For more than a hundred years, Yellowstone has drawn millions to the American West. Each year, more than 3 million people visit the park, stopping for its 19,000-year-old geysers, its million-year-old mountains and its blankets of forests that look just as dense as they do in the hundred-year-old photos in the textbooks. For many, Yellowstone is that old photo, a natural snapshot of America before Manifest Destiny.
But the forests of the northern Rocky Mountains are not static photos. They are living organisms that change with their environment. Every so often a fire breaks out and the forest resets itself, and according to a new study conducted by UW-Madison ecologists, climate change is affecting forests’ ability to recover.
According to that study, recovery from a fire is dictated by the years immediately following the fire. When faced with a severe post-fire drought, new trees have a harder time establishing themselves, leading to thinned-out forests.
“Most of the trees in these forests are not very well adapted to survive fire. They’re adapted to reproduce immediately following the fire,” said Brian Harvey, the study’s lead author who is now a postdoctoral at the University of Colorado-Boulder. “So if they’re sending out all of their seeds and it happens to be a fire followed by really warm, dry conditions, it could spell trouble for that forest’s ability to reproduce.”
The study also pointed out that seed sources, islands of surviving trees and the forest on the edge of the burn zone can direct a recovering forest. When a fire cuts through the forest, it creates an uneven mosaic of patches and islands. The larger the patch and the farther it is from seed sources, the less likely there is to be a dense recovering forest, the study said.
“If you combine that with the harsher post-fire climate, it’s a little bit of a double whammy,” Harvey said. “They have a hard time getting there, and once they get there, they have to roll the dice on a good series of climate conditions for several years… to actually establish and grow up to be old enough and large enough to withstand a [drought].”
Harvey’s study, based on samples collected on-site by a team of UW-Madison students, focused on a region of dense forests between Yellowstone and Glacier National Park. The thick mountain forests in that region, according to UW-Madison professor and coauthor Monica Turner, are built around routine intense fires.
“In the forests we’ve been looking at in Yellowstone, Glacier and Grand Teton, the fires that happen are severe fires. They kill all the trees. That’s the natural fire regime of things,” Turner said. “A lot of times people see stories about big severe fires and assume they’re not natural … but those kinds of fires have recurred in Yellowstone in intervals of 100 to 300 years.”
Historically these fires, which Turner says have occurred in the forests for thousands of years, are often the only times for a forest to adjust and reorganize.
“The opportunities for the forest to reorganize itself, either in terms of numbers of trees or in species of trees, is typically when a fire happens,” Turner said. “You kind of push the reset button, and that’s when the trees have a chance to either migrate … or change in density.”
According to Turner, the severe post-fire droughts that the study looked at were fairly infrequent.
Since the big Yellowstone Fire in 1988, only one such post-fire drought, a two-year drought starting in 2000, occurred in that burn area. But climate change is expected to shift that in the coming decades.
According to a study published in 2011, climate change is expected to increase the frequency of forest fires and droughts in the northern Rocky Mountains. Turner worked on that study as well.
More frequent droughts could dramatically affect the makeup of recovering forests, according to Harvey’s study. There is not a lot of data on how more frequent fires will affect individual populations, Turner says, but the effects on the forest will be noticeable as larger patches are burned and recovering populations thin.
“We expect hotter conditions, drier conditions, and much more fire in that part of the world than we’ve had in the past century,” Turner said. “We would have a very fundamentally different climate than what Yellowstone has seen over the past 10 thousand years … We might see more patchiness in the distribution of forests. In some places where we currently see a continuous forest that goes for miles and miles, [might] be patchier and have fewer trees.”
It is these changes that make researching forest recovery important, according to Harvey.
“Those things that, in the past, would’ve been statistical outliers … are expected to become the norm in the future,” Harvey said. “[This research] is a good crystal ball into that future.”
Harvey’s study was coauthored by Daniel Donato, a former member of Turner’s lab who now works with the Washington Department of Natural Resources, and was funded by the United States Joint Fire Science Program and the National Park Service. It was published in the March issue of the journal Global Ecology and Biogeography.