so-called mixed-grass prairies that stretch from eastern Wyoming north into Canada.
"If there is a cause-and-effect relationship between temperature increases and a decrease in blue grama, it could hold for the entire region," Detling said.
There isn’t conclusive proof that temperature changes and a longer growing season are causing blue grama to diminish, so the Colorado State team has designed further experiments to test their hypothesis. One unfinished, on-site project involves using automatic garage-door openers to pull a tent over experimental plots at night. This blocks re-radiation of heat from the soil and increases nighttime temperatures over the plots by two degrees centigrade, which will help determine blue grama’s response.
Colorado State’s Shortgrass Steppe LTER is one of 21 National Science Foundation-supported long-term ecological research areas in North America and elsewhere.
Some 25 scientists from Colorado State’s College of Natural Resources, College of Natural Sciences, Natural Resource Ecology Laboratory and other units are engaged in research at the Pawnee National Grasslands in northeast Colorado. Since 1982, Shortgrass Steppe LTER experimenters have looked at the dynamics of long-term ecological change, devising models and applying them to regional and larger systems. Recent projects have examined the effect of climate change on ecosystems.