It took less than a century after John Deere unveiled his steel-bladed plow in 1837 for the North American prairie to all but disappear. For 20 million years, a nearly 1,000-mile-wide swath of unbroken grassland belted the continent’s midsection from northern Canada to Mexico. Now, only about 5 percent is left, mainly as mixed and shortgrass prairie in the Plains states. To the east, less than 1 percent of the original lush tallgrass remains, most of it as remnants in pioneer cemeteries and old railroad rights-of-way.
Plowed up, paved over, and little lamented, the vanishing prairie found few early champions. Among them were naturalists Aldo Leopold and John Curtis, who began using Civilian Conservation Corps enlistees in the 1930s to help restore more than 110 acres at the University of Wisconsin–Madison Arboretum. One of the earliest attempts at habitat restoration, the site today has hundreds of species of native plants, birds, and small mammals.
Now, prairie restoration is attracting widespread interest among environmental scientists, conservation groups, and even the U.S. government. The first federal grassland preserve, Midewin National Tallgrass Prairie, opened 3 years ago on the grounds of the former Joliet Army Ammunition Plant near Chicago. Thousands of ordinary midwesterners are also rediscovering their long-spurned heritage, working to preserve or restore patches of prairie in fallow cornfields, quarter-acre backyard plots, and an expanding network of preserves. How-to Web sites instruct landowners in restoration techniques, and seed companies specializing in prairie species are thriving. Prairies now rank among the most popular ecosystems targeted for restoration anywhere, especially the tallgrass of the Midwest’s eastern third.
But researchers are left wondering how well the prairie renaissance is really succeeding and whether it’s actually possible to re-create a prairie. Until recently, little long-term monitoring had quantified the success rates of common restoration techniques, and few studies had compared even the most careful restorations with scarce remnant prairie habitat.
Broad-scale comparisons are complicated by the fact that restorations serve a range of different purposes. Beyond bringing back native plants, some restorations focus on conserving fresh water or creating habitat for birds. About 40 percent of North American bird species are native to prairie.
Because restoring prairies is both labor- and cost-intensive, some restorations are seeded with only a fraction of the plants that a remnant prairie holds. Seed mixes usually contain relatively few species and some of those species are difficult to grow from seed.
Real prairies are highly diverse. “In a remnant prairie, you can find 150 to 180 species of plants,” says Deborah Marr, a plant ecologist at Indiana University in Bloomington. In western Indiana, Marr and her colleagues have been comparing restored prairie with slices of the original in nature preserves and along railway rights-of-way. Across the board, the remnants have more native-plant species. Over a 4-year study period, plant diversity increased in the restored prairies, but the proportion of grasses and flowering broad-leaved plants diverged from that found in remnant prairies.
A study during the 1990s of sites around the Fermi National Accelerator Laboratory in Batavia, Ill., where restoration efforts began in 1975, also found that species richness declined over time in restored sites, but not in remnants. The Fermi restorations had never achieved the biodiversity of remnants to begin with. High diversity, a Holy Grail to prairie ecologists, so far eludes their restorations, but no one is sure why.
“A remnant is very complex,” notes Marr. Blazing wildflowers and rippling bluestems only hint at the complexity below ground. “It takes a long time for soils to build up,” she adds. Unlike cropland, prairie soils are rich in fungi, which appear to be an essential component of high diversity. In August, at a conference in San Jose, Calif., Indiana University researcher Peggy Schultz reported on field trials that suggest that adding soil from prairie remnants, or at least inoculating restorations with the kind of fungi found in remnants, can allow hard-to-establish plants to take hold.
Obtaining the variety of seed needed to start a restoration can be an arduous, months-long task involving painstaking hand picking by squads of volunteers. As a result, many restorations rely on mail-order seeds that typically include grass cultivars or wildflowers from various sources. The grass cultivars were originally bred by the U.S. Department of Agriculture to hold topsoil in place. They are now planted on millions of acres as part of the USDA’s Conservation Reserve Program, which pays farmers to plant erosion-taming native grasses on land removed from agriculture.
In the late 1990s, though, plant biologist Sara Baer, then a graduate student at Kansas State University in Manhattan, began noticing something unexpected while doing research at the 3,487-hectare Konza Prairie Biological Station in northeastern Kansas, part of the largest remnant tallgrass prairie in North America. The grass cultivars in her sites were germinating readily and growing fast and tall. In some situations, that would be desirable, but here, the robust grasses were crowding out slower-growing native flowering plants and disrupting the balance of species. Productivity was easy to restore in the prairie; diversity much less so.
At around the same time, plant ecologist David Gibson and his students at Southern Illinois University Carbondale (SIUC) began finding major genetic differences between cultivars and wild seeds. Their photosynthesis rates also differed. Furthermore, genetic differences existed between local and nonlocal wild seed.
Since then, some prairie enthusiasts, passionate about making restorations as faithful to the original undisturbed prairie as possible, have begun avoiding mail-order seed mixes, instead hand gathering wild seed only within 200 miles of their site. Gibson cautions that little research supports any particular approach.
With a 5-year grant from the National Science Foundation, Gibson and Baer (now also at SIUC) hope to tease out some guiding principles for restorations. At three sites in Kansas and Illinois, the scientists are planting wild seeds and cultivars of prairie grasses and wildflowers in multiple plots and in various proportions. “No one has ever put the same plant species, but from different seed sources, in a common environment before,” says Baer.
Cultivated seed will make up from 4 percent to 97 percent of each plot’s mix. The idea, says Baer, is to see whether tinkering with proportions can help establish and maintain a truly diverse prairie.
“Production of prairie seeds is a big business, and there are all these species that people can choose to put in,” Baer says. “Restorations are unique, in that by our decisions, we humans are an integral filter.”
Historically, bison grazing and fire were the two natural filters that shaped and maintained the prairie. Until they were nearly extirpated in the 19th century, along with the prairie itself, bison by the tens of millions ranged across North American grasslands, often in herds so big that observers compared them to roaring avalanches. Fires, set by lightning strikes and later by Native Americans, would attract bison and other herbivores, because the burned patches sprouted fresh green grasses that the animals prefer to graze on. At the same time, bison avoided the tender broad-leaved plants, or forbs. This kind of preferential grazing established a system of checks and balances, which kept grasses under control and allowed many plant species to flourish.
Researchers at Kansas’ Konza Prairie and elsewhere have begun to see how bison encourage habitat diversity by grazing very heavily on burned patches and avoiding other areas altogether. Heavily disturbed habitats, for example, attract some native birds. Other native birds prefer completely undisturbed habitats; and still others, such as prairie chickens, require a mix of habitats. The same holds true for insects and small mammals.
Over time, fire drove the bison’s behavior, which in turn shaped the prairie’s biodiversity. But fire by itself is not enough to restore diversity to the prairie, says ecologist Scott Collins of the University of New Mexico in Albuquerque. Various studies by Collins and his colleagues have shown that frequent burning by itself can reduce biodiversity. Collins’ work at Konza shows that species diversity rises in areas that are grazed and infrequently burned, and falls in frequently burned, ungrazed areas. “Diversity is much higher at all levels in the grazed areas,” he says.
“If I had a prairie to restore,” says Collins, “my recommendation would be that some kind of grazing, or at least mowing, to eliminate the big, thick grass canopy and create more light, take place.” Studies by other researchers indicate that bison and other native herbivores like to eat many nonnative, exotic plants, which helps suppress the invasions that plague grasslands.
Deron Burkepile of Yale University, who studies native herbivores in North American and South African grasslands, says, “I think that grazing is essential for restoration. More and more people are starting to adapt that mind-set.”
Those findings are borne out by the sight of newly installed bison herds silhouetted against the sky at a growing number of prairie preserves. Still, little is known, even now, about eons-old grazing patterns and fire frequencies. If annual burning causes plant diversity to fall, then how often should controlled burns occur? Studies indicate that burning every 4 years probably isn’t frequent enough to keep out trees and woody shrubs.
Adapting to the future
Ironically, figuring out historical fire frequency at Konza may not be relevant today, says Collins. Human-driven environmental change imposes new conditions on prairies that could make restoration more challenging than ever. Already, encroachment by nonnative shrubs, bushes, and other woody plants is afflicting grasslands around the world, with or without controlled burning.
And the past itself is a moving target, points out Alan Knapp, a plant ecologist at Colorado State University in Fort Collins who is also doing studies at Konza Prairie. “We have a romantic, snapshot view of the prairie when Europeans settled it,” he says. “But ecological systems are always dynamic, always changing.”
Prairies evolved under blazing summers, harsh winters, and extreme fluctuations of temperature and rainfall from year to year and within growing seasons. To adapt, prairie plants developed underground-storage structures and extensive root systems. Scientists have recently discovered that prairie species grow more variably from year to year, depending on rainfall variations, than do plants in any other North American biome. That variability makes remnant prairies, such as Konza, good natural laboratories for studying the likely effects of future climate change.
Most climate models predict extreme and variable rainfall patterns and future temperature increases. To study those hypothetical effects, scientists at Konza Prairie are manipulating rainfall and temperature under canopied shelters where native prairie grasses, such as big bluestem and Indian grass, grow. Altering the timing of rainfall from current norms can lead to significant declines in the plants’ productivity. “It’s surprising how rapid the changes have been,” says researcher Melinda Smith of Yale University.
In a new, related study at Konza Prairie, Smith is profiling the genetic activity of two grasses under simulated climate-change conditions. Some regulatory genes may become less active when the grasses are stressed by alterations in precipitation. Smith and colleagues hope to identify specific genetic changes linked to the plants’ responses to environmental changes. That should, in turn, reveal implications for large-scale ecosystem processes.
One early surprise is the large amount of genetic diversity that already exists within populations of native dominant species. “You can find 14 genotypes of big bluestem in 1 square meter,” says Smith. “Diversity within dominant species is often ignored.”
Tapping that genetic diversity may in time offer the best shot for keeping grasslands vibrant under future conditions that will be vastly different from those of today.