Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 181

Disturbance and Change over Time: Another
Important Source of Variation in Forest
Communities

disturbances in forests are especially noticeable be-
cause they commonly occur over large areas and their
impacts often last for decades (fig. 11.5). 26 i n any moun-
tain landscape, a shifting mosaic of forests responds
both to local environmental conditions and to the
effects of disturbances, some of which were recent and
others centuries old.

Forests are always changing, sometimes so slowly that
the changes are imperceptible. the ecological processes
that drive slow changes include competition among
trees for light and other resources, selective feeding
by herbivores, tree death, seed dispersal, and germi-
nation. other changes are rapid and obvious, usually
resulting from discrete disturbances that kill or injure
many of the trees and other forest organisms. exam-
ples include fire, an insect outbreak, a severe wind-
storm, and timber harvesting. Forests also change in
response to variation in the local climate. For example,
a severe but temporary drought may injure or kill sus-
ceptible plants; or a long-term directional change in
climate may lead to a gradual change in overall species
composition, with new species moving in while others
decline. in addition to the adaptations already dis-
cussed, surviving in Rocky Mountain forests requires
an ability to bounce back after the disturbances that
have occurred for millennia . 25
Landscape changes caused by disturbances and bi-
ological processes are superimposed on the patterns
that develop in response to environmental gradients
in local climate and soils. Both kinds of pattern are
important in all vegetation types, but the effects of

Forest Fires

two key aspects of all disturbances are their frequency
and severity. the natural frequency of fire varies from
decades to centuries among different forest types. Forest
fires tend to be most frequent in the foothills and lower
elevations of the mountains, where precipitation is suf-
ficient to support flammable vegetation and where sum-
mers are often dry enough to permit fires to ignite and
spread over large areas. Late nineteenth- and twentieth-
century fire exclusion reduced fire frequency in many
lower-elevation landscapes. 27 Fires became less frequent
in some areas because livestock grazing reduced the
abundance of fine-textured fuels, and roads, fields, and
towns broke up formerly continuous expanses of flam-
mable vegetation.
At higher elevations, fires are notably less frequent
because of the typically moist conditions in those en-
vironments: the snowpack melts later in the spring,
or not until midsummer. Also, summer rainstorms
often dampen the fuels, and relative humidity is often
higher than at lower elevations. Lightning ignites fires
every year in high-elevation forests, but most fail to
spread beyond the ignition point because fuels are too
wet to burn. only in unusually dry years are weather
conditions suitable for extensive fire spread. Signifi-
cantly, it's only during those historically infrequent
years that large fires occur in high-elevation forests.
compared to climatic factors, fire suppression has
had a small impact on the frequency and size of high-
elevation fires.
two related terms are fire intensity and fire sever-
ity. Fire intensity refers to the energy released in a fire,
expressed as BtUs per minute per unit area; fire sever-
ity refers to the effects of that energy release on organ-
isms or ecosystems. intensity must be measured while
the fire is in progress, which is difficult. Severity, how-
ever, can be estimated after a fire by characterizing tree

Fig. 11.5. tree-killing bark beetles of the genus Dendroctonus,
such as this mountain pine beetle, are native insects that have
coexisted with pine trees for thousands of years. the adults
are about 0.2 inch in length, though their wingspan is much
longer. Many species of bark beetles do not kill trees, subsist-
ing instead on the inner bark of trees that have recently died
from other causes. Photo by Dion Manastyrski, from the col-
lection of Lorraine Maclauchlan.