Climate change: Yellowstone’s forests could be replaced by grasslands by mid-century

The fires in Yellowstone National Park began to burn in June 1988. A natural feature of the landscape, park managers expected the flames to fizzle by July, when rains historically drenched the forests and valleys of the world’s first national park.

But the rains never came. In unusually hot, dry and windy conditions, more fires erupted, spreading 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 that schedule. Lodgepole pines like those at higher elevations in the park have pine cones that open in fire, releasing seeds to replenish a post-burn forest. But it takes time for trees to mature and the forests to recover — time that a changing climate has been depriving the forest of over the last three decades.

A new study published Jan. 17, 2019 in Ecological Monographs, led by Winslow Hansen and his former graduate adviser, University of Wisconsin–Madison Professor of Integrative Biology Monica Turner, shows that some of Yellowstone’s forests may now be at a tipping point. They could be replaced by grassland by the middle of this century.

“It’s terrifying in some ways,” Turner says. “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.”

Though Turner’s work in Yellowstone across the last 30 years helped us better understand the resilience of the forests, and aided land managers in planning for the future, Hansen became interested in whether the forests could keep pace with climate projections.

Large fires are no longer a rarity in the West, and already 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 suddenly may be overwhelmed.

“Fires are being driven by hot, dry conditions and these are trends that are expected to continue,” says Hansen, now an Earth Institute Postdoctoral Fellow at Columbia University. “We need answers now to better anticipate future change.”

To achieve this, he and Turner carefully picked sites in Yellowstone to determine how well tree seedlings can establish in warmer, drier conditions like those predicted for mid-century, based on the International Panel on Climate Change Fourth Assessment Report. Right now, lower elevations of the park provide a natural simulation of these projections.

“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,” explains Hansen. They also 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 hand-collected, local Douglas fir and lodgepole pine seeds in each patch of soil at the study plots. Over a three-year span, he measured the number and height of the tree seedlings that grew and survived.

Douglas fir trees are better adapted than lodgepole pine to grow in dry, warm soils and are more often found at lower elevations. However, the study found that seedlings of both tree species only established and survived in high-elevation plots, where temperatures at the soil surface were between 52.5 and 59 degrees Fahrenheit. At low-elevation study plots, soil temperatures regularly exceeded this range and the soils dried. Up to 92 percent of the seeds never established and those that did died within three years.

“It wasn’t just a small reduction,” says Turner, “it was a failure to establish at the lower elevations.”

In a separate experiment, Hansen hauled soils from the same post-fire areas of the park back to Madison, Wisconsin, and potted them inside a facility on campus called the Biotron that can recreate virtually any environment on the planet. He wanted to identify more precisely which combinations of temperature and soil moisture affect tree seedling establishment after fire.

In each room, he simulated the growing seasons of the early 20th century, the mid-21st century, and the late 21st century through 2080, which ranged from relatively moist to drought-like and warm. After weeks of planning, he and his wife, Ann Olsson, spent 19 straight hours setting up the rooms.

“It took an enormous amount of effort to calibrate,” says Turner.

Every four days, with undergraduate assistant Ryan Fitzsimmons, Hansen hand-watered the seeds in all 1,000 pots. Every two to three weeks, they 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.

“At the time, it felt disastrous,” says Hansen, but the Biotron experiment showed that seedlings emerged only when soils met a minimum warm temperature and that future warming could actually enhance post-fire regeneration at higher elevations.