Present-Day Environments and Climate Change - Mountains and Plains: The Ecology of Wyoming Landscapes - page 37

ing of soils. 18 Attempts to suppress disturbances may
create an environment in which some native species
cannot survive.
Fire is a well-known natural disturbance that can
occur when there is sufficient fuel. Long before humans
learned to use fire, lightning strikes provided the neces-
sary ignition, even in grasslands. the number of light-
ning strikes to the ground is surprisingly large, often
several hundred during a single summer storm. not all
of them cause fires, but once started, prior to the time of
active suppression, the fires probably burned for weeks
or months over large areas. native Americans frequently
started fires, probably most often to facilitate their hunt-
ing of big game. Whatever the source of ignition, early
explorers often saw smoke on the horizon. Meriwether
Lewis wrote about fire and the scarcity of timber along
the Missouri River in 1805:

re-established by sprouting. in the same way, grasslands
also recover in a short time, with little plant mortal-
ity. in contrast, shrublands dominated by big sagebrush
may not recover for 10-20 years, simply because big
sagebrush lacks the ability to produce new stems and
leaves (shoots) by sprouting from its root system.
Windstorms constitute another periodic distur-
bance. individual trees are often toppled, creating small
canopy gaps important to the survival of some under-
story species. occasionally, large numbers of trees may
be blown down, as occurred in the teton Wilderness in
August 1987 and the Routt national Forest in october
1997. in a few minutes, the trees over large areas were
broken or uprooted, creating a landscape reminiscent of
that after the eruption of Mount St. Helens in Washing-
ton; all other environmental factors are affected. evi-
dence of windstorms is easy to find in mountain forests.
Fire and windstorms are physical disturbances, but
biotic disturbances are just as important. Such burrow-
ing animals as prairie dogs, pocket gophers, badgers,
and harvester ants create disturbances that often cause
the death of some organisms while providing a favor-
able environment for others (see chapter 6). the gaps
created in this way are individually small, but com-
bined they modify the environment over large areas.
Furthermore, burrowing animals move about on the
landscape, subjecting a large proportion of the land
to burrowing over long periods. Such disturbances are
sometimes a nuisance for livestock owners, but there
are beneficial influences as well—better infiltration of
water in a semi-arid environment, better aeration, mix-
ing of organic matter deeper into the soil, a higher level
of biological diversity, and generally more plant growth
(see chapter 6). though difficult to value monetarily, all
are examples of ecosystem services.
Population explosions of certain insects also create
disturbances. the insects most commonly involved in
the lowlands are grasshoppers and Mormon crickets,
which can reach densities of 5-25 per square foot in
grasslands and shrublands in some years, consuming
most of the herbaceous plant material in a short time.
Similarly, several kinds of bark beetles have histori-
cally killed trees over large areas from time to time. in
the past 20 years the beetles have affected forests over
a larger area than ever before observed, probably the
result of climate change. Aside from influencing spe-

this want of timber is by no means attributable to
a deficiency in the soil to produce it, but owes its
origin to the ravages of the fires, which the natives
kindle in these plains at all seasons of the year. the
country on both sides of the river, except some of
its bottom lands . . . is one continued open plain,
in which no timber is to be seen except a few . . .
clumps of trees, which from their moist situation, or
the steep declivities of hills, are sheltered from the
effects of fire.19 19

Fires could be annual events in some of the more pro-
ductive grasslands, or they could be comparatively rare
in some high-elevation forests. in the foothills, a fire
might be expected every 5-25 years. Modern-day sup-
pression of grassland fires creates a favorable environ-
ment for the establishment and growth of some trees
and shrubs.
All fires are conspicuous disturbances, but some are
more severe than others. crown fires in forests become
extremely hot and burn through the treetops, killing
most trees and abruptly changing the nature of the
ecosystem. in contrast, surface fires that burn slowly
through the forest undergrowth often consume only
the shoots of small plants and the organic material on
the forest floor; most mature trees survive, as do most
shrubs, grasses, and forbs, because their roots and
rhizomes remain alive. Small trees may be killed, but
within a year or two, most of the herbaceous plants are