Wolves Returned To Yellowstone In 1995. Are They Responsible For The Park’s Aspen Recovery?

Above Crystal Creek in northern Yellowstone, one aspen grove holds two generations that should have overlapped but didn’t. A few dead trunks from an old cohort still stand over the stand. Beneath them grows a dense thicket of young trees, roughly 20 feet tall, with trunks 2 to 3 inches thick. Almost nothing stands in between.

That missing middle is the story of Yellowstone’s aspens. For most of the 20th century, elk ate young shoots back to under about 3 feet, and old trees died with no successors. Now, three decades after wolves returned to the park, researchers have documented the first new generation of canopy aspens in northern Yellowstone since the 1940s. Wolves often get the credit. The research suggests they earned some of it, alongside cougars, bears, hunters and a shifting elk herd, and that scientists are still arguing over how big and widespread the comeback really is.

What Changed In Yellowstone’s Aspen Stands?

A new size class emerged, not a finished forest. You’ve probably heard the appealing story that wolves returned and Yellowstone’s aspens promptly bounced back. The real handoff took decades.

The National Park Service records that 14 wolves arrived in Yellowstone in January 1995 and were held in pens before release. Thirty years later, in 2025, Oregon State University researchers reported the first documented new crop of overstory aspens in northern Yellowstone since the 1940s. Overstory trees are the ones that eventually form the forest canopy.

The study, published in Forest Ecology and Management, was a long-term observational comparison, not a controlled experiment. In 2020 and 2021, the team revisited 87 randomly chosen aspen stands on the park’s northern elk winter range. These were the same stands surveyed in 1997 and 1998 and again in 2012. In each stand, the researchers measured one sampling strip 2 meters wide and 30 meters long, then surveyed the rest of the stand.

Small trees with trunks 5 to 10 centimeters across at chest height, about 2 to 4 inches, turned up in 43 percent of stands. Saplings taller than 2 meters, about 6.5 feet, appeared in 49 percent of the sampling strips, up from none in 1997 and 1998. Recovery was uneven. Thirty percent of stands had saplings throughout, 32 percent had them in patches, and 38 percent had few or none. In a typical strip, animals still ate the growing tips of 59 percent of young aspens shorter than 2 meters each year.

Why does that count as a milestone? Age studies found that most of northern Yellowstone’s canopy aspens sprouted in the late 1800s, and almost no new trees joined them after the 1940s. For decades, elk ate young shoots back before they could grow out of reach, so aging trees died and were not replaced.

In part, though not alone. The Oregon State team credits several forces at once. After wolves arrived in 1995 and 1996, they joined bears and cougars in killing elk, while hunters continued to take elk outside the park. Together, those losses shrank the northern herd and changed where it spends the winter. Before 1994, about 80 percent of the herd wintered inside the park. In recent years, about 80 percent has wintered outside it. The researchers also point to the harsh winter of 1996 to 1997, when thousands of elk starved and many others were shot after being driven out of the park, and they say the newcomers likely had little to do with that early drop.

A 2016 article in the park’s journal Yellowstone Science, led by Utah State University ecologist Daniel MacNulty, adds an important wrinkle. It notes that the northern herd’s latest decline began by December 1994. That was three months before the first reintroduced wolves left their acclimation pens. The authors wrote that wolves almost certainly contributed to the decline, but that scientists had not reached consensus on how much.

A 2018 Ecosphere study by the same Oregon State group compared aspen inside the park with three elk winter ranges outside it, where hunting and livestock grazing occur. In most areas, the share of young aspens browsed each year fell from 80 to 100 percent in 1997 and 1998 to 30 to 60 percent in surveys from 2011 through 2015. Places with less browsing had more saplings, and the authors concluded that both predators and human hunters influenced changes in elk numbers and distribution.

How Strong Is The Trophic-Cascade Evidence?

Real, but disputed in size. Ecologists call this chain reaction a trophic cascade: predators affect plants by changing the numbers or behavior of the animals that eat them.

A 2022 Ecology Letters study led by Utah State researchers followed young aspens in 113 randomly placed plots between 2007 and 2017, pooling 18,623 stem measurements. Many earlier studies had measured only the five tallest young aspens in each stand. Compared with random sampling, that shortcut overstated the growth of new canopy trees four- to sevenfold. The tallest stems had already outgrown the height at which elk like to feed, and the method missed stands where nothing was regrowing. Random sampling still showed a cascade, just a weaker one.

The 2025 Oregon State paper also reported a 152-fold increase in sapling density, a figure that made headlines, including in The Washington Post. In a comment published in 2026, MacNulty and colleagues argued that the team had miscalculated the placeholder value it substituted for the zero starting count, and that the true increase was about 17.5-fold. They also found that the densest 10 percent of plots accounted for 54 percent of average sapling density, while the median plot had no saplings at all. The Oregon State authors issued a correction. In a published response, they revised their effect-size estimate from 5.0 to 2.9 on a natural-log scale and maintained that it still marks a strong, ecologically significant cascade.

What Does The Evidence Actually Support?

Continued but uneven recovery is the most defensible reading. New pressures complicate the picture. A 2023 Ecology and Evolution study from the same researchers documented bison bulls snapping aspen saplings and found that 18 percent of sapling stems across the northern range had been broken. Other saplings died after their bark was scraped off, probably by bison horns. Bison numbers more than quadrupled between 2004 and 2015, and the authors wrote that the animals are now holding back the young aspen recovery and have undone it in some spots. In their 2025 paper, the team also cautioned that a warming, drying climate could keep aspen from coming back in some places and could make some current stands smaller.

The clearest legacy shows up at trunk height. For most of the 20th century, elk kept young aspens in northern Yellowstone under about 3 feet tall. In parts of the range, those young aspens are now growing into saplings and small trees. Elsewhere, elk, bison, climate and local shelter such as fallen logs still decide whether the next shoot reaches the canopy.