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Whether aspens are in a boom or bust depends on how the numbers are crunched.
By Christina MacIntosh / Environmental Reporter 10 hrs ago
Over 30 years ago, federal wildlife managers reintroduced wolves to Yellowstone National Park — and the canines began dining on Elk, culling the population, leaving less wapiti to browse on aspens, willows and cottonwoods.
In the years since, observers of the Greater Yellowstone Ecosystem have likely heard some version of a commonly held belief: Wolf reintroduction reduced over browsing and helped those trees rebound. Scientists call that process a “trophic cascade,” jargon for a ripple effect down the food chain.
But, since the early 2000s, researchers studying forestry in Yellowstone have disagreed about and debated the magnitude of that effect, and whether reintroduction has, in fact, helped aspens rebound to the extent the public believes.
Nearly 25 years after researchers first started debating the issue, different factions still can’t agree. Researchers at Utah State University and Oregon State University published warring papers on the topic earlier this year.
Doug Smith retired as Yellowstone’s senior wildlife biologist in 2022, after working in the park for 28 years. During his tenure, Smith said he played “referee” between the two groups, as well as researchers from Colorado State University.
According to Smith, the groups have all been right in their own ways and get different results because they ask different questions. He finds the ongoing spat frustrating. The disagreement has led some to believe in overblown benefits of wolf recovery, and some to doubt the concept of the trophic cascade altogether, Smith said.
“We have blown a great opportunity for science communication and led to a distrust of science,” he said.
In a place as complex and dynamic as Yellowstone, it’s difficult to track the effects of one actor, like wolves, or to neatly summarize the effects across the landscape as a whole.
While the population of Elk in the park has decreased by about 60% in the last 30 years, a host of other changes have occurred in and outside of Yellowstone’s boundaries.
Wolf populations have indeed increased, but so have the populations of cougars, grizzly bears and Bison. Outside Yellowstone’s boundaries, humans increased hunting pressure on Elk in the years following wolf reintroduction. And the climate in the Greater Yellowstone is changing, leading to shifts in precipitation and moisture availability, which could impact aspens as well.
Through all that complexity, researchers agree on one point: Wolves have played some role in aspen recovery where it’s occurring, but that recovery has been inconsistent across the park. Whether those effects can be categorized as “strong” or “widespread” depends on the researcher.
Dueling papers
This winter, Dan MacNulty, a professor of wildland resources at Utah State University, published a paper detailing mathematical errors in the work of a group of researchers from Oregon State University, who popularized the concept of the “trophic cascade” in the park.
The group published a response to MacNulty’s critiques and published a correction, stating that what they thought was a 152-fold increase in aspen sapling and young-tree density was actually a 17-fold increase.
But to the Oregon researchers, the point of a strong trophic cascade still stands.
“In my opinion, it’s not a big deal,” Robert Beschta, a professor emeritus of forestry ecosystems at Oregon State University, said of the error. “It doesn’t change the science at all.”
For MacNulty, the correction and subsequent response left something to be desired.
He also has issues with the group’s research methods, which he believes obscure the reality of how spotty aspen recovery has been in the park by using averages that sway results towards high-performing stands. About 60% of the stands the Oregon group sampled had no trees, according to the group’s paper.
“In some respects, this is a little bit of a distraction,” MacNulty said. “Neither their correction or follow-up response really addresses the crux of my criticism, which is that it’s not representative of typical sapling density.”
The Oregon group also uses a method that involves measuring the five tallest trees in a stand. To MacNulty, that means the group was not measuring “representative aspens” in the stands.
But measuring the tallest trees is nevertheless an important metric for assessing whether or not regeneration was beginning to occur, Beschta said. If young trees exceed 2 meters tall, they’re above the browsing level of Elk.
“Height is a very conservative estimator of a recovering plant community,” Beschta said. “The indicator we were using was a reasonable one.”
The Oregon group has never claimed that aspens in the park have fully recovered, Beschta said.
Research by the Utah State University group indicates that most aspen stands have underperformed, contracted and gotten smaller, since reintroduction, MacNulty said. But those stands still have individual trees that are growing taller than they used to, like the Beschta group says, Smith said.
“I think both sides are right, they just come at the question differently,” Smith said. “That’s a big part of the disagreement, is that you get the answers to the questions you ask, and each group is asking a slightly different question, so they kind of miss each other.”
Earlier in the debate, the Oregon State group did attribute all regeneration to wolf reintroduction, Smith said. Subsequent criticism was valid causing confusion to remain amongst the general public, Smith said.
What’s causing the change?
The groups can agree that aspen regeneration in the park has been a mixed bag. Some stands have seen great success, others have not.
The groups have different views on what’s driving the variability. Beschta’s group believes Bison are the main factor. Their population in the park has increased from about 2,500 in 2000 to 5,300 in 2025.
“We just turned a corner from Elk being the dominant large herbivore, to Bison,” he said.
MacNulty blames differences in moisture, precipitation, fire and even soil chemistry. Stands not 50 yards from each other, facing identical environmental conditions, have seen dramatically different fates, he said.
If Elk were once the limiting factor on aspen growth, the next limiting factor kicks in once they’re removed, Smith said. For aspen, that’s usually moisture.
“If you get a reduction in browsing, but you don’t have adequate water, willow or aspen don’t grow,” Smith said. “If you don’t have water, removing Elk does nothing.”
Without wolves and cougars on the landscape, widespread browsing leveled the playing field between stands with different potential.
The disagreement is, at the end of the day, a niche one, especially given that willows and aspens make up only 5% of the vegetation in northern Yellowstone, Smith said. Over 80% is sagebrush and the other 15% is Douglas fir.
“It’s always been a shrub grassland, so they’re kind of arguing over a philosophical question that doesn’t have historical standing,” Smith said.
Simple story
Since the early 2000s, researchers have published papers capturing the nuances and caveats that accompany trophic cascades in the park, MacNulty said. Some of that work highlighted the role of humans in declining Elk numbers. But the flashiest results featuring heroic wolves get the most attention from the media and the public.
“The simple story has really penetrated the bloodstream of educators, the public and scientists,” MacNulty said. “It’s because it’s a simple, redemptive story about nature being healed.”
Aspen stands
Arthur Middleton, a professor of environmental science, policy and management at the University of California-Berkeley, has been studying wolves and Elk in Yellowstone for over a decade. He finds the simple story of trophic cascades frustrating because it obscures all the challenges the ecosystem continues to face.
“If it gives people the impression that this job is even close to being finished, that is not correct, and I’m going to call it out,” he said.
It also makes it seem like wolves need to earn their place on the landscape, Middleton said.
“I don’t need wolves to do a dance, or perform, or have a special effect, in order to be justified or in order for me to like them,” he said.
Trophic cascades are one of the most fundamental phenomena in nature, Smith said.
Visitors come to the park and see them in action, with aspen and willow regenerating in certain places. Ultimately, what matters is that positive impacts of large carnivore restoration are apparent on the landscape, Smith said.
“That’s the important question,” he said. “What they’re disagreeing about is, ‘How does the measuring affect the end result?’ And in the end it kind of doesn’t.”