Yellowstone's wolves and aspens: the famous figure was 152 times more young trees, and it has been corrected to 17.5

In January 1995, 14 wolves from Canada arrived in Yellowstone (United States). There had been none in the park since around 1930. Thirty-one years later, a new generation of quaking aspens is growing in the north of the park. But the most famous figure from the study that reported it has had to be corrected.

What the study found

An Oregon State University team led by Luke Painter visited 87 aspen stands in 2020 and 2021. It published the results in Forest Ecology and Management in 2025:

• In 43% of stands there were young trees 5 to 10 cm in diameter, something not seen there since the 1940s.
• In 1998 there wasn’t a single young aspen taller than two metres.
• The average was 1,460 young aspens per hectare.

The explanation is a trophic cascade. With wolves (as well as bears and cougars) there are fewer elk, which ate the shoots. Northern Yellowstone counted about 17,000 elk in 1995 and 6,673 in 2022.

The correction: from 152 to 17.5

The figure that went round the world was a 152-fold increase in young aspen density. A team led by Dan MacNulty, an ecologist at Utah State University, pointed out that the baseline density had been miscalculated. The authors published a correction: the real increase is 17.5-fold.

MacNulty adds that 20% of sites account for 80% of the increase and that more than half had no shoots at all. Painter replies that the corrected figure still supports their conclusions. Neither denies that wolves have an effect; they disagree on how much.

What we still don’t know

Measurements end in 2021, so it’s not yet known how many of those trees will reach adulthood. Bison also eat the shoots in some areas. And Yellowstone’s bison keep growing: from two dozen to nearly 5,300.