The Great Plains face a surprising threat: Too many trees
Grist · LC · trust 54/100
To really understand what’s going on in the Nebraska Sandhills, Shelly Kelly told me, it’s helpful to see it from the air. From the window of her father’s Bonanza A36, we looked out over the heart of one of the most rural landscapes in the United States. The Sandhills are a 19,500 square-mile swath of grassy dunes embellished with grazing cattle, serpentine rivers, and sunflowers.
Kelly has spent her life on the prairies of the High Plains. As a child, she made bouquets of flowering grasses and hunted for mudpuppies in the interdunal buffalo wallows on her parents’ ranch. Now a rancher herself, she’s trying to preserve the landscape she loves against the usual suspects — overgrazing and overharvesting — and one surprising new threat: trees.
She pointed out my window to the prairie below. “You can see it there,” she said, pointing to a neat grove of Juniperus virginiana , eastern red cedar, deliberately planted by ranchers to protect calving heifers from the wind. These windbreaks are standard features on the Plains. But to the south of their orderly rows, a dozen cedars have popped up in the middle of the grass, scattering across the nearby range like race-runners breaking free of the pack.
This spreading of trees is known as woody encroachment — or the “green glacier,” as some call it, evoking the slow creep of tree species over grasslands — and it’s got ecologists and ranchers of the Plains on high alert.
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“We’re losing our biodiversity,” Kelly said. “We’re losing our resiliency.”
For ranchers, a woody takeover of their rangeland creates an obvious problem: less land to graze. The Texas A&M Agrilife extension estimates that woody encroachment over the 10 Great Plains states represents $4 billion in lost range production. Ecologists predict biodiversity losses would affect the insects, birds, and small mammals that populate the country’s remaining prairies. Everybody worries about what those thirsty trees will do to the already stressed water supply as the green glacier expands from the wetter climate in the eastern prairie states to the arid western regions.
Prairies — on the High Plains and around the world — thrive in cycles of stability and disruption. Fire, grazing, dry seasons, and the area’s particular mix of vegetation all play a role in preserving the healthy root systems that allow a diverse mix of grasses to continue growing year after year. Some heartier grasses with root systems 20 feet deep can keep the prairie green in a drought, while healthy soil can hold dormant seeds until the conditions are right.
Fire — both naturally-occurring and prescribed burns — has long been a feature of the prairie landscape. Suppressing it is part of what has allowed woody plants to seed, spread, and develop the thick bark that makes them more likely to then withstand future fires. Meanwhile, overgrazing reduces trees’ and shrubs’ grassy competition, allowing them to grow faster. Cattle and bison don’t eat the woody plants. Goats, sheep, elk, and other “browsing” species don’t eat enough of them to control the spread, though some are experimenting with browsers as a component of a larger management plan.
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The Flint Hills of Kansas and Osage Hills of Oklahoma are home to two-thirds of the nation’s remaining tallgrass prairie, said Brian Obermeyer, The Nature Conservancy’s Great Plains grasslands strategy director. The persistence of the prairie in this region is largely because of the longstanding culture of prescribed burns, dating back to when Chisholm Trail cattle drives would divert their skinnier livestock to fatten up on the highly nutritious, tender, juicy grasses that grow back after a burn. Obermeyer even found a lease contract dating back to 1915 requiring the land to be burned regularly so that cattle would have access to the new growth. But in regions where that practice is not common, Obermeyer said, “Getting people comfortable with the culture of prescribed fire takes time.”
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