USA: YELLOWSTONE NATIONAL PARK, Wyo. — This is a tale of two forests, Densetown and Stumptown, whose paths diverged after a succession of wildfires. One illustrates the historic resilience of the dense Yellowstone pinelands; the other portends a much sparser future for these forests under climate change.
Seventeen years ago, fire swept through this area north of Jackson Lake and Grand Teton’s glaciered peaks, burning a forest of old-growth lodgepole pines that had towered for more than 200 years. In its aftermath, a new generation of pines sprouted, healthy and dense.
Then last year, wildfire again roared through. Thanks to the vagaries of wind and terrain, one thick section of the forest was untouched. A neighboring plot of trees was reduced to singed stumps and ashen pine skeletons. A crew of scientists here this summer studying the forest’s regrowth nicknamed the sections Densetown and Stumptown.
Their findings on how forests respond to fire will help guide forest management and firefighting policies as a warming climate contributes to more frequent wildfires. Fires are now raging across the West, from Southern California to Glacier National Park, with experts predicting that 2017 will go down as one of the worst wildfire seasons in decades.
Although lightning-sparked fires are a natural part of the forests’ life cycles, forests reburning at short intervals is a relatively rare phenomenon. For the past 10,000 years, these woods have burned approximately every 100 to 300 years, meaning fires typically scorched old trees. But as climate change leads to longer and hotter dry seasons, younger forests throughout the Yellowstone region may start burning more frequently. (The jury is still out on how climate change will affect wildfires in other Western conifer forests.)
These changes could play out in a couple of ways.
First, short-interval fires could overwhelm an evolutionary adaptation that in the past allowed burned lodgepole forests to regrow just as thickly as before. Many of the lodgepoles here are serotinous, meaning they grow pine cones sealed with a sappy resin that protects their seeds from flames. During a fire, the cones open and the seeds are released. Only mature lodgepoles produce these resinous cones, while younger ones yield unprotected cones that release their seeds as soon as they’re finished growing.
When fires are infrequent, the forest has time to mature and build up a stock of serotinous cones that will restart the next generation: hence Densetown. But when part of the young forest burned again just sixteen years into its regrowth, creating Stumptown, it had not yet produced many serotinous cones. Its seed stock was obliterated.
Second, the fires could burn up larger sections of forest. Small islands of forest often survive even within otherwise burned areas, said Brian Harvey, an ecologist at the University of Washington, and seeds from these preserved areas often blow into the surrounding burned forests or are carried there by animals. This reseeding method is especially important at higher altitudes where lodgepoles don’t produce serotinous cones.
“But what we’re seeing now is more homogeneous burning throughout the forests, with fewer islands of unburned areas,” Dr. Harvey said. “When that happens, there are fewer seed sources to replace the stands.”
That’s important here because lodgepoles make up 80 percent of the trees in this heavily forested region, which includes Yellowstone and Grand Teton National Parks, five national forests and a handful of outlying wildernesses and wildlife refuges. What will happen to these forests if a changing climate means not only old forests burn, but young ones, too?
That’s what Dr. Harvey and his colleague, Monica Turner, an ecologist at the University of Wisconsin, are here investigating. Yellowstone’s recent fires offer a rare natural experiment to see how forests regenerate after burning and reburning at short intervals.
On a blistering summer day in Stumptown, surrounded by blackened dirt and leafless, lifeless trees that offer no relief from the sun, a dozen students and research assistants fanned out on hands and knees. Minutes later, one shouted “Found one!” and the others converged on her discovery: an inch-high baby lodgepole pine peeking up in the shadow of a charred log.
By counting seedlings a year after the most recent fire, the team can calculate how densely new lodgepoles might regrow in Stumptown. Although there could be some late-sprouting seeds and animals might bring in new seeds in the years to come, based on their initial count, the researchers predict there will be around 400 trees per acre here. In Densetown nearby, there are some 32,000 trees per acre.
In addition to becoming sparser, Stumptown’s tree population seems to be diversifying. Aspens came in after the 2000 fire and resprouted after the 2016 fire. Now they are establishing alongside the lodgepoles.
Throughout Yellowstone’s long history of fire and regrowth, forests have tended to come back like Densetown. But climate change may be pushing even these hardy forests past their breaking point, said Dr. Harvey, and how trees regrow in Stumptown could be a sign of things to come. Taken together, their preliminary findings suggest many of Yellowstone’s dense, lodgepole-dominated forests will give way to sparser, more diverse woodlands and meadows.
“When fires burn at short intervals, we have a lot fewer trees coming back,” Dr. Turner said. “It’s still enough for a forest, but it will be sparser than before.”
There are advantages to these sparser forests. More prevalent grasses and aspens provide food for elk and deer, and bird diversity often explodes after a burn. One problem with younger forests burning instead of older ones, Dr. Hutto said, is that some birds are picky when it comes to charred habitats, preferring burned mature forests.
“If your forest doesn’t get very big before it burns, you don’t get the coolest stuff,” he said.
Roy Renkin, a vegetation management specialist at Yellowstone National Park, said that he was skeptical that young forests would burn more frequently, because they do not produce enough fuel.
“It’s an example of linear thinking to say that warmer and drier equals more and bigger fires,” he said. “There are feedbacks and interactions that change the linearity.
Climate change might be altering these fundamental forest dynamics, said William Romme, a forest fire researcher at Colorado State University.
“The big question we’re asking now is, ‘What does the future hold for these forests?’ ” Dr. Romme said. “Are we entering an era in which things aren’t going to behave like they did before?”
Stumptown itself, Dr. Turner noted, is evidence that given the right climate conditions, young forests in Yellowstone can and will reburn. Ultimately, she said, so long as wildfires are not endangering human lives, it is best to let them take their course. Rather than try to preserve forests in their current state, officials should focus on things they can control or mitigate, like pollution, habitat fragmentation and invasive species, she said.
“Change is going to happen,” Dr. Turner said. “But we’ll still have forests. We’ll still have a wide variety of native species. It will still be Yellowstone.”