Wolves have not helped the aspen trees in and around Yellowstone National Park as much as previously thought, a study to be published in the journal Ecology suggests.

The finding challenges the assertion that aspens in Yellowstone had rebounded thanks to wolves, a claim that has become symbolic of the far-reaching affects wolves have on their environment and, for some, a beneficial payoff of the controversial reintroduction.

Researchers with the U.S. Geological Survey, the University of Montana and Humboldt State University have found that elk continue to put pressure on aspen groves, regardless of whether wolves are near.

And, while the Northern Yellowstone elk herd is 60 percent smaller than it was before wolf reintroduction in 1995, "none of the aspen groves studied after wolf restoration appear to be regenerating," said Matt Kauffman, a USGS scientist and lead author of the study.

"Elk forage pretty heavily on aspen and it's really still unknown whether aspen are going to be able to recover," he said.

But while the findings refute what's become a common narrative in the wolf debate - that by eating elk, the predator has allowed aspens and other elk food to rebound from over-browsing -- Kauffman said it shouldn't affect the conversation about the overall worth of wolves.

"I don't think our study has any real bearing on the merits of wolf introduction," he said. "The reintroduction was an incredible wildlife-management success."

Instead, Kauffman said the results clarify the complex relationship between elk, wolves and aspen.

A theory amongst scientists - fleshed out in a widely circulated study published in 2007 in the journal Biological Conservation -- has been that aspen in Yellowstone were regenerating in areas of the park where wolves were common. Elk avoided those areas, the theory went, giving the young aspen a chance to grow to maturity. In other words, wolves acted as unwitting guard dogs for baby trees.

As Science Daily put it at the time, the 2007 results were "especially encouraging for the health of America's first national park, but may also have implications for other areas of the West and other important predators."

But Kauffman and other researchers have since mapped places in the northern elk herd's range where wolves posed a serious risk to elk, as determined by elk kills, and where they didn't. The researchers found aspen trees were browsed in both areas.

And, they found that the elk population in the area is still too large to allow a significant aspen rebound.

"A landscape-level aspen recovery is likely only to occur if wolves, in combination with other predators and climate factors, further reduce the elk population," Kauffman said.

Daniel Person can be reached at dperson@dailychronicle.com or 582-2665.

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