Yellowstone's Northern Range - Greater Yellowstone Science ...

THE NORTHERN RANGE
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to confirm and refine the findings of Romme et al.
(1996). Age-structure studies of aspen near the
west and south entrances of Yellowstone National
Park would provide instructive comparisons with
northern range aspen from nearby areas with
different ungulate and fire-history regimes. Agestructure
studies from Grand Teton National Park
would likewise be helpful.
Interesting avenues for experimental management
exist. It may be possible, for example,
through judicious "fencing" with downed trees and
other devices, to create local aspen refugia: to
exclude elk and other ungulates from some aspen
clones and thus ensure the growth of new aspen
trees in some locations on the northern range.
Such a management action would be relatively
nonintrusive, and would probably reassure many
enthusiasts of aspen and their dependent species.
Whether such an action is in keeping with current
National Park Service policies, or is ajustifiable
divergence from that policy, is a more difficult
question. However, the creation of a few such
aspen refugia would be more than justifiable as a
research program to test specific hypotheses about
interactions among elk, aspen, fire, climatic
change, and geomorphic processes. Such a study
would be more useful if it also included sites with
other species of interest, such as willow, cottonwood,
and other riparian species.
ASPEN, WILLOWS,
AND BIODIVERSITY
As with grassland studies, researchers have
emphasized the importance of an integrated
understanding of the many influences on woody
vegetation on the northern range. And to perhaps
an even greater extent than with grassland studies,
the role of climate in the current condition of these
species is a recurring, even overriding theme.
There remains no question that ungulate browsing
is the immediate cause of the decline of aspen and
willows on the northern range, but there is considerable
uncertainty over why that browsing has a
different influence now than it has had historically.
Aspen and willows are favored though minor
food items for elk in Yellowstone (Barmore 1980,
Singer and Renkin 1995). Even the complete
absence of these species on the northern range
would not have any significant effect on the
nutritional opportunities available to the northern
Yellowstone elk herd. Thus it might seem puzzling
at first that the decline of aspen and willows, two
of the most abundant plants in North America,
should play such a major role in the dialogues
about northern range grazing. But these species, as
well as other woody vegetation, such as cottonwood,
have important cultural values for what they
contribute aesthetically to the enjoyment of
western landscape, and they also have important
ecological values as habitat to species of animals
that would not otherwise occupy the northern range
in meaningful numbers, especially some species of
birds. Jackson and Kadlec (1994) found that "total
bird densities and total number of species were
lowest in sites in which 70 percent or more of the
willows were severely browsed, suggesting that
birds have a threshold of tolerance for browsinginduced
changes to the vegetation." They also
suggested that "intense browsing does affect the
assemblages of breeding birds in willows, but we
also speculate that factors such as food abundance,
type and gradient of adjacent plant community, and
soil-water relationships are important."
The plight of neotropical migrant birds in the
western hemisphere has been widely publicized in
recent years; they are experiencing massive habitat
degradation in wintering areas in Mexico and
Central America. Though the willows of the
northern range constitute less than one percent of
park willows, the northern range is currently home
to a more significant percentage of adult aspen
clones in the park, as well as to many of the park's
cottonwoods and some other woody species.
Changes in the status of these species can ripple
through the wildlife community in complex and
significant ways, so there is some urgency to
research questions relating to woody vegetation on
the northern range.
But as explained in the "Willows" section,
dense tall-willow habitats occur in Yellowstone in
summer ranges above 7,000 feet and in many other
places in the greater Yellowstone area. Vigorous
aspen clones also are available, particularly in the