America’s billion-dollar boom problem is expanding

Fast growing, drought tolerant trees slowly spreading across grasslands on every continent except Antarctica. Given how desperate we are to reduce carbon in the atmosphere, it might seem like a good thing that millions of new saplings are sprouting each year. But in reality, their spread across fragile grasslands and scrub is upsetting ecosystems and livelihoods. As these areas turn into forest, wildlife disappears, water resources dwindle and soil health suffers. The risk of catastrophic wildfires is also skyrocketing.

In a new study published in the Journal of Applied Ecologyresearchers have shown that forest expansion also takes an economic toll. American ranchers often rely on tree-free pastures to raise their cattle. Between 1990 and 2019, landowners in the western U.S. lost nearly $5 billion in forage — the plants that cattle or sheep eat — because of the growth of new trees. The amount of forage lost in those three decades is equivalent to 332 million tons, or enough bales of hay to circumnavigate the globe 22 times.

“Grasslands are the most threatened and least protected terrestrial ecosystem,” said Rheinhardt Scholtz, a global change biologist and associate researcher at the University of Nebraska-Lincoln. Also called steppes, pampas or plains, the grasslands of our planet have drastically declined. Less than 10 percent are still intact, according to Scholtz, as most have been plowed up for crops or bulldozed for human development. One of the most serious threats to the grasslands that remain is woody encroachment. “It’s a slow and silent killer,” says Scholtz.

Historically, the expansion of trees into grasslands was limited by regular fires, relegating 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, the trees spread. When trees encroach on grasslands, they outcompete native grasses and wildflowers by stealing the lion’s share of sunlight and water. Birds, often used as a gauge of ecosystem health, are sounding the alarm: Grassland bird populations in North America have declined by more than 50 percent since 1970, a 2019 study in Science found it.

According to Scott Morford, a University of Montana researcher who led the study on rangeland forage loss, tree cover has increased 50 percent over the past 30 years in the western half of the U.S., with tree cover growing steadily year over year. . In total almost 150,000 km driven2 once tree-free grasslands have been converted into forest. “That means we’ve already lost an area the size of Iowa to trees,” says Morford, who emphasizes that another 200,000 km2 of tree-free pastures – an area larger than the state of Nebraska – are “immediately threatened” because they are close to seed sources.

To figure out the amount of forage production lost due to forest expansion, Morford and his team used satellite imagery combined with meteorological data, topography, and information about soils and vegetation on the ground to estimate the change in herbaceous biomass (that is, non-woody plants, such as grasses) in relation to tree cover over time. “Our computer models allow us to raise or lower the tree cover like a knob on your stereo to see how production is affected,” explains Morford.

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