One of the strategies suggested for moving toward
sustainable forest management consists in developing harvesting
methods that allow the demand for fibre to be met while minimizing
clearcutting, a method that often stirs up public controversy. Partial
cutting holds promise for sustainable management, since this approach
can be used to shape the forest to produce timber along with other
socially beneficial products. In keeping with this broad goal, the
silviculture researchers at the Laurentian Forestry Centre (LFC)
of the Canadian Forest Service are focussing their efforts on developing
silvicultural systems adapted to mixed and coniferous stands, and
on studying the effects of silvicultural practices on forest growth
and production.
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| SHELTERWOOD
CUTTING IN BALSAM FIR STANDS: AN EFFECTIVE TREATMENT |
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Under an even-aged management regime, shelterwood cutting is considered
an ideal method for tending forest stands and promoting stand regeneration.
Very little information is available on the application of shelterwood
cutting to balsam fir–yellow birch stands in Quebec or on
the effects that this method has on the growth and development of
residual trees.
Experimental preparatory cutting— the first step in the
shelterwood method— was carried out in a second-growth balsam
fir stand in the Lower St Lawrence Model Forest, with the objective
of evaluating the effects of this type of cutting on the growth
of residual trees five years after treatment. This work is part
of the research program headed by Dr Richard Zarnovican, which centres
on the effects of silvicultural treatments on forest growth and
yield.
Shelterwood cutting in balsam fir stand |
Preparatory cutting is designed to establish a reserve of seed
trees that will produce viable seed, without creating excessive
openings in the stand or allowing competing vegetation to invade
the cutover. With this approach, the cutting is concentrated on
trees in the lower strata, which are less effective as seed trees
and which show less of a growth response to thinning compared with
other trees.
The increased space that results from preparatory cutting helps
to stimulate tree growth. In the balsam fir stand under study, this
intervention produced a mean diameter increase of 8 to 18% compared
with the pre-treatment diameter. Shelterwood cutting therefore had
a beneficial effect on growth over the five-year period covered,
particularly when the cutting intensity was over 30%. The trees’
positive response to the treatment indicates that a 60-year-old
balsam fir stand can still respond positively to an increase in
the available space and an improvement in growing conditions.
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| SHELTERWOOD
CUTTING FOR HEALTHY REGENERATION |
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Forest managers are aware that, in terms of producing natural regeneration,
shelterwood method is a versatile technique that can be used to
shape stands in accordance with the requirements of most species.
Nevertheless, achieving healthy regeneration requires an abundant
store of quality seeds in the soil, and this situation is influenced
by physiological and climatic factors.
Harvesting of seeds in a trap |
In this context, Richard Zarnovican set up a study to assess the
dynamics of seed production with a view to regeneration in balsam
fir–yellow birch stands using the shelterwood method. The
study was done in a 55-year-old second-growth balsam fir stand in
the Lower St Lawrence Model Forest. Seeds scattered over the forest
floor were harvested in containers designed to keep out pests, and
soil samples were taken in order to count and analyse buried seeds..
The year the seeds were collected was a very good seed year for
balsam fir, and an abundant quantity of seeds was scattered uniformly
throughout the stand. However, buried seeds, which were concentrated
in the first two centimetres of the organic layer of soil, showed
a very low germination rate, indicating a rapid decline in germinating
capacity. This situation explains the inability of balsam fir to
produce satisfactory natural regeneration outside good seed years.
Shelterwood cutting should therefore be applied as a means of achieving
the best possible natural regeneration in balsam fir stands.
This natural regeneration technique represents a promising approach
for mixed stands of yellow birch and balsam fir as well, but in
these stands scarification of the soil is required. Another research
project, this one led by Jean-Martin Lussier’s team, has been
launched with the aim of describing the dynamics of annual seed
dispersal and seed banks in yellow birch–balsam fir stands
and evaluating the impact of regeneration cutting on these dynamics.
While the establishment of regeneration is dependent on seed availability,
the receptiveness of seedbeds is likewise important. The succession
of good and bad seed years, the effective distance of seed dispersal
and the longevity of the seeds in the soil are factors that control
establishment and regeneration. Taking into consideration the seed
production cycle of most of the species concerned, the researchers
will spread the research over four years so that two good and two
bad seed years can be covered..
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| MODERATE
THINNING TO PROTECT YELLOW BIRCH FROM FREEZING RAIN |
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In early January 1998, a major ice storm struck southern Quebec.
Some areas received 80 mm of freezing rain during the five days
of precipitation, which is an unusual amount for a storm of this
type. The weight of the ice layer on tree branches, combined with
the effect of the high winds, caused serious damage to forest stands.
An aerial survey showed that nearly 18 000 km2 of forest sustained
varying degrees of damage, with 92% of this area representing privately
owned woodlands.
Effect of freezing rain on a young yellow
birch stand |
During the summer after this unusual climatic event, Richard Zarnovican’s
team began a study in the Eastern Townships on the damage in a young
yellow birch stand that had regenerated naturally after cutting
and had undergone precommercial thinning at age 15. The birch trees
had suffered much more serious damage than the sugar maple and ash,
the other two species present in the stand. The study also showed
that the damage to yellow birch was even greater in areas where
heavy thinning had been done. The trees had breaks and a permanent
curve in their trunks—defects which spell volume and growth
losses and pose a serious threat to timber production.
In southern Quebec, where ice storms are not uncommon, forest
owners should keep the potential effects of such events in mind
when managing young deciduous stands. Because of the strong susceptibility
of yellow birch to ice storm damage, stand composition needs to
be given special attention. Moderate thinning is one of the tending
methods that should be favoured for young stands, because the more
severe the thinning, the greater the ice storm-associated damage
and volume losses.
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COMMERCIAL THINNING: LEARNING FROM THE PAST |
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While a great deal has been written about the effects of partial
cutting on European forests and those in the southern part of North
America, it is hard to find data on the response of the coniferous
stands that make up the boreal forest spanning our continent.
Recent commercial thinning in a black spruce
stand in the Réserve faunique des Laurentides |
The growing interest in this kind of treatment recently prompted
the ministère des Ressources naturelles, de la Faune et des
Parcs du Québec (MRNFPQ) to set up an extensive experimental
layout, the results of which will not be known for a number of years,
after the stands have responded to the treatments applied. A number
of specific trials have, nevertheless, been implemented over the
past 30 years in Quebec, but no systematic effort was made to compile
relevant dendrometric data following these interventions.
Jean-Martin Lussier’s team, working in collaboration with
researchers at Université du Québec in Abitibi-Témiscamingue
and Université du Québec in Chicoutimi, set out to
derive useful information from these studies by analysing the residual
trees. This approach involves measuring the trees’ response
in diameter and volume growth and determining the effect of stand
thinning on stem shape. The researchers conducted a retrospective
study of the commercial thinning operations that took place in jack
pine stands in the Abitibi region between 1960 and 1990.
Based on their research, it was determined that thinning generally
produced an acceleration in stem growth. The comparison of decadal
volume growth before and after treatment showed that during the
10 years after a thinning operation, volume growth doubled. Small
and medium sized trees benefited the most from the increased availability
of light, nutrients and soil water following thinning. The large
stems showed a lesser response, since they already had access to
the resources necessary for healthy growth. In these stands, treatments
that tend to preserve dominant trees, such as low thinning, appear
to induce less of a response in the residual trees than free thinning,
which is aimed at harvesting trees from all diameter classes.
The research team also noted that thinning tends to alter and
modify the shape of the trees. The findings suggest that, as a result
of the reduction in stand density, the trees are more exposed to
the effects of the wind and they accelerate radial growth at their
base to compensate for these effects. Nonetheless, the repercussions
on timber production are negligible.
In the jack pine stands of the Abitibi region, thinning is a silvicultural
practice that should be encouraged for timber production because
it induces a considerable increase in the diameter growth of residual
trees. Jean-Martin Lussier plans to undertake additional retrospective
studies of silvicultural trials in black spruce stands and balsam
fir stands. Studies of this type could ultimately be implemented
more broadly in the eastern boreal zone and encompass Ontario and
the Atlantic provinces.
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| FOCUS
ON MIXED FORESTS |
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Sustainable forest management calls for silvicultural practices
that permit the production of quality wood in sufficient quantity
to meet needs and that respect the structure and natural dynamics
of stands. Forests such as balsam fir–yellow birch stands
pose significant challenges in this regard. These are complex forests,
which represent a transition between the southern hardwood forest
and the boreal forest; they consist of a mosaic of mixed stands
comprising about ten species and their dynamics are not well understood.
Researchers generally agree that stand structure and composition
evolve as a function of the natural disturbance regime. This aspect
needs to be documented to a greater extent in relation to balsam
fir–yellow birch stands, especially since these forests have
undergone numerous partial cuts since the start of the century.
This has modified their structure and, in many cases, led to a reduction
in stand value.
Balsam fir–yellow birch stands pose
particular management problems. Yellow birch establishment appears
to be favoured by the creation of canopy gaps, combined with
some soil disturbance, whereas balsam fir regeneration seems
to originate mainly from a pre-existing seed bank and canopy
openings of variable intensity without any soil disturbance. |
Researchers generally agree that stand structure and composition
evolve as a function of the natural disturbance regime. This aspect
needs to be documented to a greater extent in relation to balsam
fir–yellow birch stands, especially since these forests have
undergone numerous partial cuts since the start of the century.
This has modified their structure and, in many cases, led to a reduction
in stand value.
Jean-Martin Lussier’s team has initiated a study dealing
with the effects of silvicultural practices on the structure and
productivity of mixed stands of balsam fir and yellow birch, in
collaboration with the forest research directorate of the ministère
des Ressources naturelles, de la Faune et des Parcs du Québec
(MRNFPQ), Université Laval, FERIC and Gestofor. It is part
of the Silvicultural Systems Adapted to Mixed Forests project. This
study should make it possible to determine the role that past natural
and human-caused disturbances have played in shaping current stands,
particularly with regard to species mixture, age structure and fibre
production per hectare. Another aim of the research is to determine
the factors that govern the establishment of regeneration and its
access to the dominant stratum.
In a second phase of research, the researchers will evaluate the
effects of current silvicultural practices on stands’ fibre
production from the standpoint of sustainable development. The analysis
will deal with the response of residual trees to partial cuttings,
the condition and growth of advance regeneration, the establishment
of regeneration under the canopy and after harvesting as well as
the change in stand structure following harvesting. The results
of this work will be used to develop new silvicultural practices
that are better adapted to these forests.
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Silvicultural systems project
A number of experts with an interest in mixed
forests have pointed up the lack of silvicultural treatments
specifically suited to the healthy yellow birch–conifer
stands found in Quebec’s mixed forest zone. They therefore
set up the Silvicultural Systems Adapted to Mixedwoods project,
which is aimed at developing silvicultural practices that
can maximize the value of processed wood products while
ensuring the sustainable use of mixed forests.
The partners involved in this project hope to ensure optimal
use of financial and technical resources for the project,
avoid overlapping studies, promote collaborative work among
the researchers and the industry and provide a layout that
can be shared for a variety of studies.
Approximately 10 studies are currently under way in the
Portneuf region, which was chosen for the project on account
of its abundance of healthy yellow birch–conifer stands.
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Balsam
fir’s susceptibility to windfall can be attributed
to stem decay, to the superficial anchoring of the root
system or to these two factors combined. To shed some light
on this issue, Richard Zarnovican conducted an inventory
of the trees that had been thrown down by the wind in a
60-year-old balsam fir stand two years after commercial
thinning.