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

WOODY VEGETATION
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willows in Yellowstone
National Park. Some are
difficult to identify when
specil11:ens are available for
examination. Thus, historic
photographs rarely allow for
species identification, but do
provide gross evidence of
changes in abundance or
distribution. Many observers
have attributed the decline in
willows on the northern range
to overbrowsing by elk
(Grimm 1939, Kay 1990,
Chadde and Kay 1991, Wagner
et al. 1995a). Other suggested causes include
declines in beaver since the 19208 (Singer et al.
1994), the colonization of the northern range by
moose early in the 1900s (Houston 1982; Chadde
and Kay 1988, 1991), fire suppression activities by
park managers since the late 1800s (Houston 1973,
1982), and climatic variations, especially the
drought of the 19308, which, tlu-ougb changes in
groundwater levels or other processes, might have
either directly killed the plants or reduced their
ability to produce secondary defensive chemical
compounds that make them less palatable to
grazers (Houston 1982; Singer et al. 1994, 1996b).
Elk are usually the proximal factor in the
decline of willow stands (Figure 4.3). Elk very
likely accelerate willow declines that might be due
to other causes. However, not all declines in
willows are due to ungulate browsing. Houston
(1982) pointed out that an entire stand of willows
on tl,e lower northern range was defoliated by
Disonycha pluriligata, a native beetle whose
presence on the northern range was unknown until
his study; he suggested that it would have been
easy for the casual observer to mistake the aftereffects
of this defoliation for heavy browsing by
ungulates. In another instance, willow stands and
individual willows died in the summers of 1988-
1989 apparently as a consequence of the severe
drought of 1988. Those dying stands were measured
for water pressure in 1988 and had the lowest
measurements of any of the many willow stands
measured that year, verifying the drought stress
(Singer et al. 1994).
A third alternate factor in willow declines is
fire. Willows on several marked transects burned
and were either killed outright in 1988 or never resprouted
or recovered (Norland et al. 1996). This
is contrary to what others have observed. According
to Wolff (1978) and MacCracken and Viereck
(1990), willows virtually always vigorously resprout
and reseed following fire. Some burned
Yellowstone willow stands were immediately
replaced with grasslands or forb vegetation. We
interpret this as additional evidence that willows
cannot recruit in many areas of the northern range
and are failing on some sites under existing climate
conditions or because of low surface/ground water
control.
Popular perceptions of willows, as with
aspen (see next section), seem to be of a progressively
declining group of species that has been
fading from the northern range since early in this
century. However, over the course of this century,
most willow declines on the northern range
occuned in the 1930s. Most ofthese occuned
during the extended drought of that period, and in
the face of elk numbers much lower than at present
(Houston 1982, Singer 1996b). These declines
included the disappearance of some willow stands
and suppression of the heights of some other
willow stands. Since then, there has been little
change in willow status, perhaps lending credence
to the idea that climatic forces, rather than ungulate
browsing, may be the most important factor in
Figure 4.3. Willows
are protected from
allllngll/ate
brolVsing ill the
Junction Butte
exclosltre. Casual
observation of this
and other excloSllres
has led to a mistaken
impression that all
vegetation all the
northern range
"should" have the
same appearance as
that inside
exclosures. The only
way the vegetation
within the exclosllres
could be replicated
across the whole
northern range
would be by
completely
eliminating
IInglllates from the
range. NPS photo.