South Dakota's stand against the 'Green Glacier'

South Dakota State University researchers and Extension specialists are fighting to protect North America's most endangered ecosystem: grasslands. 

It starts slowly. A tree here, a shrub over there.

Then a few years go by. Pastures that once lasted until September are now overgrazed by early August. Suddenly, trees and bushes are everywhere, and there's less grass available for cattle.

This is how the "Green Glacier" — a term coined by ecologists to describe the encroachment of unwanted trees — is slowly burying grasslands under a canopy of evergreen trees throughout the Great Plains. What started in Texas more than 70 years ago has slowly made its way up to South Dakota, and South Dakota State University researchers are in the midst of a challenging fight to halt this ecological disaster in its tracks.

It started with shelterbelts
In 1934, President Franklin D. Roosevelt initiated the "Great Plains Shelterbelt" project to protect farms and crops from severe dust storms. The nightmares of the Dust Bowl — the greatest man-made ecological disaster in history — left farmers scarred, and the federal government believed that by planting trees on the perimeters of farms, the soil would retain its moisture and be protected from wind erosion. By 1942, 220 million trees were planted, from Texas to the Canadian border, and many were eastern redcedar — an adaptable evergreen native to the eastern United States that is resistant to drought, heat and cold.

The trees were well-suited for the Great Plains. They could survive the tough and variable conditions and grew quickly in a variety of soils. They fit perfectly in the treeless landscape of the grasslands as they provided a sanctuary for livestock and a "living" snow fence during harsh winters. 

Unfortunately, eastern redcedars are proficient in propagating seedlings that spread out a few hundred yards away from the parent tree. Research shows that a mature female tree can produce over 1.5 million seeds annually. Their berry-like cones also attract certain types of birds, which spread the seeds even further out from shelterbelts.

The first signs of the Green Glacier were in the mid-1900s and appeared in tandem with the U.S. Forest Service's forest fire prevention campaign. The advertising campaign, highlighted by Smokey the Bear, is widely considered one of the most successful in history and created an "anti-fire" culture. But as researchers soon found, not all fire is bad, and for grasslands, fire may even be necessary.

Fire suppression allowed redcedars to sprout and flourish in rangelands that, prior to settlement, would otherwise have burned.

Ranchers were slow to react to the spread, and eventually, they lost large percentages of pastures and even entire ranches. This process, which SDSU professor and plant ecologist Lan Xu describes as "woody encroachment," spread north into Oklahoma, Kansas and Nebraska.

The endangered grasslands 
When crossing the Missouri River near Chamberlain, a dense canopy of cedar trees can be seen on either side of the riverbank. Fifty years ago, those trees weren't there. This is the most glaring evidence of the Green Glacier's march north into South Dakota.

"It's just awful," Xu said. "And it gets worse each year."

Researchers estimate that less than 40% of historical grasslands, that once stretched undisturbed from Canada to Mexico, remain today. Most are fractured and have been converted into segments of croplands, golf courses, suburbs or infrastructure. The grasslands that remain are found mainly on private land, used primarily for ranching and are under threat from woody encroachment.

Grasslands are one of the world's most under appreciated ecosystems. While not as revered as forests nor as picturesque as oceans, grasslands are crucial to climatic patterns and wildlife — especially in the U.S. Grasslands are high in biodiversity and absorb vast amounts of carbon dioxide in the roots of native plants and grasses. They also provide habitat for many bird species, like pheasants, and mammals, like bison and the black-footed ferret (the most endangered mammal species).

The encroachment of eastern redcedar is a threat to the entire grassland ecosystem. This invasion is especially concerning for native plants. Research shows that grassland species richness declines by 88% in the face of woody encroachment.

A recent study from Xu found eastern redcedar canopies reduce biomass production and alter the composition of plant communities, including native grasses.

The destruction of native grasses creates less habitat for wildlife. Research has found that grassland birds have suffered a roughly 50% decline in population since 1960. Mammals suffer as well. When woody encroachment cover exceeds 30%, the majority of small mammal species disappears.

These findings show that woody encroachment will throw off the entire balance of the ecosystems.

Woody encroachment creates less rangeland for cattle and other livestock. This is most evident in Texas, where large portions of rangeland are now forested. The Working Lands for Wildlife organization estimates the cumulative forage production lost to tree encroachment (between 1990 and 2022) equates to approximately 370 million tons. In South Dakota, rangeland production loses are estimated to be 209,671 tons in 2022 alone.

The Great Plains Grasslands Extension Partnership estimates that 22.4 million tons of rangeland production are lost annually to woody encroachment each year. And losses continue to increase each year.

Eastern redcedars also deplete soil moisture and can reduce prairie streams significantly.

Woody encroachment represents the single greatest threat to livestock production in the Great Plains and to the overall long-term viability of grasslands, considered by researchers to be North America's most endangered ecosystem.

South Dakota's stand
South Dakota is currently home to some of the largest remaining grasslands in the U.S. Unlike Texas, Oklahoma, Kansas or Nebraska, woody encroachment is just beginning here.

This is why Alexander Smart, an SDSU professor and SDSU Extension agriculture and natural resources senior program leader, says there is still time to stop the Green Glacier in its tracks. 

"This is a long-term problem," Smart said. “The hardest part is to recognize the slow change on the landscape. We have the opportunity to protect those core areas that haven’t been invaded yet.”

While earning his doctorate in agronomy and range management from the University of Nebraska-Lincoln, Smart learned one of the most crucial techniques in stopping woody encroachment: prescribed burns. Fire, as Smart explains, was one of the ways that grasslands remained relatively pristine prior to settlements. In tallgrass prairies — which is what South Dakota's grasslands are primarily composed of — conditions would be right for a fire every three to five years. Farther west in shortgrass prairie, fire would happen every decade or so.

These fires, caused by lightning strikes or by Native Americans looking to attract buffalo, would essentially refresh the grass and prevent trees and shrubs from growing. As settlers moved onto the prairie, fire suppression became the norm.

Now, ranchers are learning that prescribed fires are the best way to prevent them from losing their pastures to woody encroachment. The challenge, however, is overcoming the stigma — some may call it fear — of fire while also learning how to do the burns correctly.

"We didn't really have the knowledge here in South Dakota on how to do these prescribed fires," Smart said. "We had to learn from what happened in the southern plains."

Prescribed burns require a coordinated, six- to eight-person team to conduct correctly and safely. In the southern plains, ranchers were forced to create community burn teams to fight against the Green Glacier. In South Dakota, similar groups have formed.  
The Mid-Missouri River Prescribed Burn Association was the first group to embrace prescribed burns in South Dakota. Formed in 2015, the group started as a landowner-led effort to combat the spread of eastern redcedars along the Missouri River in Gregory, Charles Mix, Brule and Lyman counties. Since its inception, the nonprofit has completed prescribed burns on roughly 20,000 acres of both public and private land.

Sean Kelly is an SDSU Extension range management field specialist located in Winner. He serves as a liaison for the Mid-Missouri River Prescribed Burn Association and helped bring together the public-private agreement that allowed for the association to get as much done as it has already. The group's success has created more interest in prescribed burns, which is helping to fight the problem.

In 2023, SDSU Extension received a $1 million contribution agreement from the U.S. Department of Agriculture's Natural Resources Conservation Service to further combat the spread through annual prescribed burn trainings led by SDSU Extension specialists like Smart, Kelly and Pete Bauman. These outreach efforts are helping to spread knowledge surrounding prescribed burns and the challenges woody encroachment presents.

"We're doing a lot of outreach effort to get this knowledge to the people who need it," Smart said. "We are trying to create a 'culture of will' that works together as a community to solve this problem."

Native plant diversity 
Burns not only control tree infestations, but they also promote the growth of native plants and forbs. When eastern redcedars take hold in a pasture, their dense canopies coupled with thick needle duff layer nearly cover the ground entirely. This kills off native herbaceous vegetation by blocking sunlight reaching the ground, limiting photosynthetic ability for herbaceous plants to grow. But once the eastern redcedars are removed, the herbaceous plants, including grasses and forbs, return. This is because the seed bank and bud bank, as Xu describes, remains in the soil, which facilitates recolonization and herbaceous vegetation succession.

"We have found that the seed bank, which retains the native grasses and forbs, remains in the soil even under eastern redcedar canopy," Xu said. "Once the canopy is removed, the native grasses sprout again."

Grassland regeneration creates a newfound diversity, Xu's research has found, which creates a more resilient rangeland. This benefits the soil, wildlife and even livestock. A resilient rangeland can withstand, and recover, from droughts better than monoculture pastures, which occur during woody encroachments.

Combatting with goats
While burns are the most successful control method, they can be intimidating, expensive and time-consuming. SDSU researchers are exploring other feasible solutions to combat the problem.

In 2021, Smart and Xu conducted a study to see if goats could be used as a biological control. Goats are a common farm animal, but they are unique in their feeding tendencies. Because of their four-chambered stomach, goats can digest woody plant matter and have been known to strip bark off trees. This ability is why goats are known as "browsers."

The researchers found that after other food options, like leaves, shrubs and grasses, have been exhausted, goats will start stripping bark off eastern redcedars. This process, known as girdling, effectively kills the tree.

While Smart considers this a promising tool in the toolbox for the fight against the Green Glacier, more research needs to be completed to gain a more comprehensive understanding of this biological control.

Future outlook 
In November 2021 at the South Dakota Stockgrowers Association Convention, Dirac Twidwell, a professor of agronomy and horticulture at the University of Nebraska-Lincoln and a leading voice in the fight against the Green Glacier, provided an unfortunate truth: no county anywhere in the Great Plains has restored production yield after eastern redcedar or woody encroachment started coming in heavily, and no county has ever prevented it from happening. Not a single one.

While Twidwell's message illustrates the grim situation in the southern plains and what is potentially to come for South Dakota, Smart is more optimistic about the state's chances in stopping the Green Glacier. For one, researchers know prescribed fires — when done repeatedly — will eliminate eastern redcedars from the landscape. This information is crucial for focusing the strategy and actions needed for dealing with the Green Glacier as it makes its way into South Dakota.

While other states were slow to the react to the invasion, South Dakota has learned from its neighbors to the south and has taken a more proactive approach. The state now has multiple burn associations, and thanks to Extension specialists like Smart, more and more ranchers and farmers are learning the technique each year.

While South Dakota is not "out of the woods" in its fight against woody encroachment, these initial signs show the state is poised to give the Green Glacier its toughest battle yet.