When Forests Don’t Come Back After Fire: What Happens to Yellowstone’s Understory

Lodgepole pine forests in Yellowstone and Grand Teton usually grow back after wildfire, but in a meadow in Grand Teton in July 2021 the only trace of a forest was tiny stumps hidden in the wildflowers. That meadow, burned in the 2016 Berry Fire, led Nathan G. Kiel and Monica G. Turner to ask what grows on the forest floor when the trees do not return. They report their answer in Park Science (Volume 37, Number 2, Winter 2023).

Why these forests normally recover

Yellowstone and Grand Teton sit at the center of the Greater Yellowstone Ecosystem. Yellowstone covers 2.2 million acres, and Grand Teton's peaks rise to over 12,000 feet. High-severity, stand-replacing fires have burned subalpine forests here since the glaciers receded more than ten thousand years ago, historically once every one to three centuries in very hot, dry summers. The 1988 fires burned nearly one-third of Yellowstone, driven by winds above 60 miles per hour.

Research since has shown that even such large fires are not necessarily ecological catastrophes. Lodgepole pine dominates much of the forest, and some varieties have serotinous cones sealed by resin. A tree stores more seeds each year, and when fire heats the cones they open and shed seed onto blackened soil, an ideal seedbed. With plenty of sun and moisture the seedlings grow fast, and after a decade or two most signs of the fire have faded.

Many young trees that burned in 2016, though, were too young to have serotinous cones, and the unburned forest around was full of immature trees that produced no seed. Forests recovered nowhere near as well as after earlier, long-interval fires. The year 2016 was the biggest fire year in the two parks since 1988.

The understory

Most plant diversity in North American forests grows in the understory, beneath the canopy. A Greater Yellowstone forest might have fewer than 10 tree species but dozens of species of flowers, grasses and shrubs. Many of these are perennials whose roots and underground parts survive fire, because fires do not burn deep into the soil, so within a few years the understory is green again. Bill Romme, a retired fire ecology professor, said most of the herbaceous plants there are not seedlings but plants that were there before the fire. A fire planner with the National Park Service, Diane Abendroth, said understory communities are far more than window dressing: they are habitat and what many visitors come to see.

Silvery lupine, common in the northern Rocky Mountains, became more abundant after short-interval fires where tree cover was low, probably because it fixes nitrogen and needs a lot of sunlight. Photo: NPS / Nathan Kiel.

What the researchers did

About 16 percent of the 1988 burn had not come back as forest. Those stands were dominated by spruce, fir and non-serotinous lodgepole, which need seed from living trees outside the burn.

An area burned in 1988 that had not recovered to forest more than three decades later. It is high up and was once dominated by fire-sensitive spruce and fir. Photo: NPS / Nathan Kiel.

What they found

• Resilience. Shade-tolerant plants persisted despite more sun and warmer, drier soil after short-interval fires. Native perennials resprouted and recovered, and some, like silvery lupine and elk sedge, grew more abundant. Communities stayed diverse and largely resisted invasion by non-native plants such as Canada thistle and yellow toadflax.
• Change. Where trees had not returned after 1988, meadow and grassland plants such as sticky geranium and alpine bluegrass were more abundant. After short-interval fires there were more species suited to drier, sunnier conditions and more usually found lower down, perhaps moving upslope as the climate warmed.
• A warning. The team found cheatgrass, a noxious invader, where the 2016 Maple Fire burned after the 1988 North Fork Fire. Fire can help it spread in Greater Yellowstone.
• High elevations. Communities in historically snowy high places looked the most vulnerable, a surprise, since cold-loving plants can thrive with modest warming. The team's data suggest communities adapted to cool, wet conditions may change as the growing season gets longer and warmer. Annual snowfall in Yellowstone has dropped by 3.5 inches per decade since 1950.

Sticky geranium, more common in open meadows than in closed forest, may spread if burned areas stay unforested. Photo: NPS / Nathan Kiel.

Why it matters

Climate change is making hot, dry summers more frequent. The authors cite projections that after 2050 most years could see large fires like 1988, and that swaths of forest may be lost by the end of the century. Paul Hood, a fire ecologist at Grand Teton, said that in a warmer world there may be a point at which there is too much fire. Fire planners such as Becky Smith say monitoring and research both matter, giving managers an idea of where climate change is heading. Abendroth added that the parks need to know as much as possible about the future environment and are running out of time to act.

Whether forests recover will matter for wildlife, for future fires and for the views visitors know. It may also change visitor experience, since people want to know whether there will be shade on a hot summer day. The authors add that nature adapts, and change is not inherently bad: the plant communities where forests do not return may be better suited to the warmer, drier future than the forests before them.

Wildflowers fill an alpine meadow: red paintbrush, goldenrod, silvery lupine and fireweed. Photo: NPS / Eileen Mavencamp.

Sources

• Nathan G. Kiel and Monica G. Turner, “When Wildfire Makes the Forest Understory the Star,” Park Science, Volume 37, Number 2, Winter 2023, National Park Service: https://www.nps.gov/articles/000/when-wildfire-makes-the-forest-understory-the-star.htm; rewritten in hubnx's own words.