Defenders' Experts
Disturbance Regimes
Disturbances are generally defined as events and processes
that kill or
remove substantial portions of trees in a forest: wind,
flood,
landslides,
insect and disease outbreaks, and fire. The term
disturbance "regime" is most
commonly associated with fire,
which often
has a fairly predictable severity,
season of
occurrence, interval
between events, extent, etc. in a given region
and forest type. These
can then be referred to as, for
example, a frequent,
low-severity fire
regime, typical of
ponderosa pine in the West or long-leaf
pine in the
Southeast.
Thick, fire-resistant bark allows these species to
persist
despite frequent fires that historically thinned out smaller trees and
shrubs, producing stands described as open and park-like,
dominated by
older
pines.
In more moist areas, infrequent, high-severity
fire may kill all or
nearly all the trees in a given area.
Despite lurid coverage on the
nightly
news, forests are not
"consumed" or "destroyed" by fire. Even
when all the trees
are
killed, it's more accurate to say a forest has
been changed by fire
(albeit
changed rather dramatically from our
perspective).
Forest soils and biological
legacies (including dead
standing
trees [snags], logs, seeds, living underground
rootstocks, and
many animals) persist and contribute to the growth and renewal
of the
green stands that we more readily recognize as a
forest.
This total mortality is what some advocates of intensive timber management have in mind when they claim that clearcutting mimics fire. What this analogy overlooks is that fire doesn't remove trees, nor does it build roads, mechanically disturb soils or spray colonizing ("competing") shrubs with herbicides. Modified "clearcuts," with irregular borders and retained biological legacies such as snags, logs and large green trees, more closely approximate the effects of a high-severity fire.
Many scientists have suggested that management that
mimics or
approximates natural disturbance regimes is more likely to
maintain
the ecological functions, habitat quality and biodiversity
benefits we
seek.
This mimicry can take place at the stand level, where it will influence selection of a silvicultural approach, ranging from selectively cutting individual trees, to small patch cuts, to large "clearcuts" covering tens to hundreds of acres. These choices would also influence the frequency with which a manager would return to an area; in general less extractive approaches such as selective logging would require returning to an area every few years, whereas as much as a few hundred years might pass before an area would be "clearcut" again.
In this section. . .
Forests, in all their variety, provide benefits to biodiversity and management opportunities and challenges that are different from other ecosystem types.
Natural forests are a mosaic of stands of varying sizes and ages which supports a great diversity of wildlife.
Disturbances are events and processes that kill or remove substantial portions of trees in a forest: wind, flood, landslides, insect and disease outbreaks, and fire.
Forests depend on the rich variety of tiny living things residing in or on the soil of the forest.
