|HERALD AND NEWS, Klamath Falls, Oregon - Sunday, November 19, 2000|
found a missing link between aspens, elk - and wolves.
is typical of the park's aspens, which are withering in the absence of wolves.
|By John Bragg
IT'S EASY to spot aspens in autumn. The combination of red, gold,
yellow and pale green leaves splashed across a white-barked thicket is
unmistakable, especially when seen against dark evergreen forests that
usually adjoin them.
Wolves play among aspen in the Lamar River
Valley in northern Yellowstone National Park.
William J. Ripple, professor of forestry at the Oregon State University Forest Resources Department's Environmental Remote Sensing Applications Laboratory (ERSAL) in Corvallis, studied the decline of aspens in the northern range of Yellowstone. He and Eric J. Larsen, of OSU's Department of Geosciences and ERSAL, believe the elimination of the gray wolf in the early 20th Century may have altered the ecology of the park. Freeing elk from the risk of predation may have encouraged them to move into aspens, where they could browse unmolested on young suckers.
The elk may have eliminated young, replacement trees that should have repopulated the aspen groves.
Their research was recently published by the journal Biological Conservation.
Aspens do not commonly reseed, but instead usually grow by sending up suckers from an extensive root, or clone. The trees are only the above-ground projections of a larger organism.
Ripple and Larsen studied changes in the distribution of mature aspens. They used aspen cores to determine growth rates of existing trees, then applied those rates to aspen tree recorded in a 1926 study in the park. Those stands appeared to have originated between 1751 and 1920.
A bull elk in Yellowstone.
| In comparison, most contemporary stands appear to have originated
between 1871 and 1920. About 85 percent of aspens growing today date from
that time. Ten percent originated before 1871, but only 5 percent of mature
aspens originated after 1921.
"Successful aspen 'overstory' recruitment occurred on the northern range of Yellowstone National Park from the middle to late 1700's until the 1920's, after which it essentially ceased," the researchers concluded.
The interplay between elk, aspens and wolves may be a cause of the decline. Park officials and other research confirms elk like to browse aspen suckers. Given enough elk, the suckers never live long enough to gain mature tree height. But elk have browsed Yellowstone's aspens for centuries. Why would their browsing make a difference now?
Ripple and Larsen think the removal of wolves might be the answer.
"Overstory recruitment" ceased when wolves were eliminated from the park, said Larsen. Wolves lurking among aspens likely killed elk or modified their movements and browsing patterns. Once the wolves were gone, the elk were free to browse among the aspens unhindered. The presence of wolf-killed elk also may have attracted grizzly bears, which prey on newly born elk. Wolves and grizzlies likely were enough to cause elk to shun the dense aspen groves.
Elk stripped the bark from these aspens near
Mammoth Hot Springs, leaving blackened trunks.
It was called "market hunting."
Yellowstone was declared the nation's first national park in 1872. Hunting was made illegal in 1883, but it went on regardless: A relentless slaughter of wildlife.
"All large wild animal populations in the park were being decimated," Ripple said. "Bison, elk and other large animals were shot for their hides and the carcasses were baited with poison to eliminate the coyote and the wolf."
The Army assumed park administration in 1886 and put an end to poaching. By the turn of the century people were beginning to feel newly-protective of Yellowstone's wildlife, especially of its bison, deer and elk. The herds began to recover. Bereft of the browsers, aspens had grown unhindered for a while. A 1995 study confirmed most of the park's mature trees originated during the market-hunting heyday.
The wolf population also recovered - until the decision was made to extirpate them.
Park records document 138 wolves killed, although the total was likely more, Ripple said. The last den of wild wolves was destroyed in 1923.
Since 1921, there has been little or no replacement of mature aspens. "Our aim was to investigate if the absence of wolves was potentially an important factor in the decline," Ripple said. They eliminated other factors such as fire suppression, invasion by conifers, climate change or development
The first hunts reduced all the game, and aspen suckers grew without browsing. The second round eliminated wolves - and changed the browsing patterns of the elk.
The disturbance to predator-prey relationships, especially between wolves and elk, has been a major factor in (the park's) aspen decline, Ripple concluded.
Wolves were reintroduced to Yellowstone in 1995. That may be of long-term benefit to aspens.
An elk roams in sagebrush near Mammoth.