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A long-term study by Colorado State University ecologists suggests warmer nights are producing a lengthened growing season and changes in prairie vegetation in the shortgrass steppe of eastern Colorado and surrounding states. The researchers studied 23 years of climate and vegetation data which suggests a warming trend which could lead to longer growing seasons. The longer growing season seemed to favor cool-season grasses and weeds over native warm vegetation like blue grama grass. Colorado State doctoral student Richard Alward (now at the University of Nebraska); James Detling, professor of biology; and Daniel Milchunas, research scientist in the university's Natural Resources Ecology Laboratory and rangeland ecosystems science department, studied how warmer nights have affected blue grama grass amounts. A key factor for the warming is an increase in nighttime cloud cover which traps heat near the earth's surface, Alward said. "Blue grama is a very drought and grazing tolerant grass, and under normal conditions it'll hang in there," Detling said. "It's very sensitive to water and is more productive under dry conditions. But the results of our study indicate it may be sensitive to temperature as well, particularly minimum temperature." The researchers found the average annual minimum temperatures are increasing at about twice the rate of average annual maximum temperatures at the Shortgrass Steppe Long-Term Ecology Research site (LTER), where the data was collected. The data shows the last killing frost of the spring is occurring earlier and the first killing frost of the fall is coming later. "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. "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," said Alward, who received a NASA Global Change Fellowship to support his research. "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 and may eventually out-compete blue grama in the short grass steppe ecosystem." 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. There is not conclusive proof 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 the 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 Nation- al Science Foundation-supported long-term ecological research areas in North America and elsewhere. |
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