A Brief Summary of Silviculture Issues in B.C.’s Boreal forests (BWBS)
The Boreal White and Black Spruce Zone (BWBS) covers about 10% of British
Columbia, mainly in the northeastern corner of the province (DeLong et al. 1991).
On poorly drained sites, black spruce (Picea mariana) is usually the dominant tree
species. Tamarack (Larix laricina) may also be present, and black spruce-lodgepole pine
(Pinus contorta) mixtures are common on imperfectly drained sites. On well-drained
sites, lodgepole pine often occurs in pure stands, or as the dominant tree species with a
minor component of trembling aspen (Populus tremuloides) and occasionally black
spruce. For both the well-drained and poorly drained sites, fire is the most common
disturbance agent. Fire disturbances generally result in regeneration of the same tree
species.
Both pure and mixed forests of trembling aspen and white spruce (Picea glauca)
are common on moderately well drained, fresh to slightly dry sites (DeLong et al. 1991).
Lodgepole pine and balsam poplar (Populus balsamifera) occur as pure stands, and as
intermixtures or minor components of BWBS mixedwood stands on these medium sites.
The occurrence of pure lodgepole pine stands on medium sites increases with elevation.
Paper birch (Betula papyrifera) and subalpine fire (Abies lasiocarpa) may also be found
as minor components.
Upland stands commonly originated after fire, and can range from pure stands of
conifer to pure aspen, and mixtures of the two in varying proportions. There can be
horizontal mixtures or vertical mixtures where white spruce are growing under taller
aspen. Low intensity surface fires favor aspen regeneration (suckering) from lateral roots.
If a seed source is available, spruce seedlings may establish immediately, or over a period
of 10 to 60 years (Lieffers et al. 1996a). A severe fire, which consumes the forest floor,
creates a seedbed of mineral soil and ash, which is very favorable for white spruce
regeneration. The aspen regeneration after a severe fire is the same age as the white
spruce, but rapidly overtops the spruce, due to the fast juvenile growth rate of aspen.
Complexity in stand development for boreal mixedwoods on upland sites arises
because of differences in recruitment strategies, shade tolerances, juvenile growth rates
and life span or time to maturity of the main species (Lieffers et al. 1996b). The structure
and composition of mixedwoods are also influenced strongly by time since disturbance,
amount of forest floor removal, survival of root stocks and seed sources (Lieffers et al.
1996b).
On alluvial sites along the major river systems (Peace, Liard and their major
tributaries) pure and mixed stands of white spruce and balsam poplar are dominant.
Erosion and deposition in the river valleys are the major disturbance agents initiating
succession. Fire may occur, but only as a minor disturbance agent.
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Site index of merchantable aspen is commonly 16 - 20 in the BWBSmw1, and 18
or greater in the BWBSmw2 near Fort Nelson. White spruce site index and volume are
greatest on alluvial sites in the BWBWmw2, where historical white spruce stands had
heights up to 50 m, and total volumes over 500 m3/ha. White spruce site index ranges
from 14 to 18 in merchantable upland stands. Extensive areas of small (< 20 m total
height), lower productivity lodgepole pine are present, and are increasingly being utilized
by modern mill technologies.
With very few exceptions in the BWBS, the amount of white spruce natural
regeneration after harvesting has been poor and insufficient to replace the spruce that had
been present. Stands which had a broadleaf component, usually aspen or balsam poplar,
have become dominated by broadleaf species aspen and balsam poplar after harvesting or
wildfire (eg. Butt 1988, Peterson et al. 1989).
The B.C. experience with white spruce natural regeneration is consistent with
other boreal jurisdictions (eg. Greene et al. 1999). Many of the FRDA era research trials
in the BWBS (Iron Creek, Stewart Lake) were established on sites which had a partial cut
history where natural regeneration of white spruce had been unsuccessful, and/or a
clearcutting history where natural regeneration (seed tree reserves) sometimes in
combination with artificial regeneration, had also been limited in success .
Establishment of conifers, particularly white spruce, was the focus of research
efforts in the 1970’s and 80’s. This included stock type, site preparation and vegetation
management trials. The results of these research efforts contributed to improvements in
recognizing limiting factors in silviculture prescriptions, larger stock types of higher
quality, appropriate site preparation techniques, prompt planting of disturbed sites, and
improved vegetation management techniques; which have the combined result of
improving conifer reforestation successes over that of the early 1980’s.
Harvesting and utilization of broadleaf began with alluvial balsam poplar stands
in Fort Nelson in the late 1970’s. Aspen harvesting for oriented strandboard (OSB),
pulp, and veneer products began in the mid-1980’s. The annual allowable cut for
broadleaf’s in northeastern B.C. is similar to that of conifers. Harvesting of aspen under
appropriate conditions consistently results in prolific regeneration from lateral roots.
Inadequate aspen regeneration is most commonly associated with zones of repeated
machine traffic, such as roadside processing areas. Prevention through assessments of
hazards, planning and harvesting supervision is more cost effective than rehabilitation.
Rehabilitation treatments would need to include remediation of limiting factors (eg. soil
compaction), and broadleaf artificial regeneration (seedling container stock), while
recognizing that these approaches are supported by very limited research or operational
experience.
The focus of silviculture research has shifted from initial establishment to longerterm information needs such as free-growing benchmarks, growth and yield, biodiversity,
stand dynamics and stand structure (Biring et al. 1999). Re-examination of older
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vegetation management trials in the BWBS (Harper et al. 1997, Biring et al. 1999,
Boateng et al. 1999) have consistently shown that a single glyphosate application early in
the development of the stand have dramatically improved growth of white spruce in
treated areas over that of controls. The glyphosate treated areas meet current conifer freegrowing definitions while maintaining a broadleaf component, and without adversely
affecting vascular plant community diversity.
There are a spectrum of potential silviculture systems that could develop boreal
mixedwood stands with a range of compositions, structures and value (Lieffers et al.
1996b). Timing of stand entries and the level of management inputs can dramatically
shift species composition, and future availability of outputs such as timber volumes. This
abundance of choices, and complexity of outcomes is both the opportunity and the
dilemma of BWBS silviculture.
Clearcutting, and more recently, clear-cuts with reserves, has been the dominant
silviculture system in the BWBS for all forest types. Retention of white spruce advance
regeneration has occurred to a limited extent, notwithstanding the availability of decades
of boreal research and information from permanent sample plots (eg. Yang 1989, Ball and
Walker 1997). The influence of the size, stocking and distribution of retained white
spruce on aspen regeneration has been vigorously debated in the context of long term
land use objectives. Harvesting systems which retain advance regeneration, in
combination with underplanting of aspen dominated stands with conifers may be required
to maintain or encourage development of desired mixtures on a landscape level
The short-term and long-term effects of different densities of broad-leaved species
on the development of mixed species stands is a fundamental information need which has
dramatic implications for management of boreal forests. Initial regeneration methods
may include both natural and artificial regeneration strategies. Species selection,
densities and spatial arrangement of artificial regeneration may vary depending on
management objectives and ecological conditions. Better information on species
interactions, the effects of species densities (and how these change with age), and of stand
dynamics (crown development, self-thinning, interspecific competition) is required. A
mixture of short-term and long-term studies are required to explore species interactions,
effects of stand tending, and stand dynamics.
Improvements in management prescriptions requires a deeper understanding of
structural and functional ecological relationships at both the landscape and stand levels.
The dominant disturbance agent of wildfire and resulting early successional patterns need
be further examined for biologically appropriate management and regeneration strategies.
Studies of natural processes that address understory light, microclimate and nutrient
cycling are needed to help understand the interactions between species in mixedwood
stands.
Developing of plans and actions needs to be carried out on larger temporal and
spatial scales in the BWBS. This would include stand level modeling tools which can
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incorporate the existing stand level knowledge. Landscape and larger spatial analyses
which incorporate successional dynamics, harvest scheduling, and random events such as
wildfire are needed to properly co-ordinate timber supplies of broadleaf and conifer
within a sustainable system. Improved inventory is an essential requirement to: 1)
increase the value of landscape or higher level analyses, and 2) reduce uncertainty of
modeled outcomes.
The priority issues requiring study are:
1) information is needed to make informed decisions regarding appropriate amounts of
broad-leaved and coniferous trees that should be maintained at various stages of stand
development in order to achieve different management objectives
2) alternative silviculture systems, specifically retention of white spruce advanced
regeneration in mixedwood stands has not been adequately evaluated for i) implications
for aspen regeneration, ii) impacts on wildlife habitat, and iii) applicability in meeting
long term goals for timber production, biodiversity, and sustainability in boreal
ecosystems
3) integration of stand level techniques within a larger spatial and temporal context so
that managed forests retain more of the features of natural boreal forests
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