Benchmark Ecosystems
One of the most salient values of biological reserves is
to provide benchmarks or controls for research and monitoring programs designed to determine the ecological effects of
various forest management practices. An example is the control watersheds used at experimental sites such as Hubbard
Brook Experimental Forest in the White Mountain National
Forest in New Hampshire and Coweeta Ecological Research
Forest in the Southern Appalachians (see Box). Given our
current understanding of population, ecosystem and landscape processes, it is clear that such control areas need to be
large, well distributed, and representative of the kinds of
ecosystems that we are managing. Ecological reserves provide us with a context for evaluating the rest of the landscape that is subject to our manipulation.
Recreational and Aesthetic Values
It is also important to note that other attributes of reserves give them unique value, albeit beyond the purview of
ecological science. Many people value reserves for spiritual,
aesthetic, and recreational reasons simply because they are
places that are not routinely modified by human hands (Figure 6). Maintenance and enhancement of these forest values
has clearly become increasingly important to society and a
necessary part of sustainable forest management.
Are reserves more vulnerable to natural disturbances?
Two related premises are stated or implicit in some forest proposals: one, that reserves are more vulnerable to natural disturbances than actively managed landscapes; and two,
that natural forests are more susceptible to disturbances than
managed forests. A corollary is that we can, therefore, create managed forest landscapes that will be less susceptible
to disturbance than natural forest landscapes.
There is no evidence to suggest, however, that natural
forests are more vulnerable to disturbances than managed
forest stands. Indeed, there is considerable evidence to the
contrary, evidence that natural forests are actually more
resistant to many types of both small- and large-scale disturbances. This is a very complex issue, yet in most cases, the
natural landscape proves to have the greater natural resistance to disturbance. We consider here evidence from three
types of disturbance: wildfire, windthrow, and pests.
Wildfire
It has been an article of faith in forestry for many decades that a managed landscape is less susceptible to wildfire than a wild landscape. Indeed, conversion of old-growth
forests in the Pacific Northwest has sometimes been justified
on grounds that it reduced the potential for catastrophic
fire. Scientific investigation has shown that, of all of the
forest ages and conditions, unmanaged old-growth forests
in this region are least likely to burn catastrophically. The
resistance of such forests to fire is related to a variety of
factors, including the cool, moist, windless microclimate characteristic of old-growth forests. Old growth forests do contain immense fuel loads, and when they do burn, fire suppression may be very difficult. This is the source of their
bad reputation with foresters because in the early part of
the twentieth century, policy required forest managers to
fight fires in these forests. Essentially all of the large catastrophic fires in Pacific coastal old-growth forests during
the last half of the nineteenth and first half of the twentieth century were of human origin. For example, the famous Yacholt Burn of 1902 in Washington and the Tillamook
Burn of 1933 in Oregon originated outside of massive oldgrowth forests and spread into them under very dry and
windy conditions.
Some of the greatest wildfire risks and most difficult
fire-control situations occur in landscapes that contain intimate mixtures of both young forestseither managed or
natural and old-growth forests. Young stands are more
likely to burn than old-growth stands, particularly if the
young stands result from earlier wildfires and incorporate
large amounts of dead fuel from earlier stands. If the young
stands are the result of regrowth after timber harvest, human access to the region is likely to have been dramatically
increased by road construction, and this has mixed consequences: it provides improved access for fire suppression, but
it greatly increases the chances of both accidental and intentional human ignitions, which are now the most important source of ignitions in many forests. Some landscape
models suggest the concept of mixed landscape is risky (e.g.,
Franklin and Foreman 1987). These models are supported
by empirical evidence from fires in southwestern Oregon and
northern California in which cutover areas with young coniferous stands burned catastrophically while intervening residual old-growth forest patches experienced reduced fire
intensities and partial or complete survival (Perry 1998).
Windthrow
In some forest regions in the West, evidence indicates
managed landscapes containing mixtures of forest conditions
and age classesincluding high contrast edges where forest stands meet cutover patchesare more vulnerable to
catastrophic windthrow than natural landscapes. For example, a massive blowdown in Oregons Bull Run River watershed in the early 1980s was primarily a result of dispersed-patch clearcutting and resulting high contrast edges
between cutovers and roads and residual old-growth forest
stands (Franklin and Forman 1987). In the case of hurricane damage in the Northeast, however, older stands,
especially mature white pine forests, are at higher risk of
blowdown.
Insects and Disease
Managed landscapes may also create conditions that
are more favorable for outbreaks of insect pests and disease.
For example, the creation of large pine plantations in the
southeastern United States has provided optimal conditions
for large-scale outbreaks of the southern pine beetle. The
fact that managed landscapes tend to be less diverse and
provide large contiguous blocks of one or a few susceptible
species and age classes makes catastrophic outbreaks of
many pathogens more likely. In contrast, however, the older
spruce stands in the Canadian boreal forests are most susceptible to spruce budworm outbreaks. In this case, it may
be that management efforts to reduce natural disturbance
maintain these spruce forests beyond their natural life
span.
Summary
There is no easily generalized evidence that a managed
landscape will be more resistant to catastrophic disturbance
than a natural landscape. Since forest managers and researchers both have had limited success in predicting the
occurrence of catastrophic events much before they occur, it
is not practical to attempt to preempt the role of natural
disturbances by harvesting stands prior to their occurrence.
SUBSTITUTING SILVICULTURE FOR NATURAL
FOREST PROCESSES
One implicit assumption in some analyses and policy proposals is that management of forest stands by silvicultural
manipulation can fully substitute for natural forest processes
in maintaining the full ecological values of forests. This includes substituting management practices for the effects of
the natural disturbances and successional changes in forests
over decades to centuries.
Duplicating natural disturbance processes
A tenet of forestry for many decades has been that regeneration harvest techniques are modeled on, and essentially mimic, natural disturbance processes. For example, it
is commonly stated that clearcutting is comparable to the
destruction of a forest stand by wildfire. In light of current
ecological knowledge, we consider here whether it is justified to assume that timber harvest practices can duplicate
and substitute for the effects of natural disturbance.
Consider first what we know about the character of
natural forest disturbance processes. It is useful to recognize
two broad classes of disturbance: chronic disturbances, which
are of moderate intensity and produce low to moderate levels of mortality of existing dominant trees; and catastrophic
disturbances, which result in death of the majority of existing dominant trees and regeneration of a new tree cohort.
It is necessary to consider both types of disturbance since
they have different implications for management potential
and problems.
Chronic disturbance
Forests subject to frequent, light- to moderate-intensity
disturbances tend to absorb the effects of the disturbances
and incorporate them into the basic fabric of the stand. Examples include forests subject to repeated fires of light to
moderate intensity, such as the pine forests of interior western North America, and forests subject to frequent gap-scale
wind disturbances, such as many of those found in the Northeast. Such disturbance patterns create and maintain structurally complex forests with multiple canopy layers and uneven tree ages over large areas for long periods of time.
Ecologically, these forests are mosaics of small structural units.
While location of individual structural types changes over
time, the forest as a mosaic is stable (Bormann and Likens
1979).
Typically in such ecosystems, late-successional forests
permanently occupy very large percentages of the landscape.
While individual patches are dynamic, the forest as a whole
is very stable since it is rarely subject to a large-scale catastrophic disturbance. Traditionally, we have failed to appreciate the stability of such late-successional forest ecosystems.
These stands are often characterized as very “dynamic” and
“unstable” because each of the small structural patches is
seen as a “stand” rather than being recognized as part of a
stand mosaic. The pine and mixed-conifer forests of the interior Columbia Basin and the Sierra Nevada range of California provide excellent examples of this. These forests were naturally subjected to frequent light- to moderate-intensity wildfire,
which produced complex late-successional forest mosaics consisting of small patches of contrasting structural conditions (Figure 7: Two cross-sections of forest stands
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