The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 217

izes general fuel conditions and potential fire behavior
in conifer forests during four stages of an outbreak—the
green stage early in the attack; the red stage, when the
trees are covered by dead needles; the gray stage, when
the dead needles have fallen but most of the dead tree
boles are still standing; and the old stage, when the dead
trees have fallen and a new overstory is developin g. 66
their model incorporates data on surface fuels, which
support a surface fire of varying intensity; crown bulk
density or total mass of needles and small twigs on the
trees, which can support the burning of tree crowns;
and ladder fuels , which are small living trees that can
carry fire from the surface fuels into the canopy. two
types of crown fire are possible: torching (the burning
of some individual tree crowns) and active crown fire
(in which the flames spread from tree crown to tree
crown and through most of the forest canopy, killing
the majority of trees).
Hicke and colleagues found no substantial change
in surface fuels or canopy bulk density during the red
stage, but the dry, dead needles at this stage increase the
probability of torching and active crown fire. crown
bulk density is greatly reduced during the gray stage,
which reduces the likelihood of active crown fires. How-
ever, surface fuels increase during the gray stage, lead-
ing to a potentially more intense surface fire, which in
turn may produce torching of individual tree crowns.
in the old stage, the fallen boles of the beetle-killed trees
contribute to a potentially intense surface fire. More-
over, the ladder fuels that have developed at this stage
increase the potential for crown fires.
two primary caveats are necessary when consider-
ing this conceptual model. First, there is disagreement
about many of the model's interpretations, partly
because bark beetle effects are variable and therefore
workers in different settings will reach somewhat dif-
ferent conclusions . 67 Second, although the conceptual
model focuses on the effects of changes in fuel condi-
tions, weather conditions at the time of a fire are as
important—or more important—than subtle differ-
ences in fuels. When fuels are dry, temperatures are
high, and winds are strong, any of the four stages
are easily burned. indeed, the patterns described in
the model may be relevant only during intermedi-
ate weather conditions, that is, when not too wet and
not too dry. When wet, nothing will burn and when

extremely dry, everything will. 68 Although beetle-
affected forests appear highly flammable for several
years after infestation, green forests can burn just as
intensely under hot, dry weather conditions.
Most forest fires occur during relatively extreme
weather conditions, especially in lodgepole pine and
spruce-fir forests. consequently, it is not surprising that
the influence of beetles on flammability has been small
or undetectable. one study of the 1988 Yellowstone fires
found that forests were more likely to have burned in
1988 where high beetle-caused tree mortality occurred
15 years earlier—though only about 10 percent more
likel y. 69 An examination of all fires in northwestern
colorado and southern Wyoming during the past cen-
tury found that forests where bark beetle outbreaks had
occurred previously were no more likely to have burned
with high severity than forests with no previous his-
tory of beetle activity . 70 in contrast, years with large
high-severity fires were all drier than years without
fires. overall, drought and wind—not beetles—are most
highly correlated with high-severity fires.
the implication of these recent studies is that large
fires and extensive insect outbreaks tend to occur
together, not because one causes the other but because
both respond to similar climatic conditions—warmer
winter and summer temperatures and earlier melting
snowpack. these conditions create a longer fire sea-
son and greater drought stress for trees in late summer.
Stressed trees are less able to defend themselves against
bark beetles, and the beetles themselves are better able
to survive in the winter, because long cold periods are
less frequent—a convergence of conditions that favor
both fire and beetle disturbance and that is likely to
become more frequent in the future. 71

Some Management Options

in large roadless areas, such as the backcountry of
Yellowstone national Park, managers often allow fires
and insect outbreaks to spread without interference.
Few resources are threatened and such disturbances
bring about ecological benefits. native plants and ani-
mals are adapted for such events; the ecosystems are
resilient. 72 t his is true even if there are more fires in
coming decades. Studies of charcoal and pollen pre-
served in lake sediments reveal that the Yellowstone