Climate-Driven Fires Could Turn Yellowstone Forests to Grassland By Midcentury

Adapted from a news release by Kelly Tyrrell of the University of Wisconsin-Madison.

The fires in Yellowstone National Park began to burn in June 1988. A natural feature of the landscape, they were expected to fizzle out by July, when rains historically drenched the forests and valleys of the world’s first national park. But the rains never came. It was unusually hot, dry and windy that year, and more fires erupted in and around the park until September. By then, 36 percent of Yellowstone was affected. Firefighting efforts topped $120 million.

Yellowstone experiences large fires every 100 to 300 years, and its flora and fauna are adapted to them. For instance lodgepole pines, at higher elevations in the park, have cones that open in fire, releasing seeds to replenish a post-burn forest. But it takes time for trees to mature and forests to recover — time that a changing climate has been depriving the forest of the last three decades.

A new study just published in the journal Ecological Monographs shows that some of Yellowstone’s forests may now be at a tipping point, and could be replaced by grassland by the middle of this century.  The paper’s lead author is Winslow Hansen, an Earth Institute postdoctoral fellow based Lamont-Doherty Earth Observatory, who did the research for his PhD. at the University of Wisconsin, Madison.

Hansen’s advisor and coauthor, ecologist Monica Turner, said of the findings, “It’s terrifying in some ways. We are not talking many years away. Today’s college students will be mid-career. It feels like the future is coming at us fast.”

Large fires are no longer a rarity in the West, in large part due to warming temperatures. Yellowstone is nearly 2 degrees Fahrenheit warmer on average than it was just 60 years ago. The park’s forests may be at the brink of abrupt change, where their resilience may suddenly be overwhelmed, say the researchers. “Fires are being driven by hot, dry conditions and these are trends that are expected to continue,” said Hansen.

He and Turner selected sites to determine how well tree seedlings can establish in warmer, drier conditions like those predicted for mid-century. Right now, lower elevations of the park provide a natural simulation of these projections for higher terrain. “We found places on the landscape where today’s climate is representative of what we expect to see in subalpine forests 50 years from now,” said Hansen. The researchers included sites in recently burned forests at higher elevations as controls.

Hansen collected soils from two recently burned areas, analyzed the soils for qualities like texture and pH, and then placed them in the ground at the low- and high-elevation study sites. He also installed sensors to collect hourly data on soil surface temperature and moisture, and planted locally collected Douglas fir and lodgepole pine seeds in each patch of soil. Over a three-year span, he measured the number and height of the tree seedlings that grew and survived.

Douglas firs, more often found at lower elevations, are better adapted than lodgepole pine to grow in dry, warm soils. However, the study found that seedlings of both tree species established themselves and survived only in high-elevation plots, where surface temperatures were between 52.5 and 59 degrees Fahrenheit. At the low-elevation plots, soil temperatures regularly exceeded this range, and the soils dried. More than 90 percent of the seeds never sprouted, and those that did died within three years.

In a separate experiment, Hansen hauled soils from the same post-fire areas of the park back to Madison, and planted seeds in 1,000 pots at a facility on campus called the Biotron, which can recreate virtually any environment on the planet. In each room, he simulated the growing seasons of the early 20th century, the mid-21st century, and the late 21st century through 2080, ranging from relatively moist to drought-like and warm. Every two to three weeks, he measured seedling emergence, height and mortality. Though they had not planned for it, the soil temperatures in the Biotron were up to 10 degrees Fahrenheit cooler than those in the field, primarily because the soil samples in the field were insulated by surrounding soils. Incidentally, this allowed them to examine how both tree species responded to conditions similar to the high elevation regions of Yellowstone today.

The study captured the entire range of growing season temperatures at which seedling establishment reliably occurs and found that projected mid-century temperatures fall well outside that range. “This is pretty concerning if we expect to have reliable tree regeneration in the future across much of Yellowstone,” said Hansen.

“Experiments help you nail a part of the mechanism but you also need field observations and computer simulations to explore a range of conditions you can’t do in the field,” said Turner. “With a threshold, the best way to know you’re there is to cross over it.”

The study was funded by the U.S. National Science Foundation, the Joint Fire Science Program and the University of Wisconsin–Madison Vilas Research Trust.