CORVALLIS, Ore. –
An analysis of 24 studies over a 15-year period has confirmed that
wolves and their influence on elk represent a major reason for the
recovery of trees that had previously been declining for decades in
Yellowstone National Park.
“For decades, nothing had been growing into the smaller age classes of trees because of intensive elk browsing,” Beschta said.
Despite
long-term trends of increased temperatures and reduced precipitation,
trees such as cottonwood, willow, aspen and other woody species have
been showing signs of accelerated growth in many areas since wolves were
restored to the park in 1995. Beavers and riparian songbirds are also
showing signs of coming back to areas where they had been missing or in
decline since the 1930s.
Still,
it will likely take many years for established shrubs and trees to
reach a size sufficient to produce the abundance of berries and seeds
that support a diverse ecosystem.
Those
are among the conclusions reported today in the journal Biological
Conservation by Robert Beschta and William Ripple, two professors in the
Oregon State University College of Forestry. They analyzed the results
of 24 studies of streamside vegetation published since 2001 and reviewed
long-term trends in temperature, precipitation, snowpack and stream
discharge.
“When
I first started studying this in 2001,” said Beschta, “I was skeptical
that elk, a native ungulate, could stop nearly all cottonwood
recruitment. But it was the elk that had damaged plant communities
during the period when wolves were absent, and the reductions in elk
browsing, since wolves have returned, are allowing them to begin
recovering.”
In
subsequent studies, Beschta and Ripple, as well as other researchers,
measured the diameter of cottonwoods and aspen in the park’s northern
range. They found young trees almost completely missing.
In
their latest assessment, Beschta and Ripple reviewed 11 published
studies of willow, six of aspen and five of cottonwood as well as one
each of service berry and thinleaf alder. All but two of the studies
showed increases in height, diameter, canopy cover or recruitment for
these species. The area of land covered by willow, for example, doubled
between 1991 and 2006. By 2003, young aspen trees in many areas were
starting to grow measurably higher.
More
than half of the reviewed studies also measured browsing effects on
plants, caused principally by elk. Those studies concluded that tree
recovery had begun mostly because of a decrease in browsing.
“Climate
may influence whether trees recover more quickly in some areas than in
others to some degree, but the real issue for plants growing in
Yellowstone is, how often are they browsed by ungulates?” Beschta added.
Elk
numbers in Yellowstone have declined by more than two-thirds since
1995, from a high of nearly 20,000 to less than 5,000 today. The numbers
and impacts of deer and pronghorn are relatively small, but in the past
decade, bison herds have grown, and they tend to reside in valley
bottoms much of the year. Bison grazing has prevented cottonwoods,
willow and other plants from successfully recovering in parts of the
Lamar Valley, he said.
Over
the past 20 years, mean temperatures and precipitation in the northern
range have changed in comparison to the long-term mean going back to
1895, when recordkeeping began. As measured at the Mammoth weather
station in Yellowstone, annual mean temperatures today are more than 2
degrees Fahrenheit warmer than in 1895 and annual precipitation almost 3
inches lower.
“Research
results following wolf reintroduction are generally supportive of the
concept that the contemporary carnivore guild has, via a trophic
cascade, mediated the effects of elk herbivory on riparian plant
communities,” the authors wrote. “The
ongoing reduction in elk herbivory has thus been helping to recover and
sustain these plant communities in northern Yellowstone, thereby
improving important food-web and habitat support for numerous
terrestrial and aquatic organisms.”
About the OSU College of Forestry:
For a century, the College of Forestry has been a world class center of
teaching, learning and research. It offers graduate and undergraduate
degree programs in sustaining ecosystems, managing forests and
manufacturing wood products; conducts basic and applied research on the
nature and use of forests; and operates 14,000 acres of college forests.
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