FORT COLLINS--Bison trampling across the Great Plains for the past 10,000 years helped condition an entire ecosystem for livestock grazing today, a Colorado State University research team has discovered in a long-term study.
Results from a 15-year study on 200,000 acres of grassland in eastern Colorado suggests livestock grazing has little or no effect on the shortgrass steppe ecosystem, a finding that could be applied to other areas of the Great Plains with similar ecosystems. The study shows that plants adapted to grazing by developing extensive root systems and short, bushy features above ground. Even some animal species, such as prairie dogs and mountain plover, rely on grazed portions of the shortgrass steppe for habitat.
"Through our experiments we consistently find that, contrary to widespread belief, this particular ecosystem is well-suited for livestock grazing," said Associate Professor Ingrid Burke, the project's lead researcher. "Both plants and animals have adapted survival strategies in light of a long-term presence of bison in the ecosystem."
Colorado State's Shortgrass Steppe Long Term Ecological Research (LTER) project is one of 20 funded by the National Science Foundation. The study, which began in 1982, involves the Central Plains Experimental Range and Pawnee National Grasslands. Researchers focused on the semiarid Great Plains because it is a major wheat-producing region that also sustains livestock grazing. In addition, researchers are interested in the region because current atmospheric models indicate that climate changes caused by increased greenhouse gases will be greater in the grasslands than in most other parts of temperate North America.
The LTER project, which involves more than a dozen Colorado State and international scientists and 45 scientific studies worth about $12 million, dispels many myths about the shortgrass steppe's complex ecosystem. By reconstructing eastern Colorado's climate over the past 10,000 years through in-depth studies of the soil, the LTER project sheds light on how human activities and different land uses alter the atmosphere and ecosystem.
LTER experiments proved that key plant species serve as the "glue" that holds the grassland ecosystem together. Blue grama grass comprises as much as 80 percent of shortgrass steppe vegetation; without it, the soil becomes infertile and susceptible to erosion. While blue grama grass is resistant to drought and livestock grazing, it does not withstand cultivation agriculture so easily. It was once thought blue grama grass did not return to an area where it had been disturbed, but LTER scientists showed that blue grama grass recovers over many decades.
"We now know how this plant reacts when it is disrupted by human activities and the critical role it plays in the sustainability of shortgrass ecosystems," said Bill Lauenroth, another lead LTER researcher. "These kinds of findings, generated from long-term observations of the ecosystem, have important consequences for managing areas disturbed by human activity."
Using techniques to date C-14 and stable isotopes of carbon left by plants, scientists also determined the region was cooler and wetter 10,000 years ago than it is today--resulting in a landscape dominated by cooler-climate cottonwood trees, western wheatgrass and similar plants. About 8,000 years ago, eastern Colorado shifted dramatically to warmer and drier conditions that ushered in grasses and other plants suitable to a warmer environment. A second climate shift occurred 3,000 years ago, which caused the formation of widespread sand dunes in northeastern Colorado.
"We've tried to piece the history of this land together by studying what's right beneath our feet," said Gene Kelly, a soil scientist leading this area of research. "The whole idea is to be able to better understand what happened in the past as a window to what may happen in the future."
Kelly is testing a technique to measure the ratio of oxygen isotopes in the soil as a way to pinpoint average temperatures, amount of rainfall and where rainfall originated over the past 10,000 years. Rain coming from the Gulf of Mexico has a different oxygen isotope ratio than rain from the Pacific Northwest, allowing researchers to trace the origin of ancient weather patterns.
Human activities also affect the region's climate. LTER studies conducted by atmospheric scientist Roger Pielke Sr. show that land use patterns change the atmosphere. Pielke's research shows that irrigation causes more evaporation into the atmosphere, ultimately triggering more rain-producing clouds and thunderstorms. Pielke also uses computer models to study how land surfaces affect weather and climate, and at the same time how weather and climate alter vegetation and soil dynamics.
"We're not just looking at how these ecosystems work, but how human behavior has affected them in the past and may affect them in the future," Burke said. "A great deal of the Earth is in semi-arid regions like the one we are studying, so much of what we are able to find is applicable to those areas as well."
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