Woody Encroachment
More trees aren't always good...
Woody species are one of the biggest threats to conserving native rangelands.
Trees provide lots of benefits, but they don't belong everywhere. In sagebrush country and the grasslands of the Great Plains, encroaching trees are degrading wildlife habitat, reducing the productivity of valuable grazing lands, and fueling more frequent and severe wildfires.
The examples below highlight how trees are transforming our grass and shrublands. This threat is affecting every state from Texas to Washington.
Tree cover change across non-forest lands in the western U.S. (1990-2020). Areas of expanding tree cover are shown as color transition from white to yellow to red. Credit: Kristopher Mueller/WLFW.
Blackfoot River, Montana
Encroaching trees degrade conservation legacy.
Trees threaten ranchers’ livelihoods and degrade prime wildlife habitat.
- W113.106, N46.957
- 3.5 sq. miles
The Blackfoot River is world-renowned for its trout fishing. And the ranchers who live alongside it are well-known as models for how to work collaboratively to caretake the land and water. Unfortunately, an under-appreciated threat is gobbling up the sagebrush valleys that support their livestock, as well as the grizzlies, elk, trumpeter swans, and sandhill cranes that roam through the Blackfoot.
In the 1950s, the rangelands west of the river were thriving meadows of bunchgrasses, sagebrush and wildflowers. Historic soils data show this land was a place where trees did not grow. But as European settlers suppressed fires, woody species began to creep in. Fast-growing trees like western juniper outcompete native grasses by stealing the lion’s share of water and sunlight.
Ranchers throughout the western U.S. lost out on nearly $5 billion worth of forage (plants that sheep or cattle eat) between 1990 and 2019 due to encroaching trees. In the riverside pasture pictured above, the amount of forage decreased by nearly 60% since 1990, which translates to less money for the ranchers trying to raise livestock in the Blackfoot’s valleys.
As woody encroachment degrades rangelands, agricultural landowners are more likely to convert native habitat into crops like hay to make a profit or sell portions of the ranch for subdivision development. Both of these outcomes are even more detrimental to wildlife.
Loess Canyons, Nebraska
Invading trees threaten ranching communities.
The spread of eastern redcedar reduces grassland productivity and increases the risk of catastrophic wildfire.
- W100.571, N40.915
- 1.8 sq. miles
In the Great Plains, woody species used to grow only in wet or rocky places because regular fires kept them from spreading onto prairies. But as European settlers suppressed fires and planted thousands of trees to provide windbreaks for their homes and livestock, trees began to invade grasslands.
The image above shows how quickly eastern redcedar have spread out of wet hollows or draws onto prairie pastures over the past half-century. As once-productive prairies turn into dense woodlands, it causes all sorts of problems for landowners in western Nebraska. Ranchers lose out on livestock forage and grassland wildlife like prairie chickens are displaced.
Plus, the chance of a catastrophic wildfire occurring skyrockets as woody species encroach on rangelands. This puts people and property at risk.
Research from the Loess Canyons Experimental Landscape found that the distance embers can travel from a wildfire to ignite a new fire is 450% greater in juniper-dominated woodlands than in grasslands. This means that a wildfire where trees have encroached exposes 2 to 10 times more land to wildfire risk compared to a fire on a tree-free prairie.
Northeastern New Mexico
Trees dry out already arid habitat.
In the desert Southwest, woody encroachment threatens water availability and range health.
- W105.538, N36.93
- 2.25 sq. miles
Fire suppression efforts have also allowed trees to expand in the American Southwest. This image from New Mexico shows how woody species historically grew at higher elevations, along ridgetops, or in cooler, wetter parts of this arid landscape. But over the past century, junipers have expanded down onto sagebrush rangelands.
In the desert, the proliferation of fast-growing, drought-tolerant trees is worrisome for yet another reason: they steal precious water from springs. Their roots siphon water from the ground before it can refill springs or creeks. For instance, model simulations from the central Great Plains suggest that complete conversion of rangelands to redcedar woodlands would reduce streamflows by 20-40 percent.
The New Mexico landscape pictured above is already extremely dry, and currently in the midst of a megadrought. Continued expansion of water-sucking woody species doesn’t bode well for the people and wildlife who live in this desert.
Conservation in Action
Success Stories
For eight years, SGI and partners have conducted a long-term conifer removal project in the Warner Mountains in SE Oregon. In addition to removing encroaching trees, university researchers studied how the removal affected sage grouse.
A new report highlights how landowners in the Loess Canyons of Nebraska have successfully reversed tree encroachment in grasslands while also facilitating co-produced science to inform and improve future management of the Great Plains’ grassland biome.
Learn more about woody encroachment and how YOU can help!
Woody encroachment is one of the biggest threats facing both the sagebrush and Great Plains grassland biomes.
Partnerships that include federal agencies like the USDA-NRCS, state and local conservation organizations, universities, and landowners are helping address this threat through proactive, cross-boundary, spatially targeted conservation.
Learn more about how woody plants are threatening these landscapes and the conservation solutions that are tackling this threat below.