Wolves helping aspens

By Cory Hatch, Jackson Hole, Wyoming

January 2, 2012

Fifteen years after the release of wolves into Yellowstone National Park, the predators’ impact on the elk population has helped aspens, willows and cottonwoods thrive, new research suggests.

Oregon State University researchers charted the growth of young aspens, willows and cottonwoods in the park’s northern range at various times since the 1995-96 reintroduction of wolves to see how many plants grew to a certain size and how many had been browsed by ungulates.

Yellowstone’s northern range elk population declined from 15,000 animals in the early 1990s to roughly 6,100 animals in 2010, partly due to wolf predation. During the same period, researchers found that aspen, willow and cottonwood were generally allowed to grow taller and thicker, and fewer trees showed signs of browsing.

Researchers also reviewed the literature on “woody browse” species and found that 12 of 13 field studies reported taller plants since wolves returned to the ecosystem. Two remote sensing studies showed increases in willow canopies, too.

The findings, which appeared Dec. 23 in the online version of the journal Biological Conservation, support earlier studies that showed wolves have improved habitat in the park.

The study was launched in 1997 in an attempt to understand why aspen trees were at the time declining in Yellowstone, said study coauthor William Ripple, an ecology professor at Oregon State University.

“There was no clear scientific answer,” he said. “The topic of predators and wolves was not on my radar.”

After coring aspen trees and counting tree rings, “we found that there was a major decline ... starting in the first half of the 20th century,” Ripple said. “Aspen continued its decline all the way up to our study period, in 1997. At that point, we looked at the records. We found that wolves were extirpated early on, and the last wolf was killed in 1926.”

Since wolves were returned in 1995 and ’96, Ripple and his colleagues formed a hypothesis that aspens and other plants would start growing again into tall trees.

Indeed, the percentage of aspen trees browsed in one habitat type ­­— upland areas without logs — dropped from 84 percent to 24 percent from 2006 to 2010. In 97 aspen stands, the mean height of the five tallest trees increased from 60.6 inches to 100.7 inches during the same four years.

Ripple emphasized that he measured the tallest trees in these stands, not the entire population.

In 2001 and 2003, researchers could find no cottonwood trees that reached 2 inches in diameter at breast height. In 2010, researchers found 156 trees that had attained 2-inch diameters at breast height. Willow stems were as much as twice as thick after wolf reintroduction.

The research provides a “pretty convincing case that the introduction of wolves has changed the Yellowstone ecosystem,” said Aaron Wirsing, an assistant environmental and forestry professor at the University of Washington.

Wirsing cautioned, however, that ecosystems are very complex, “so it’s really hard to isolate the effect of any one change.”

Changing climate and wildfire management are just two of many factors that could influence willow, aspen and cottonwood growth.

“But it’s hard to ignore the evidence suggesting that wolves have played at least a strong role in driving changes,” he said.

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