Fast-growing, drought-tolerant trees are slowly spreading across grasslands on every continent except Antarctica. Given how desperate we are to reduce carbon in the atmosphere, millions of new saplings sprouting each year might seem like a good thing. But in reality, their spread across vulnerable grasslands and shrublands is upending ecosystems and livelihoods. As these areas transform into woodland, wildlife disappears, water supplies dwindle, and soil health suffers. The risk of catastrophic wildfire also skyrockets.
In a new study published in the Journal of Applied Ecology, researchers have shown how woodland expansion also takes an economic toll. American ranchers often depend on tree-free rangelands to raise their livestock. Between 1990 and 2019, landowners in the Western US lost out on nearly $5 billion worth of forage—the plants that cattle or sheep eat—because of the growth of new trees. The amount of forage lost over those three decades equates to 332 million tons, or enough hay bales to circle the globe 22 times.
“Grasslands are the most imperiled and least protected terrestrial ecosystem,” says Rheinhardt Scholtz, a global change biologist and affiliated researcher with the University of Nebraska-Lincoln. Also called steppes, pampas, or plains, our planet’s grasslands have dwindled drastically. According to Scholtz, less than 10 percent are still intact, as most have been plowed under for crops or bulldozed for human development. One of the most dire threats facing the grasslands that remain is woody encroachment. “It’s a slow and silent killer,” Scholtz says.
Historically, tree expansion onto grasslands was checked by regular fires, which relegated woody species to wet or rocky places. But as European settlers suppressed fires and planted thousands of trees to provide windbreaks for their homes and livestock, trees proliferated. When trees invade grasslands, they outcompete native grasses and wildflowers by stealing the lion’s share of sunlight and water. Birds, often used as a bellwether for ecosystem health, are sounding the alarm: North America’s grassland bird populations have declined more than 50 percent since 1970, a 2019 study in Science found.
According to University of Montana researcher Scott Morford, who led the study on rangeland forage loss, tree cover has increased by 50 percent across the western half of the US over the past 30 years, with tree cover expanding steadily year on year. In total, close to 150,000 km2 of once tree-free grasslands have been converted into woodland. “That means we’ve already lost an area the size of Iowa to trees,” says Morford, who emphasizes that an additional 200,000 km2 of tree-free rangelands—an area larger than the state of Nebraska—are “under immediate threat” because they are close to seed sources.
To figure out the amount of lost forage production due to woodland expansion, Morford and his team used satellite images in combination with meteorological data, topography, and information about soils and on-the-ground vegetation to estimate the change in herbaceous biomass (that is , non-woody plants, like grasses) in relation to tree cover over time. “Our computer models allow us to turn up or turn down the tree cover like a knob on your stereo to see how production is impacted,” explains Morford.