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

THE NORTIlERN RANGE
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Figure 4.5. Aspen
inside a northern
range exclosure
grows without the
effects of ungulate
browsing. The most
recent study of aspen!
fire/ungulate history
on the northern range
suggests that there
has only been one
briefperiod in the
past two centuries
(roughly 1870·1895)
when aspen was able
to grow to tree height
(Romme et al. 1995).
NPSphoto.
declined, that there has been
little growth of new aspen trees
since about 1900, and that
aspen are now reaching "old
age" and dying (Figure 4.5). In
many cases the aspen clone is
still viable; it is now in a shrub
form, apparently unable to
escape ungulate browsing and
grow to tree height. The
reasons for this change have
been the subject of study and
sometimes heated discussion
for many years (Barmore 1980;
Tyers 1981; Houston 1982;
Despain et al. 1987; Kay 1990, 1993; St. John
1995; Wagner et al. 1995a). Continued decline of
aspen is expected under the current combination of
high ungulate use in the park, combined ungulate
and livestock use outside the park, and warmer and
drier climatic conditions.
In a study of aspen on the northern winter
range in the Gardiner Ranger District of the
Gallatin National Forest immediately north of
Yellowstone National Park, St. John (1995)
confirmed heavy ungulate use of aspen stands
there, as well as significant impacts by livestock.
Aspen stands in scree communities (which are
more difficult of access for ungulates and livestock)
and within 1,600 feet (500 m) of main roads
(from which hunted ungulates are often displaced
by human activities), recruited new stems more
successfully than other stands that were accessible
to ungulates.
The decline in aspen has for many years been
used as proof of elk overpopulation on the northern
range (Pengelly 1963, Kay 1990, Wagner et al.
1995a). But other investigators have suggested
that the decline of aspen might be more the result
of changing climatic conditions and fire suppression
(Houston 1982, Despain et al. 1986) Recent
research has introduced important new elements
into this debate.
For most of the history of the northern range
issue, mbst positions on aspen implicitly assumed
that prior to the establishment of the park in 1872,
aspen was a regular part of the setting, reproducing
consistently. A variety of recent studies now
challenge that assumption, and dramatically change
our understanding of aspen as a member of the
northern range plant community. Engstrom et al.
(1991, 1996), in an analysis of pollen in pond
sediments on eight northern range ponds, determined
that aspen has been a marginal species on
the northern range for thousands of years.
In a tree-ring study of northern range aspen,
Romme et al. (1995) showed that since the early
1800s, there has been only one period during which
aspen grew to adult height in large numbers; that
period was from the 1870s to the 1890s, when most
of the aspen that are now growing old on the
northern range began their growth (Figure 4.6).
Romme et aI's. work was anticipated by Warren
(1926), who dated aspen near present Roosevelt
Lodge and found that these also dated from a short
period of time, with a pronounced peak in the
1870s (Figure 4.7). These trees, then, were part of
the same period of growth documented later by
Romme et al. (1995). The work of Romme et al.
(1995) and Warren (1926) suggests that in the early
1800s, aspen were typically unable to grow to tree
height; it seems most probable that some combination
of factors, including elk browsing and fire,
prevented that growth.
Despain (U.S. Geol. Surv., unpubl. data)
sectioned 50 dead aspen trees on the northern
range, and discovered that 49 of them showed signs
of annual browsing in their first three years of
growth. The work of Despain, Romme et al.