Vol.3, No.3, 215-223 (2013) Open Journal of Ecology
http://dx.doi.org/10.4236/oje.2013.33025
Stand age structural dynamics of conifer,
mixedwood, and hardwood stands in the boreal
forest of central Canada
Jennifer M. Fricker1, Jian R. Wang1*, H. Y. H. Chen1, Peter N. Duinker2
1Faculty of Natural Resources Management, Lakehead University, Thunder Bay, Canada;
*Corresponding Author: jian.wang@lakeheadu.ca
2School for Resource and Environmental Studies, Dalhousie University, Halifax, Canada
Received 18 April 2013; revised 26 May 2013; accepted 28 June 2013
Copyright © 2013 Jennifer M. Fricker et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
To study the effects of stand development and
overstory composition on stand age structure,
we sampled 32 stands representing conifer, mix-
edwood, and hardwood stand types, ranging in
ages from 72 to 201 years on upland mesic sites
in northwestern Ontario. We defined the stages
of stand development as: stem exclusion/cano-
py transition, canopy transition, canopy transi-
tion/gap dynamics, and g ap d ynamics. Stand age
structure of conifer stands changed from bimo-
dal, bimodal, reverse-J, and bimodal, respe ctive-
ly, through the stages of stand development.
Mixedwood and hardwood stands revealed simi-
lar trends, with the exception of missing the ca-
nopy transition/gap dynamic stage in mixedwoods.
Canopy transition/gap dynamic stage in hard-
woods showed a weaker reverse-J distribution
than their conifer counterparts. The results sug-
gest that forest management activities such as
partial and selection harvesting and seed-tree
systems may diversify standard landscape-level
age structures and benefit wildlife, hasten the
onset of old-growth, and create desired stand
age structures. We also recommend that the de-
termination of old-growth using the following
criteria in the boreal forest: 1) canopy break-
down of pioneering coh ort is comp le te and stand
is dominated by later successional tree species,
and 2) stand age structure is bimodal, with do-
minating canopy trees that fall w ithin a relatively
narrow range of age and height classes and a sig-
nificant amount of understory regeneration.
Keywords: Time Since Fire (TSF); Stand
Development; Old-Growth F orest; Conife rs;
Hardwoods; Mixedwood; Boreal Forests
1. INTRODUCTION
Forest stands have long been described by their age
structure and diameter distribution. Ecosystem dynamics
in the Canadian boreal forest are closely tied to natural
fire regimes. Fire types, intensity and time since fire
(TSF) are fundamental to forest species composition, age
structure and forest succession [1-3]. Forest managers
are challenged and mandated by law in some jurisdic-
tions to preserve and emulate natural disturbances such
as fire in the boreal forest region. Therefore the dynamic
patterns of age structure are the basis for landscape level
planning in Northwest Ontario.
Age structures of natural forest stands change over
time [4,5]. Research has found that forest stands change
from an even-aged, relatively homogeneous tree height
structure to a two-cohorts, bimodal height structure to
where tree heights are relatively heterogeneous as TSF
increases [5-8]. Young stands are primarily composed of
early successional species that grow quickly in open ar-
eas in full light [9]. However, as stand age increases,
stands become increasingly composed of later succes-
sional species that can establish under a closed canopy
with limited light.
Age structure has been examined for conifer [10], mix-
edwood [9], and hardwood forest types [11,12] in a spe-
cific successional stage. However, few studies thus far
have compared age structure for different boreal forest
cover types with similar environmental characteristics
(i.e., soils, topography and climate) along a successional
gradient. As many stand characteristics have been found
to differ with stand composition (i.e., productivity, coarse
woody debris) [13-16], we hypothesized that the differ-
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 2 15-223
216
ent stand cover types would vary in stand age structural
dynamics.
Old-growth forests have potential wildlife habitat and
biodiversity benefits that link to unique structural char-
acteristics i.e., larger trees, a multi-layered canopy, cano-
py gaps, and higher tree species richness [17]. However,
few definitions exist to identify old-growth in the boreal
forest, and this lack of information and confusion sur-
rounding the old-growth condition is hindering manage-
ment planning and implementation [18].
The goal of this study is to determine how stand age
structure changes in the boreal forest over time. Specifi-
cally, we addressed how stand age structure varies with
forest cover type, conifer, mixedwood, and hardwood and
developmental stages, stem exclusion/canopy transition,
canopy transition, canopy transition/gap dynamics, and
gap dynamics [1].
2. MATERIALS AND METHODS
2.1. Study Area
We conducted this study in northwestern Ontario bo-
real forests north of Lake Superior in the Superior (B.9)
Forest Region [19], Ontario (48˚ 22’ N, 89˚ 19’ W, 199 m
altitude) [20] in the Spruce River Forest. Climatically,
the area is influenced by Lake Superior and has a moder-
ately dry, cool climate with a short summer. The average
annual precipitation for Thunder Bay is 712 mm with an
average annual temperature of 2.5˚C. Topographical fea-
tures were formed during the retreat of the Laurentide Ice
Sheet approximately 10,000 years ago [20].
The area is characterized as containing tree species of
paper birch (Betula papyrifera Marsh.), trembling aspen
(Populus tremuloides Michx.), balsam fir (Abies bal-
samea (L.) Mill.), white spruce (Picea glauca (Moench)
Voss), black spruce (Picea mariana (Mill.) BSP), jack
pine (Pinus banksiana Lamb), and eastern white cedar
(Thuja occidentalis L.) on sites [19]. Common shrubs
and herbs found in this area were mountain maple (Acer
spicatum Lam.), beaked hazel (Corylus cornuta Marsh.),
labrador tea (Ledum groenlandicum Jacq.), Canada fly
honeysuckle (Lonicera canadensis Bart. Ex Marsh.), nor-
thern star flower (Trientalis borealis Raf.), rose twisted
stalk (Streptopus roseus Michx.), bunchberry (Cornus ca-
nadensis L.), and wild lily of the valley (Maianthemum
canadense Desf.). The natural stand-initiating distur-
bance in the area is predominately stand-replacing fire
with a fire return interval of approximately 100 years
[21].
2.2. Sampling Design
Thirty two stands were sampled throughout the study
area in a stratified (by forest type) random manner. We
sampled three overstory types: 1) conifer dominated by
jack pine at early stages of development with a mixture
of black spruce, white spruce, and balsam fir at later
stages of stand development; 2) hardwood dominated by
trembling aspen at early stages of stand development and
paper birch at later stages; and 3) mixedwood dominated
by a mixture of jack pine and trembling aspen in early
stages of stand development and a mixture of black
spruce, white spruce, balsam fir, and paper birch in later
stages. The sampled stands aged from 72 to 201 years
TSF (Table 1).
Stand type was determined through a modification of
methods used by [22]. Stands were assessed as belonging
to a specific stand type based on the density (stems/plot)
of conifer trees that dominated the overstory of the stand.
Stands with a greater than 75% conifer component were
classified as “conifer type”, stands with a 25% - 75%
conifer component were classified as “mixedwood type”,
and stands with less than a 25% conifer component were
classified as “hardwood type”.
All sampled stands were fire-origin on prevailing me-
sic, upland sites in order to represent the forests in the re-
gion and limit soil variability. Soil order and texture were
determined by excavating three soil pits using methods
outlined by British Columbia Ministry of Environment &
British Columbia Ministry of Forests [23]. Soil assess-
ment followed [24,25] to ensure that sites met the selec-
tion criteria described previously. For all sites, soil order
was Brunisol while soil texture was sandy loam, sandy
clay loam, or clay loam.
2.3. Field Measurements
Within each stand, a 400 m2 circular plot was estab-
Table 1. Description of 32 sampled stands in northwestern On-
tario.
Density
(trees/ha)
Number of
stands
Stand
type*Stage
TSF (year) Canopy trees Regeneration
3 C 2 - 372 1217 (253) § 3955 (1142)
3 C 3 90 1208 (158) 3022 (387)
2 C 3 - 4139 487 (62) 2666 (133)
3 C 4 201 908 (260) 1288 (437)
3 M 2 - 372 1083 (72) 2311 (823)
3 M 3 90 1108 (144) 2222 (898)
3 M 4 201 1066 (96) 3777 (1646)
3 H 2 - 372 1158 (375) 1111 (512)
3 H 3 90 675 (14) 177 (117)
3 H 3 - 4139 716 (41) 2088 (270)
3 H 4 201 1025 (203) 2866 (1404)
*Stand type: C = conifer, M = mixedwood, H = hardwood; Stand develop-
mental stage: 2 - 3 = stem exclusion/canopy transition, 3 = canopy transition,
3 - 4 = canopy transition/gap dynamics, 4 = gap dynamics; Canopy trees
are 10 cm diameter at breast height (DBH), regeneration are trees <10 cm
DBH; §Numbers in brackets equal one standard error of the mean.
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 215-223 217
lished to represent the stand. Within each plot, the di-
ameter at breast height (DBH) (1.3 m above the root col-
lar) and species of all trees (DBH 10 cm) were meas-
ured and recorded. The DBH measurements were then
grouped into DBH classes (10 - 14.9 cm, 15 - 19.9 cm,
20 - 24.5 cm, 25 - 29.9 cm, and 30 cm) to facilitate tree
selection for height and age measurement. Five trees (if
available) were randomly selected from each 5-cm DBH
class for each species. For each sample tree, its height
was measured using a clinometer, and an increment core
was taken at breast height. Increment cores were stored
in a freezer until they processed.
Three circular 25 m2 subplots were randomly selected
within each 400 m2 plot to evaluate natural regeneration
(<10 cm DBH). This would also include trees that had
not yet reached breast height. Within each subplot, di-
ameter at the root collar, height, and species of all trees
were measured and recorded. Further, trees were grouped
into height classes (<0.29 m, 0.3 - 0.99 m, 1.0 - 1.9 m,
2.0 - 4.9 m, and 5 m), and 5 trees were randomly se-
lected from each height class, and a disk was taken at the
root collar for accurate aging of each sapling and seed-
ling.
2.4. Tree Ring Counting
In the lab, increment cores from trees DBH 10 cm
were each mounted on a wood strip and sanded with grit
sandpaper until the rings were visible. Growth rings were
then counted using a hand-held magnifier. To estimate
the age of each tree from root collar to breast height, a
species-specific number of years was added to each tree’s
growth ring count as outlined by [26] (jack pine = 8
years, trembling aspen and paper birch = 7 years, black
and white spruce and balsam fir = 18 years). Balsam fir
and white spruce were based on a conservative estimate
of the ages of trees using black spruce because it is more
shade-tolerant. For trees DBH <10 cm, the growth rings
of each disk were counted using a hand-held magnifier or
under a microscope. As the disk was taken at the root
collar, no ages had to be added.
2.5. Data Analysis
Ages for the remaining trees were estimated using spe-
cies-specific non-linear regression models as outlined by
[27] developed from the paired age and diameter meas-
urements,

100110
LogLog diameterAa a
where A is tree age (years), a0 and a1 are parameters, and
diameter is diameter at (a) breast height (if tree DBH 10
cm) or (b) root collar (if tree DBH <10 cm). The para-
meter estimates for the age-diameter models were pre-
sented in Table 2.
Table 2. Species-specific parameter estimates of non-linear
age-diameter at breast height models using Daniels et al. (1995)
(
01
10^ (* Log10diameterAaa ) where A is tree age
(years), a0 and a1 are parameters, and diameter is diameter at (a)
breast height (if tree DBH 10 cm) or (b) root collar (if tree
DBH <10 cm). For trees DBH <10 cm, no jack pine were sam-
pled, while the sample size for trembling aspen was very small.
= Therefore, no age-diameter models for those species were
developed.
Parameter
Species a0 a1 MS R2
Trees (DBH 10 cm)
Bf 1.7265 0.0958 71.6835 0.0496
Bw 1.4499 0.3034 101.1264 0.2529
Pj 1.4472 0.309 178.5015 0.1715
Po 1.2813 0.4364 156.3646 0.3833
Sb 1.6816 0.1729 153.9571 0.1015
Sw 1.5596 0.2406 116.9068 0.3256
Trees (DBH <10 cm)
Bf 1.2965 0.459 48.4667 0.5297
Bw 1.1645 0.2477 57.9858 0.2668
Sb 1.3301 0.5539 24.2526 0.6614
Sw 1.2944 0.4908 11.235 0.6537
Trees by species were then grouped into age classes as
follows: 1) 0 - 9 years, 2) 10 - 19 years, 3) 20 - 29 years,
4) 30 - 39 years, 5) 40 - 49 years, 6) 50 - 59 years, 7) 60 -
69 years, 8) 70 - 79 years, 9) 80 - 89 years, 10) 90 - 99
years, 11) 100 - 109 years, 12) 110 - 119 years, and 13)
120 years and scaled up to stems per hectare. Bar charts
were constructed to show the density of trees (trees/ha)
by age class and species in each stand developmental
stage and forest cover type.
3. RESULTS
3.1. Conifer Stands
Stand age structure in the stem exclusion/transition
stage was largely bimodal, having a younger cohort in
the 2 and 3 age classes and an older cohort in the 7 and 8
age classes (Figure 1(a)). The younger cohort was com-
posed of mainly balsam fir and black spruce which re-
cruited in the understory at various times after the stand-
replacing fire. The older cohort was composed of mainly
jack pine canopy trees with some black spruce, balsam
fir and paper birch that established shortly after fire,
therefore falling within a relatively narrow range of age
classes. Canopy tree density in this stage was 1217 trees/
ha, while regeneration density was 3956 trees/ha (Table
1).
The age structure in the canopy transition stage of
stand development was similar to the stem exclusion/ca-
nopy transition stage with a bimodal age structure as well
(Figure 1(b)). The older cohort of canopy trees was com-
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 2 15-223
218
(a)
(b)
(c)
(d)
Figure 1. Density of trees in conifer stands by age class (1 = 0 -
9 yrs, 2 = 10 - 19 yrs, 3 = 20 - 29 yrs, 4 = 30 - 39 yrs, 5 = 40 -
49 yrs, 6 = 50 - 59 yrs, 7 = 60 - 69 yrs, 8 = 70 - 79 yrs, 9 = 80 -
89 yrs, 10 = 90 - 99 yrs, 11 = 100 - 109 yrs, 12 = 110 - 119 yrs,
13 = 120 + yrs) and species (Sw = white spruce, Sb = black
spruce, Po = trembling aspen, Pj = jack pine, Bw = paper birch,
Bf = balsam fir) for (a) stem exclusion/canopy transition; (b)
canopy transition; (c) canopy transition/gap dynamics; and (d)
gap dynamics.
posed of a mixture of jack pine, black spruce, white
spruce, balsam fir, and paper birch. This cohort aged
between 60 - 80 years old and probably recruited shortly
after the fire. The younger cohort of 20 - 30 years old
represented the understory regeneration of balsam fir and
black spruce with minor components of white spruce and
paper birch (Figure 1(b)). We believe that birch seed-
lings seeded into these stands, as the birch trees appeared
to be distributed randomly throughout the stand and no
canopy trees were present for birch seedlings to sprout
off. Canopy tree and regeneration density decreased to
1208 trees/ha and 3022 trees/ha respectively (Table 1).
During the canopy transition/gap dynamics stage of
stand development, the age structure of the stands be-
came largely uneven-aged and the distribution of trees
resembled a reverse-J age structure (Figure 1(c)). All age
classes from 1 to 13 (0 to 120 year TSF) were repre-
sented with the exception of trees being absent in age
class 12 (110 - 119 year TSF). Canopy trees were largely
jack pine, white spruce, black spruce, and balsam fir with
minor component of paper birch while the understory
was mainly balsam fir and black spruce (Figure 1(c)). A
few jack pine and paper birch trees were present in the
oldest age class (13 age class = 120 years TSF), which
recruited immediately after the last stand-replacing fire.
Canopy tree and understory density have decreased, in
comparison to the canopy transition conifers, to 488 trees/
ha and 2667 trees/ha respectively (Table 1).
During the gap dynamic stage of stand development,
age structure had become largely bimodal once again (Fig-
ure 1(d)). There is a significant contribution to stand age
structure of exclusively balsam fir regeneration forming
the younger cohort in largely the 1 and 2 age classes (0 -
19 years old). White spruce, balsam fir, and paper birch
formed the older cohort in the 7 and 8 age classes (60 -
79 years old). However, there is a sparse number of white
spruce, black spruce and paper birch in older age classes
9 to 13 i.e. 80 120 years old (Figure 1(d)). Canopy
tree density in this stage had increased in comparison to
the canopy transition/gap dynamic conifers to 908 trees/
ha, while regeneration density decreased to 1289 trees/ha
(Table 1).
3.2. Mixedwood Stands
In mixedwood stands during the stem exclusion/ can-
opy transition stage of stand development, stand age
structure was bimodal, having a younger cohort in the 2
and 3 age classes (10 - 29 years old) and an older cohort
in the 6 to 9 age classes (50 - 89 years old). Similar to the
conifer forest cover type, the younger cohort represented
the understory regeneration of black spruce and balsam
fir. The older cohort represented canopy trees and was
predominantly jack pine and trembling aspen, with some
black spruce and paper birch (Figure 2(a)). Canopy tree
density during this stage was 1083 trees/ha, which was
lower than the density of trees in the conifer stem exclu-
sion/canopy transition stage (Table 1). Regeneration den-
sity was 2311 trees/ha, and also was less than the re-
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 215-223 219
(a)
(b)
(c)
Figure 2. Density of trees in mixedwood stands by age class
(as in Figure 1) and species (Sw = white spruce, Sb = black
spruce, Po = trembling aspen, Pj = jack pine, Bw = paper birch,
Bf = balsam fir) for (a) stem exclusion/canopy transition; (b)
canopy transition; and (c) gap dynamics. No data is available
for the canopy transition/gap dynamic mixedwoods.
generation occurring in the stem exclusion/canopy tran-
sition conifers (Table 1).
The age structure in the canopy transition stage of
stand development was similar to the stem exclusion/
canopy transition stage (Figure 2(b)). It was character-
ized by a strong bimodal age structure. A younger cohort
represented balsam fir and black spruce regeneration with
minor components of white spruce and paper birch that
were in the 2 and 3 age classes (10 - 29 years old). The
older cohort was composed of jack pine, trembling aspen,
black spruce, white spruce, balsam fir, and paper birch
trees that were within the 6 to 9 age classes i.e. 50 - 89
years old (Figure 2(b)). The density of canopy trees
(1108 trees/ha) was higher than that occurring in the stem
exclusion/canopy transition mixedwoods, which was low-
er than that in the canopy transition conifers. As well, the
regeneration density was lower (2222 trees/ha) compared
to both the stem exclusion/canopy transition mixedwoods
and the conifer canopy transitions (Table 1).
We were unable to locate stands of the mixedwood co-
ver type in the canopy transition/gap dynamics stage to
sample. Therefore only three development stages for
mixedwood cover types are presented. During the gap
dynamic stage of stand development, age structure was
somewhat bimodal (Figure 2(c)). There was a significant
contribution to stand age structure of regeneration form-
ing the younger cohort in the 1 and 2 age classes i.e. 0 -
29 years old. Canopy trees that formed the second small
peak in the 7 and 8 age classes i.e. 60 - 79 years old were
a mixture of paper birch, balsam fir, and white and black
spruce (Figure 2(c)). The understory regeneration was pre-
dominantly balsam fir with a small component of white
spruce. Canopy tree density has decreased in comparison
to canopy transition mixedwoods to 1067 trees/ha, which
was higher than that in the gap dynamic conifers (Table
1). Regeneration was 3778 trees/ha, higher than the den-
sity of regeneration in both the canopy transition mixed-
woods and the gap dynamic conifers. In general, when
present in the stands, balsam fir, white spruce and black
spruce were found in most age classes.
3.3. Hardwood Stands
During the stem exclusion/canopy transition stage of
stand development in the hardwoods, stand age structure
was typical bimodal, having a younger cohort in the 1, 2
and 3 age classes (0 - 30 years old) and an older cohort in
the 6 to 9 age classes (50 - 90 years old). The older co-
hort was composed of largely trembling aspen and black
spruce, with some paper birch and balsam fir, while the
younger cohort was the understory regeneration of black
spruce and balsam fir (Figure 3(a)). Canopy tree density
was 1158 trees/ha, and was similar to the canopy tree den-
sity in the stem exclusion/canopy transition mixedwoods
but lower than the conifers of this stage (Table 1). Rege-
neration density was 1111 trees/ha, and was lower than
both the conifer and mixedwood stem exclusion/canopy
transition stands.
Unlike the age structure in conifer and mixedwood co-
ver types, the age structure in the canopy transition hard-
woods was weakly unimodal. The major canopy tree co-
hort was composed of largely trembling aspen within the
7 to 10 age classes (60 - 100 years old) regenerated
shortly after the fire (Figure 3(b)). Understory regenera-
tion density was very low (178 trees/ha) and spread across
the 1 to 4 age classes (Table 1, Figure 3(b)). Canopy tree
density was 675 trees/ha lower than the density of cano-
py trees in the stem exclusion/canopy transition hardwoods
and the canopy transition conifers and mixedwoods.
During the canopy transition/gap dynamics stage of
stand development, the age structure of the stand appears
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 2 15-223
220
(a)
(b)
(c)
(d)
Figure 3. Density of trees in hardwood stands by age class (as
in Figure 1) and species (Sw = white spruce, Sb = black spruce,
Po = trembling aspen, Pj = jack pine, Bw = paper birch, Bf =
balsam fir) for (a) stem exclusion/canopy transition; (b) canopy
transition; (c) canopy transition/gap dynamics; and (d) gap dy-
namics.
to be weakly reverse-J to weakly bimodal, with majority
of trees in the younger cohort in the 2 and 3 age classes
(10 - 20 years old). There were trees in every age class
from 1 to 13 (0 to 120 years old) (Figure 3(c)). The
older canopy trees were mainly trembling aspen and
black spruce, while the younger canopy trees were large-
ly balsam fir with some white and black spruce, paper
birch, and trembling aspen. Regeneration cohort was lar-
gely balsam fir and black spruce (Figure 3(c)). Canopy
tree density increased compared to the canopy transition
hardwoods to 717 trees/ha, while being higher than their
conifer counterparts (Table 1). Regeneration density also
increased compared to the canopy transition hardwoods
to 2089 trees/ha, but was lower than their conifer coun-
terparts.
Age structure has become largely bimodal once again
for the gap dynamic stage of hardwood stand develop-
ment. There was a significant understory cohort of rege-
neration dominated by white spruce and balsam fir and
paper birch in the 1 to 3 age classes (0 - 30 years old).
Canopy trees were largely paper birch with minor com-
ponents of balsam fir, white spruce, and trembling aspen
forming the older cohort in the 6 to 8 age classes (50 - 79
years old) (Figure 3(d)). Canopy tree and regeneration
density both increased (1025 trees/ha and 2867 trees/ha,
respectively) compared to the canopy transition/gap dy-
namics hardwoods (Ta bl e 1 ). Canopy tree and regenera-
tion density were both lower than their mixedwood but
higher than their conifer respective counterparts.
4. DISCUSSION
After Large, mature white birch were dispersed throu-
ghout the hardwood stands comprising less than 1% of
the total stem density and 5% of the stem basal area.
Even though white birch is traditionally considered an
“early successional” species, this species can persist wi-
thin old-growth boreal coniferous stands [9]. Its prolific
seed-producing ability, combined with the availability of
suitable microsites within sufficiently larger gaps, proba-
bly maintains the presence of white birch.
Different stand types should also be managed differ-
ently. In hardwood stands, clear-cut logging appears to
emulate natural disturbance processes quite well, since
aspen and white birch successfully sprout back immedi-
ately following both fire and logging. In contrast, clear-
cut harvesting of jack pine and black spruce is more pro-
blematic, since the seed source is largely removed espe-
cially if whole-tree harvesting is followed by roadside
chipping.
Stand age structural development in conifer stands pro-
ceeded from a bimodal structure in the stem exclusion/
canopy transition and canopy transition stages to a re-
verse-J age structure in the canopy transition/gap dyna-
mics stage to a bimodal structure once again in the gap
dynamics stage. In the stem exclusion/canopy transition
and canopy transition stages, the canopy was dominated
by jack pine established immediately after the stand-re-
placing fire, as significant age-related mortality yet oc-
curred. In turn, self-thinning that occurred in earlier stages
of development would have opened up growing space
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 215-223 221
and freed up nutrients [5,27], thus contributing to the sig-
nificant regeneration of conifers such as black spruce and
balsam fir.
By the canopy transition/gap dynamics stage of stand
development, most of the pioneering cohort had died off,
as only a few jack pine trees remain living in the stands
(average 42 trees per hectare) because the age of these
stands (139 years TSF) is beyond jack pine’s average life
span [28]. As well, trees that were suppressed in earlier
stages of stand development were released to undergo ra-
pid growth and moved into the canopy and subcanopy
positions. Mortality of the pioneering cohort of jack pine
would free up additional space for further understory re-
generation to establish. This resulted in a reverse-J age
structure in these stands, as observed by [29], who found
that old (>120 year-old) spruce stands in sub-boreal British
Columbia show either a reverse-J or bimodal age structure.
During the gap dynamic stage of stand development,
pioneering cohort of jack pine had completely died off,
and the canopy was dominated by late successional coni-
fers. These conifers were younger than the jack pine that
dominated in the preceding stage, causing the oldest age
classes to disappear. As such, the age structure became
bimodal, with a canopy dominated by later successional
species and an understory with young regeneration of co-
nifers, largely balsam fir. The very low density of balsam
fir in age classes 3 to 6 (20 - 59 years) may have been
caused by periodic spruce budworm (Choristoneura fu-
miferana (Clem.)) outbreaks. The most recent one peak-
ed in 1986 and had collapsed approximately ten years la-
ter [30]. These outbreaks would have killed a significant
amount of host-specific balsam fir and, to a lesser extent,
white and black spruce trees [31-34].
While differing from conifer stands, hardwood stands
were dominated by trembling aspen, hardwood stands in
the stem exclusion/canopy transition stage of stand deve-
lopment with a bimodal age structure with a small black
and white spruce component established shortly after the
stand-replacing fire. Understory contained largely conifer
regeneration dominated by balsam fir. However, hard-
woods in the canopy transition stage had a unimodal
stand age structure with largely a single cohort of trem-
bling aspen trees established after the disturbance and
marginal amounts of conifer regeneration. Stands domi-
nated by trembling aspen often developed a dense shrub
layer of mountain maple (Acer spicatum) [35] and beak-
ed hazel (Corylus cornuta) [36]. This was observed in all
sampled hardwood stands in the canopy transition stage.
Furthermore, dense shrub layers have been found to hin-
der understory conifer regeneration [35-37], thus contri-
buting to the unimodal age structure in the canopy transi-
tion hardwoods.
In contrast to the conifer stands, hardwood stands had
many more trembling aspen trees (200 trees·ha1) from
the pioneering cohort still living in the canopy transi-
tion/gap dynamic stage of stand development even thou-
gh jack pine is generally a longer-lived tree species com-
pared to trembling aspen [28]. This may be due to the
hardwood sites in the canopy transition/gap dynamic stage
of stand development being more productive than the
conifer sites that were sampled. However, as in the coni-
fer stands, hardwoods had a relatively reverse-J stand age
structure.
Paper birch dominated the hardwoods in the gap dy-
namic stage of development. Paper birch has been shown
to be able to live for well over 200 years [9], and is the
only hardwood species in this area of the boreal forest
that could form dominant stands by this stand age. Fur-
ther, the ability of paper birch to allow light to pass
through to the forest floor, and the sparse shrub layer that
was found, would allow for a significant amount of un-
derstory regeneration to establish. Therefore, the age struc-
ture of gap dynamic hardwoods were similar to that of
the gap dynamic conifers with a bimodal age stand struc-
ture caused by a canopy of paper birch trees falling into
the older age classes and regeneration of conifers form-
ing the younger age classes.
With the exception of mixedwood stands having cano-
pies composed of a mixture of conifers and hardwoods
that met the sampling criteria (25% - 75% conifer com-
ponent), stand age structure of mixedwood stands devel-
oped similarly to that of conifer stands. This was caused
by the conifer component limiting light to the forest floor
and preventing a dense shrub layer from developing [35].
Without this dense shrub layer, regeneration would have
responded similarly to what was occurring in conifer
stands, thus causing a similar age structure to develop.
We believe that successional trajectories in mixedwood
stands may be headed towards conifer dominance due to
the composition, as indicated by the composition of rege-
neration. However, budworm outbreaks tend to occur about
every 20 years [30], therefore making this uncertain.
Regardless of stand cover type or developmental stage,
regeneration in all the stands was almost exclusively bal-
sam fir and spruce, indicating that successional trajecto-
ries in the study area are likely proceeding towards coni-
fer dominance on most sites. This is likely a consequence
of the silvics of these species and the availability of near-
by seed sources. Black spruce is moderately shade-tole-
rant, allowing it to establish under the cover of other trees,
while also being able to reproduce by layering [38]. Bal-
sam fir is a shade-tolerant species, and has seeds that are
readily dispersed by wind [39] and enter a stand from
nearby areas.
5. IMPLICATION FOR OLD-GROWTH
MANAGEMENT
Old-growth forests have been found to provide many
Copyright © 2013 SciRes. OPEN A CCESS
J. M. Fricker et al. / Open Journal of Ecology 3 (2013) 2 15-223
222
values from an ecological, aesthetic/recreational, and eco-
nomic perspective [8,40]. However, management deci-
sions surrounding old-growth are hampered by the lack
of a clear definition on what old-growth is in the boreal
forest [40,41]. While some studies including this study
use the disappearance of the pioneering cohort as the point
at which an old-growth structure is reached [1,8] defini-
tions of old-growth vary depending on the study [42]. We
recommend that old-growth in this region of the boreal
forest be considered when the following criteria are met:
1) canopy breakdown of the pioneering cohort is com-
plete and the stand is dominated by later successional
tree species such as balsam fir and spruce, and 2) the age
structure of the stand is bimodal, with dominating can-
opy trees that fall within a relatively narrow range of age
classes and a significant amount of understory regenera-
tion.
Selection harvesting could be used to hasten the onset
of old-growth and/or create a reverse-J stand age struc-
ture if applied to stands that are in approximately the
stem exclusion/canopy transition stage of stand develop-
ment or even earlier in the stem exclusion stage. We sug-
gest that selectively removing canopy trees would (a) re-
lease suppressed trees, (b) allow canopy trees to grow
even faster, and (c) allow trees to establish in gaps cre-
ated by the removal of canopy trees. This would promote
the movement of a unimodal or bimodal age structure
into a reverse-J age structure while increasing the later
successional species in the stand thereby hastening old-
growth onset.
Sustained efforts must be made to ensure that harvest-
ing rotations are such that various age and size structure
of aspen stands are maintained at the landscape level. By
age 100, jack pine stands in this area consist of an uneven-
aged mixture of many species, making them less desir-
able for logging. The harvest of large patches of aspen in
the complex canopy matrix could help perpetuate the hard-
wood component and thereby promote landscape-level
biodiversity.
6. ACKNOWLEDGEMENTS
The work was supported financially by the Natural Sciences and En-
gineering Research Council (238891-01) and the Sustainable Forest
Management Network of Centers of Excellence. We thank the review-
ers for their constructive comments.
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