| Forest ecosystems are subjected
to and must respond to many natural and human-induced disturbances,
from wildfires, insect outbreaks and ice storms through logging
operations. An ecosystem’s ability to recover from a
disturbance, whether slowly or quickly, is referred to as
its resilience.
A number of researchers are studying ecosystem resilience
with the aim of identifying forest management techniques that
preserve this attribute. At the Laurentian Forestry Centre
(LFC) of the Canadian Forest Service (CFS), Dr. David Paré
is currently conducting research on the resilience of ecosystems
in the boreal forest. In this article, he paints a portrait
of the issues related to boreal forest resilience and gives
an overview of his research work.
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| ECOSYSTEMS:
ORGANIZATIONS CONDITIONED BY THE ENVIRONMENT |
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The distribution of ecoregions on a map gives an indication
of ecosystem resilience. The boundaries separating the different
vegetation types are fairly distinct. A particular assemblage
of plant and animal species is associated with each of the
different types of forest. The species composition of these
ecosystems and their functioning is regulated by various environmental
factors, the most important being climate, soil and disturbances.
Recent studies have shown that disturbances play a much more
important role than previously suspected. Ecosystem resilience
requires that these environmental variables be held within
certain limits. The modification of one environmental factor
alone may lead to changes in the ecosystem’s composition,
structure and functioning. During glacial periods, these vegetation
zones shifted southward, but maintained their inherent organization.
Arctic tundra once existed where New York City is now, and
boreal forest was present in Virginia. Ecosystems do not have
a completely static composition; species invasions and extinctions
occur as do changes in ecosystem structure. However, these
changes, unless they result from environmental change, generally
take place very slowly.
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| DESTABILIZING
FORCES THAT ENSURE RESILIENCE |
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Small rocky islands covered with red
pine at Lake Duparquet. |
The effect of the climate on vegetation is well understood
for the most part, and all gardeners are familiar with the
hardiness zones which indicate the places where particular
plant species can be grown. Factors such as the length of
the frost-free period are also important. However, what is
less well known is that climate has an indirect effect on
plant distribution in natural ecosystems. By interacting with
vegetation, climate influences the frequency of disturbances.
This factor has a decisive effect on the composition of ecosystems.
Lake Duparquet in the Abitibi region can be used to illustrate
this idea. In this environment situated in the heart of the
mixed boreal forest, there are some small rocky islands covered
with red pine and juniper forests. These forests are typical
of more southerly regions, and normally can only be found
hundreds of kilometres south of the area, such as in the Ottawa
Valley. Why then do these red pine and juniper forests exist
in the Abitibi? There is a simple explanation. The small rocky
islands in Lake Duparquet capture lightning like veritable
lightning rods. The high frequency of lightning-generated
fires that occurs on the islands as compared with the situation
in forests around the lake, combined with their low intensity,
helps to maintain this type of vegetation, which was present
in the area several thousand years earlier.
The prairies of Western Canada used to burn in various locations
every 10 years. Fire prevention has transformed these prairies
into forests. These examples and many more illustrate the
important role that disturbance agents play in maintaining
ecosystem properties. In a context where climate is constant,
a modification in the frequency of disturbances causes major
changes in ecosystem functioning and composition. These destabilizing
forces are important for maintaining the resilience, diversity
and even the productivity of ecosystems. The ecosystem impacts
depend on the disturbance attributes of size, spatial distribution,
intensity and frequency.
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| NATURE
WITH OR WITHOUT HUMANS? |
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Forestry operations can alter the character of the forest.
There is increasing support for the idea that forest management
should mimic natural processes and maintain landscapes, species
composition and the processes that occur in the absence of
human activity.
Boreal forest landscape. |
There are two opposing, albeit not mutually exclusive, views
in this regard. According to the first view, portions of the
region should be left intact and other areas should be managed
intensively using an approach aimed at maximizing productivity.
This could comprise a range of interventions extending to
the introduction of exotic species and the use of pesticides
and fertilizers. This approach resembles agriculture. With
the second approach, the aim is to manage the forest so as
to generate a profit, while seeking to emulate nature as much
as possible.
The first approach is often favoured by countries that have
almost no remaining natural forest and whose small reserves
of intact forest represent a precious and untouchable asset.
They typically make claims of the following type: "This
wood does not come from virgin forest, but rather from plantations."
This argument, which may be seductive for the general public,
deserves close scrutiny. Is it really wise to obtain timber
supplies exclusively from artificial forests which often contain
exotic species that cannot support the local plants and animals,
and to keep only a small percentage of virgin forest in the
region? There is no guarantee that these remnants of virgin
forest will play their role in preserving biodiversity, since
they are so very small.
These two visions are not mutually exclusive, since it is
conceivable to manage small zones for timber culture (eg,
agriculture model), carry on intensive forestry in other zones
by managing second-growth forests there, and finally maintain
areas with complete protection. The debate continues on this
topic, and some countries—those whose wood production
comes almost exclusively from plantations of exotic trees—will
continue to claim that virgin forests should not be harvested.
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| MANAGING
IN THE FACE OF UNKNOWNS |
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Soil conditions following a very intense
fire. |
The question is how can we manage our forests by emulating
nature, as the second approach recommends. It has become clear
that management cannot focus on individual species and seek
to ensure that each species has what it needs to survive.
The lack of knowledge of various species (for example, many
insects and microorganisms have not yet been identified) and
of organismal biology, coupled with the complexity of ecosystems,
makes this kind of approach impossible. Forest management
that seeks to promote stand composition and structure similar
to those characterizing natural environments should permit
the maintenance of biodiversity and the essential functions
of forest ecosystems. A problem frequently encountered with
this approach relates to the natural variability of ecosystems.
The functioning of an ecosystem cannot be compared with that
of a finely tuned machine. Unpredictability is the basis of
ecosystem dynamics. For example, some very intense fires destroy
all the trees and eliminate all the organic matter in the
soil, whereas other fires leave a large proportion of trees
intact. Some fires spread across an area comparable to that
of certain European countries, while others are small. It’s
when we consider these phenomena operating in large territories
that patterns emerge. The approach that involves managing
ecosystems not only by considering what is happening at the
site scale (for example, on a hundred hectares), but also
in extensive regions where the forests are at different successional
stages is called landscape ecology. Fragmentation of ecosystems,
the representativeness of the different stages of development,
the presence of islands or corridors of forest in recently
disturbed zones are all properties that can only be perceived
when examining vast territories; they are all important in
terms of the resilience of the boreal forest.
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| A
MATTER OF SCALE |
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The site
At the site scale, a clear-cut is not an ecological disaster
but rather a phenomenon that has similarities with the natural
processes that take place in the boreal forest. This practice
permits the resilience of biodiversity and the system’s
productivity. However, a clear-cut creates very homogenous
conditions, in contrast with the variety of patterns resulting
from a wildfire. It is believed that some cut-overs would
benefit from being further disturbed through burning or scarification
because this would renew the soil as some fires do. It is
a myth that the lightest possible impact is the best solution;
this does not hold true when we analyse the situation in the
field.
Soil conditions after a cut. |
The effects of a clear-cut may resemble those of a disturbance
caused by fire in many respects, at the site level. All the
marketable trees are removed, and this will result in an even-aged
stand (containing trees of the same age). There will also
be lots of light reaching the ground and the mineral soil
will be exposed to a greater or lesser extent. A few differences
exist as well. The organic soil layer will be thinner after
a fire, and there will be more snags and deadwood, along with
intact islands of vegetation that cover about 5% of the total
surface area. Our studies have shown that, when the cutting
does not expose the mineral soil in black spruce stands, the
floristic composition and soil fertility will be different
from that found after a fire. We have also observed that fire,
in contrast with cutting, promotes the maintenance of certain
nutrients, in particular, potassium, calcium and magnesium,
as well as a higher soil pH level, although nitrogen losses
are greater. The issue of nutrients is not critical at all
sites, but in many cases branches and foliage should be left
on the site, along with parts of the tree that are rich in
nutrients, at the time of harvesting. Through field work and
modelling we can identify sites that are likely to be adversely
affected by the use of the whole-tree harvesting method. Problems
related to physical disturbances (roads, soil compaction,
etc) at the site are connected with inspection and regulation,
rather than a lack of knowledge.
The landscape
Research has shown that, at the landscape scale, fire cycles
vary regionally and have also varied over time. Thus, some
regions of Quebec currently have a fire cycle of 75 years,
whereas other areas have a cycle of nearly 300 years. It is
generally recognized that fires strike forests of all ages
randomly. Thus, the fire cycle creates forests in which the
different successional stages are all represented, and this
representativeness is well known. It differs from the representativeness
sought in the industrial model of forest harvesting: the normalized
forest. According to this concept, the forest should comprise
equal areas of the different age classes. For example, if
the forest takes 100 years to mature, each age class should
be represented on 1% of the territory, thereby ensuring a
constant supply of fibre. However, since forest fires strike
randomly, in a forest that is subject to natural processes
young stands will be over represented, forests of ages close
to the fire cycle will be well represented and there will
be a small abundance of stands whose age exceeds that of the
fire cycle. In the industrial model, these old-growth forests
do not exist at all. In reality, these forests, although not
well represented in areal extent, exist and often have an
irregular structure with trees of varying ages. In regions
with a long fire cycle, for example on the North Shore, it
has been estimated that prior to the start of logging more
than 50% of the region was occupied by forests with an irregular
structure.
The region
At the scale of a region, clear-cutting entails some problems
even if the rule of sustained yield is applied (ie, cutting
cannot exceed productivity). This practices does not allow
old-growth forests to subsist nor even forests resembling
old-growth ones, and the distribution of forests of varying
ages is often very different from the pattern found in nature:
the natural forest is more like a mosaic, whereas management
by clear-cutting often results in a landscape which, aside
from a few strips and small islands of forest, resembles a
carpet that has been rolled out.
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| SOLUTIONS |
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A great deal of research work remains to be done, at both
the site and landscape scales, to protect the resilience of
the boreal forest.
At the site scale, it is important to choose harvesting and
site preparation methods that are suited to the conditions
present there. It is also essential to determine the intensity
of the disturbance that will result from these operations.
A disturbance of insufficient intensity could promote the
growth of mosses and ericaceous plants and lead to stagnating
soil fertility, in addition to creating biodiversity problems.
In fact, many species need high intensity disturbances because
they have evolved in such an environment over a period of
thousands of years. In contrast, an overly intense disturbance,
such as the excessive removal of humus, could also reduce
the site’s potential for productivity.
A landscape-scale model proposed recently by a group of Quebec
researchers, including Dr. Sylvie Gauthier of the Laurentian
Forestry Centre (LFC) of the Canadian Forest Service (CFS)
could make it possible in applying clear-cutting and partial
cutting to conserve a forest landscape that is similar to
the natural landscape in the boreal forest, while maintaining
timber harvesting operations. Ensuring greater heterogeneity
in forest interventions also appears to be a way to emulate
natural processes more closely, for example, by varying harvesting
intensity, the surface area of cuts and the extent of soil
disturbance. There is every reason to believe that, by mimicking
nature, we will be helping to conserve the known and as yet
unknown attributes of the boreal forest. It is important for
the general public to take part in this debate. Sustainable
forest management is not rigid and can allow plenty of latitude.
In one region conservation might be promoted, in another fibre
production and in yet another moose production. There is a
widely held view that nature will provide a maximum of all
it has to offer humans even without the wise use of resources.
This myth needs to be dispelled. The forest can be managed
for a specific purpose, but when we want to make use of several
resources, compromises are often required. The choices to
be made are societal decisions, not choices determined by
science. Research on ecosystem functioning sheds light on
the limits we must not overstep if we want to ensure sustainable
forest management and on the ways to optimize production.
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