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

THE NORTHERN RANGE
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able uncertainty about the extent to which elk were
reduced on the northern range in the 1 870s and
l880s (Schullery and Whittlesey 1992). As already
noted, the market-hunting harvest was quite high in
the mid-1870s, but the slaughter seemed to have
dropped off by the end of that decade (Schullery
and Whittlesey 1992). It also appears that if elk
were markedly reduced during the 1870s, they
began to recover by the early 1880s. Houston
(1982) and Coughenour and Singer (1996a)
document the quickness with which the northern
elk herd recovered following the reductions of the
1960s, and there is no reason to believe elk could
not do the same in the late 1800s. As already
mentioned, Schullery and Whittlesey (1992)
published contemporary accounts of 5,000 elk
reported in mid-winter (February) on the northern
range between Mammoth Hot Springs and Cooke
City as early as 1883 and continuing after that year.
During the most recent period of extreme elk
population reduction (1963-1969), when there were
usually fewer than 5,000 elk on the northern range,
Barmore (1975) observed that they were still
numerous enough to browse almost all aspen
suckers (75 percent leader use). Some set of
circumstances, almost certainly involving climate,
enabled the aspen of the 1870s and l880s to grow
to tree height despite browsing, something they
were unable to do during the 1960s. Houston
(1982) reviewed climatic reconstructions based on
tree-ring analyses, concluding that "the highest
winter precipitation for the entire 161-year (1750-
1910) period occurred from about 1877 to 1890."
It seems most likely that a variety of factors
influenced aspen success in the 1870s and 1880s.
Besides climate, other factors that may have played
a part include fire history, commercial trapping of
beaver in the 1870s and 1880s, and reduction of elk
numbers or displacement of elk from some areas
by human activities, especially hunting. It may be
that all aspen clones that developed tree-size
growth in that period were not successful in doing
so for exactly the same reasons.
Whatever causes are eventually identified for
the success of aspen in the 1870s and 1880s, it is
clear now that the traditional viewpoint-that an
abrupt change in elk numbers is solely responsible
for the aspen decline-is incorrect. Elk prevent
aspen from reaching tree size today because elk eat
the young trees, but that does not mean that elk are
the sole cause. Research has resulted in "new
hypotheses that are being investigated: 1) under
proper growing conditions, aspen suckers can grow
large enough in a single season and can produce
enough defensive chemicals to keep elk from
completely debarking and preventing young plants
from attaining tree size. If proper conditions do
not exist, the clone is kept in a shrub or perennial
forb stage by browsing, but the clone itself-its
root system-is not eliminated; and 2) aspen show
an opportunistic sexual reproductive strategy
following burning.
The fires of 1988 provided an excellent
opportunity to test fire's role in the reproduction of
northern range aspen. In the year following the
fires, an unprecedented amount of aspen growth
from seedlings was documented (Kay 1993,
Renkin et al. 1994, Renkin and Despain 1996b,
Romme et al. in press). However, on the northern
range few if any of these seedlings have been able
to escape browsing and grow to adult height,
despite their tremendous numbers. Large fires
alone were unable to enable aspen to escape elk
and grow to tree height on the northern range
(Figure 4.8).
Seedlings have been much more successful
on Yellowstone's summer ranges at higher elevations.
The fires of 1988 led to the widespread
establishment of aspen Hin extensive portions of
YNP where there was no aspen before the fires"
(Romme et al. in press). Establishment sites were
primarily in burned lodgepole-pine forests,
especially in the west-central part of the park; this
event may have important implications for understanding
the long-term persistence of aspen in the
park. Genetic studies revealed that these new
seedlings derived from some distant seed source
presumably to the west of the park, not from
existing aspen clones in the park. The fires of 1988
thus resulted in an apparently substantial increase
in genetic diversity in the aspen populations of
Yellowstone National Park. These new seedlings
are persisting: "They are elongating slightly and
increasing in density in at least some places despite