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On Old Growth forests: [excerpts]
While the species composition of old growth forests
may be duplicated in other
forest phases, certain structural features are found only in old
growth. These
special structural attributes include large trees, snags, large
fallen trees,
and large accumulations of forest biomass. We will see later why
these
structures are essential to fully functioning forest landscapes.
Working together, compositional and structural
attributes support the unique
ecological functions of old growth forests. Storage of carbon,
filtration of
water, and capture of nitrogen are three functions carried out
better in old
growth than in other types of forests. Old growth forests maintain
large,
uniform, high quality timber volumes over time, and support stable
or slowly
evolving communities of plants and animals (Haynes 1986, Franklin
and DeBell
1988). Thus, old growth forests are steady state ecosystems.
Although individual organisms die and are replaced,
the dynamics and biomass of
the ecosystem remain constant over time, until some external disturbance
damages
or destroys the forest cover. The natural forest then begins advancing
through
successional stages until it once again reaches the old growth
phase (Bormann
and Likens 1979).
Why are old growth forests important? Old growth
forests are a significant and
unique part of the diverse ecological web formed by natural forest
landscapes.
Human activities or natural disturbances in one part of the forest
landscape can
affect many other parts of the landscape. As an important part
of the landscape
ecology of natural forests, old growth provides unique resources
for plants and
animals(including people) within the landscape (Harris 1984, Franklin
et al
1986).
The timber industry generally encourages logging
of "decadent" old growth
forests so that they can be replaced by "vigorous" young
tree farms. However,
the unique structure and composition of old growth forests facilitate
several
critical ecological functions which cannot be duplicated in short
rotation
commercial tree farms. For example, according to Franklin et al
(1981):
1. Old growth forests generally contain an accumulation
of biomass on the forest
site, which enriches the forest soil over time.
2. Old growth forests provide high quality, naturally
filtered water from stable
creeks.
3. Old growth forests provide a continuous supply
of diverse, durable structures
(large trees, snags, and fallen trees) which provide habitat for
diverse plant
and animal communities. These structures remain after natural
events destroy an
old growth forest and provide diversity in younger forests.
Old growth forests are also important because
we do not fully understand their
functions, the life forms they support, or their importance to
the ecology of
commercial forests. The genetic information which ancient forests
contain has
never been assessed. Both common sense and present knowledge indicate
the danger
of eradicating old growth from the forest landscape, or even reducing
the
proportion of old growth forests beyond a certain, unknown point.
We do know
that forests are a major global ecosystem and that the natural
forest landscape
was largely old growth. From this, we can safely conclude that
global,
landscape, and stand level needs exist which require a landscape
dominated by
old growth. Our failure to understand these needs is no justification
for
greatly altering the proportion of old growth forests in a region
or on the
planet (Thomas et al 1987, Franklin and Spies, 1989). Even with
improved forest
management techniques, it is unlikely that humans will ever be
able to create
old growth forests. Ethically, the human species does not have
the right to
cause the extinction of the old growth ecotype and the many species
which it
contains (Juday 1988).
OLD GROWTH COMPOSITION - OTHER SPECIES
A variety of life forms other than trees comprise
the old growth community, or
make use of old growth forests. These forest organisms go through
successional
patterns similar to those already discussed for forest trees.
The common pattern
of ecological succession for mammals, birds, and invertebrates
begins when a
large number of generalist species occupies the early forest,
before the young
forest achieves crown closure. As the crown closes, the number
of species
present in a forest normally dwindles. The fewest number of species
inhabit the
forest during the period when the canopy is closed (approximately
from age 25 to
150-200 years). Species diversity then rebounds to near pre-closure
levels as
the old growth phase begins and the forest canopy begins to open
up. However,
the kinds of species which comprise an old growth forest community
are
significantly different from those which inhabit a seral community,
a young or
mature forest, or a tree plantation.
The species which inhabit old growth forests
tend to be specialists, adapted to
old growth conditions and requiring specific types of habitat,
which are often
found only in old growth forests (Franklin 1990). In contrast,
the species which
inhabit seral forests (early forest stages) tend to be aggressive
generalists--
hardy species which can adapt to many different conditions and
ecosystems.
The high degree of specialization common to old
growth organisms is a logical
survival strategy for species which share a stable biological
community.
However, these animals are not equipped to compete with the aggressive,
adaptable, transitory species which exploit the brief period between
the
destruction of a forest and the closure of the forest canopy in
the replacement
stand (Wilcove 1988). If ecologically viable habitat is not protected
for their
use, old growth dependent animal and plant species are in danger
of
extinction (Ruggiero et al 1989, Thomas et al 1987, Connor 1989).
On Water Quality:
3.6.1 Human Use Of Water - Water Quality, Quantity
And Timing Of Flow
The suitability of water for human use is determined
by the quality, quantity
and timing of flow of the water resource.
3.6.1.1 Quality
Some forms of water pollution and siltation can
be rectified by treatment, but
obtaining water that is pure in the first place is preferable
to extensive
treatment. Old growth forests produce extremely pure water because
of the
structures and biological functions which occur within the forest.
The terrestrial ecosystem of the old growth forest
is an immense recycling
plant. Dead plant and animal material is attacked by the small
animals, insects,
microorganisms, and bacteria of the forest floor community. The
stable, cool,
moist environment beneath the forest ensures that these organisms
are always
present, and that decomposition proceeds in slow, uniform fashion
(Franklin et
al 1981). The by-products from decomposition enter the soil, where
they are
absorbed by the fungi/plant root community of the rhizosphere,
and transferred
back to the standing vegetation (Amaranthus et al 1989). As a
result of these
processes, nutrients are tightly retained within old growth ecosystems,
and the
level of dissolved or suspended materials in water flowing out
of the system is
low. The development of such a closed cycle is a gradual process,
which
progresses from the time a forest is established until a severe
disturbance
starts the cycle again.
Organic debris which falls directly into streams
is also largely retained within
the old growth ecosystem due to the structure of streams in old
growth forests.
Fallen trees and large pieces of woody debris form small dams
in streams which
act as sieves, retaining organic matter long enough for microbes
and insects to
break down and use the organic inputs.
Old growth forests also effectively prevent erosion
and resultant siltation. The
extensive root systems of the trees, the regulation of soil water
levels caused
by transpiration of water through the forest canopy, the loose
or friable soil
structure maintained by the rhizosphere, and the permeable soil
surface provided
by organic layers on the soils surface work together to prevent
soil saturation,
overland flow, and downslope movements.
Organic debris also controls channel erosion
in old growth forest streams. The
dams formed by the organic debris (particularly large fallen trees
or "large
organic debris") break the stream into a series of short
falls, riffles and
ponds.
This "stepped" configuration absorbs
or diverts most of the energy of the moving
water, and prevents stream bank and channel erosion. Thus, the
high quality of
the water produced in the old growth forest is preserved as it
flows through
creeks stabilized by large organic debris.
3.8.2 Forest Diversity And Soil Productivity
Research in the last decade has indicated that
diversity, at both the small-
scale stand level and the larger landscape level, is vital to
maintain the
productivity of forest soils: The old growth and herb shrub forest
stages
possess the only opportunities for adding nitrogen to the soil.
The intermediate
conifer establishment and dominant stages are essentially a soil
nitrogen mining
state. In managed landscapes and managed stands, where we're trying
to condense
the (natural) successional sequence into a tree crop species management
cycle,
we essentially remove those stages that are improving soil fertility.
(Schowalter 1990)
The level of available nitrogen is the primary
nutrient related growth limiting
factor in most forested ecosystems. Nitrogen is the most common
gas in the
atmosphere, but is in a form which is unusable by plants. Atmospheric
nitrogen
must be "fixed" with oxygen or hydrogen in order to
be accessible to plants
(Spurr and Barnes 1980). This process is carried out by certain
species of
bacteria. Some of these bacteria live in symbiotic relationships
with various
species of lichens and herbaceous plants. Other "nitrogen
fixing" bacteria are
free living in rotting wood.
Large fallen trees are a location for nitrogen
fixation in the forest. As trees
decompose, nitrogen compounds are released. These compounds are
conserved and
concentrated by various organisms, and reenter the nutrient cycle
of the forest
(Maser et al 1988). Anaerobic bacteria fix atmospheric nitrogen
into a form
usable by plants within the moist interior of the log. While the
net rate of
nitrogen accumulation per volume of fallen tree is thought to
be relatively low,
the large volumes of biomass within which the bacteria can function
and the long
time spans involved result in biologically significant nitrogen
accumulations
(Maser et al 1979).
The types of organic matter accumulated in old
growth forest soils are unique,
as are their functions. While young forest litter (leaves, needles,
twigs) takes
only 10 to 50 years to decompose, large Douglas-fir logs can take
400 years to
become incorporated in the soil. Thus, the nutrient pool of a
large fallen tree
cycles on a time scale which can easily include the first four
centuries in the
life cycle of a young, replacement forest. Large fallen trees,
which provide a
long lasting biological legacy, are produced only by old growth
forests. Small
fallen trees from younger forest are more likely to burn in a
fire, decay much
more rapidly than large fallen trees, and are not as biologically
valuable (Maser et al 1979).
3.8.3 Forest Diversity And Insect Pests
Schowalter (1990) has observed that old growth
forests have large insect
populations, yet these forests are relatively free of catastrophic
insect
damage. Insects and forests have evolved together over millions
of years. Given
ideal circumstances, insects are able to reproduce rapidly, but
natural forest
ecosystems have evolved a wide variety of ways to prevent "ideal
circumstances"
from occurring. In-tree chemical defenses, chemical confusing
agents, insect
predators, and diversity of tree species all combine to keep insect
populations at endemic levels most of the time.
3.9 HUMAN USES OF OLD GROWTH FORESTS
The liquidation of old growth forests is a large
scale experiment, carried out
within our province---our biosphere---for which we do not know
the outcome. From
our limited view, we can "see" that everything is going
well. Although most of
the old growth forest is gone, few ill effects are noticeable.
Few negative
impacts can be "proven" to rigorous scientific standards.
We pride ourselves on mimicking the European
approach to forest management,
which after only 400 years (about one life cycle for an average
forest) is
collapsing (Maser 1988, Plochmann 1990). We do not consider factors
such as
climate change, cumulative effects, landscape ecology alterations,
and loss of
genetic diversity in our "long range" 5 or 20 year forest
management plans.
Perhaps we ignore these factors because their
meaning and importance tends to
exceed our own lifespans. People are challenged to be able to
relate to the time
and space of forested landscape. Forests operate on cycles of
200 to 1000 years-
-indeed, forests are a continuum. If we are lucky our lives may
last 100 years.
If our governments are lucky they last four years. Our corporate
institutions
function on one year profit and loss statements. We are one tenth
the height of
a short tree. More that 40 people would have to stand on each
other's shoulders
to reach the top of a moderately tall Sitka spruce tree. The person
at the top
would only be able to see more tree tops, not the whole forest.
A moderate sized
watershed (e.g. 5000 hectares) would require months for two people
to explore,
to map, and to begin to understand the relationships within this
landscape. The
imbalance between human cycles and forest cycles has led us to
the brink of
destroying the very forests that sustain us.
Old growth forests mean different things to different
people. However, our hope
for survival and the survival of forests is inextricably linked
with our ability
to appreciate and accommodate the vast differences in scale between
people and
forest landscapes. We are the only organism who ever wanted to
dramatically
change the forest landscape, and the changes we have made now
threaten the
survival of the forest itself. It is now time to find our place
within the
forest web . . . at both the stand and landscape levels.
The full paper can be downloaded here:
http://www.planputnam.org/highlands/resources/Old%20Growth%20Ecology.pdf
Jeff Green
PlanPutnam
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