# Relationships between Structure, Composition, and Dynamics of the Pristine Northern Boreal Forest and Air Temperature, Precipitation, and Soil Texture in Quebec (Canada)

Authors: Louis Duchesne; Rock Ouimet
Journal: International Journal of Forestry Research
 (2009)

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## Abstract

This study reports on the contemporary structure, composition, and dynamics of the pristine northern boreal forest in Quebec, Canada, associated with air temperature, precipitation, and soil texture, using 147 permanent sample plots located at the limit of continuous forest in Quebec. The results show that tree species composition of stands is associated with stand age, soil texture, air temperature, and precipitation regime. After establishment of the pioneer cohort, the postsuccessional stand dynamics differed among temperature and precipitation regimes, probably because of their influence on tree growth. Our results support the hypothesis that shifts in forest composition related to stand dynamics and the subsequent senescing phase associated with the old growth stage generally occur sooner and proceed faster on more fertile sites due to quicker growth and the subsequent earlier mortality of pioneer species. This study suggests that climate warming should accelerate the successional dynamics of these ecosystems through its positive influence on tree growth.


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## Body

## 1. Introduction

The circumboreal forest is the most
extensive terrestrial biome in the world, encompassing some 14 millionkm2,
or 32% of the earth's forest cover, and constituting one of the largest reserves
of untapped wood fibre [1]. At the global level, the boreal forest is subjected
to temperature and precipitation gradients that largely control the range of
major tree species and the distribution of the main boreal zone [2]. While the
causal chains between tree growth dynamics and the simple climatic variables
are somewhat lengthy and involved, two of these climatic variables, air temperature
and precipitation, explain more than 60% of some dominant species' growth
variation, as measured by tree ring chronology [3]. Tree growth and forest
dynamics are also affected by factors other than climate, such as soil water
and nutrient availability. A previous study has shown the possibility of deriving
site quality indices from biophysical variables to model tree growth and yield for
the main boreal forest tree species [4].Forest fire is recognized as the main
disturbance agent in the boreal coniferous forest, and as being of great
importance for plant succession, ecosystem processes, landscape structure, and
biological diversity [5]. The frequency, size, intensity, seasonality, and type
of fire depend on air temperature, precipitation, and soil characteristics, in
addition to forest structure and composition [6]. The American boreal forest is characterized by relatively short
fire cycle periods and by a mosaic that is mainly composed of postfire
even-aged stands [7]. However, long fire intervals that allow changes in canopy
dominance and the development of uneven-aged forests have also been reported,
particularly in the eastern boreal forest [8–10]. Also, it has been demonstrated that the postfire dynamics of the black
spruce forest of the clay belt region in Quebec and Ontario differ among soil
substrates [11–13].Currently, there are no reported large-scale
field observations documenting the combined effect of precipitation and
temperature regimes, along with soil characteristics, on the dynamics of the northern
boreal coniferous forest. In a changing environment, it is generally recognized—in addition to modeling
efforts—that adequate
monitoring protocols are needed to establish rates of change in environmental
drivers and in species and community responses to those changes [6, 14]. Detailed studies are needed in a variety of
environments to detect recurrent patterns and underlying processes, and to improve
our comprehension of the interactions among climate, disturbances, and
successional trajectories [6]. The objective of this study was to analyze
the relationship between air temperature and precipitation regimes, along with
soil texture, and the contemporary structure, composition, and dynamics of the
pristine northern coniferous boreal forest in Quebec, Canada. We wanted to test the hypothesis that the dynamics
of the continuous northern boreal forest differed among air temperature and
precipitation regimes, along with soil characteristics. We hypothesized that
tree growth was greater, and that the successional dynamics of the northern boreal
forest are faster on warmer, wetter and more fertile sites. This analysis
should provide a better comprehension of the possible effect of climate and
soil characteristics on the contemporary northern boreal forest. Such
information is a prerequisite for adequate conceptualization and framing of forest
growth and yield models used in forest management planning.


## 2. Material and Methods

Since 1970,
approximately 12 300 permanent sample plots have been progressively established
and periodically inventoried in the continuous forest range of Quebec by the provincial
Ministry of Natural Resources and Wildlife to improve the knowledge base of
provincial forest resources, notably forest dynamics. Among them, 7155 plots were
established during the first decade following the initiation of surveys (1970–1977). By 2005,
approximately 300 plots located above latitude 50°N had been measured
four times, with a time interval between successive surveys ranging from 4 to
11 years. These plots were distributed a few kilometres above and below the current
northern limit of the forest management area in Quebec (50 to 52°N
and −68 to −78°W), mainly within the black spruce-moss bioclimatic
domain [2]. Along this transect, maximum elevation and
precipitation increase from west to east from 125 to 800 m, and from 700
to 1100 mm yr−1, respectively, while mean
annual temperature decreases from 4.5 to 0°C [15]. The fire cycle differs considerably among
regions and the time period considered. Cycles of less than 100 years to more
than 500 years have been reported [16]. Most of the area is dominated by Picea mariana, which tends to form monospecific stands.
These plots represent the pristine northern boreal forest in Quebec, given that
forest operations have not yet reached these latitudes.


### 2.1. Dendrometric and Site Data

Permanent
sample plots were circular, covering 400m2 (radius = 11.28 m). At the
time of establishment and successive measurements, every tree having a diameter at breast height (DBH; 1.3 m above the
highest root) larger than 90 mm was numbered with paint, classified according to
its species and condition (recruit, alive, standing dead, fallen, or harvested),
and its DBH was measured using a diameter tape to the nearest millimetre. Nine dominant
trees were also sampled systematically nearby each plot with an increment borer
at breast height (at the first sampling) for tree age approximation based on tree
ring count. Their height was also measured using a clinometer or a hypsometer. Main
soil characteristics (forest floor thickness, depth of the B horizons, drainage
class) were recorded, and mineral B horizons were sampled for particle size composition
(sand, silt, and clay content), following the Bouyoucos method [17]. Visual
indications of perturbation (insect defoliation, windfall, fire, or cutting)
were recorded when more than 25% of the stand basal area were affected. Spruce budworm defoliation
was recorded in only one plot during the first period. Partial windfall was
recorded in three plots and seven plots during the first and the second period,
respectively, while a partial burn was observed in only two and three plots
during those periods.


### 2.2. Meteorological Data

Daily air temperature
and precipitation from 1977 to 2001 were computed for each permanent plot using
BioSIM, a system designed to model insect development at the landscape level on
the basis of daily weather time series [18], and subsequently used for
biophysical site index development [4]. This system performs two main functions:
(1) matching georeferenced sources of weather data to the specified locations,
and (2) adjusting data from the selected stations for differences in latitude,
longitude, elevation, slope, and aspect between the source of weather data and a
specified location. Data were available from 378 normal-generating weather
stations scattered throughout Quebec, but mainly distributed in the southern
part of the province. Data from 42 weather stations located in the northern
part of the province were used to generate seasonal climatic conditions at the
studied plots (see Figure 1).Figure 1
Location of the 147 forest
permanent sample plots (white circles) located above the 50th parallel, and
meteorological stations (black circles) used to generate climatic conditions at
each plot. Note that several plot locations are superimposed due to the map
scale. The shaded area corresponds to the black spruce-moss bioclimatic domain,
while the grey hatched line corresponds to the limit of the current managed
forest area in Quebec, Canada.


### 2.3. Data Handling and Statistical Analysis

Each record
of the permanent plot dataset was compared with previous and successive
evaluations, and logical consistency was used to check and correct the data, if
necessary, according to the validation procedure outlined in Duchesne and
Ouimet [19]. We selected all plots that were forested at the time of the first
survey and for which information on vegetation, stand age, and soil texture was available (organic
soils were not considered). The final database contained 147 plots with over 6800
trees for the purpose of this study (see Figure 1).For each plot, parameters reflecting initial stand structure and
composition were computed, including basal area proportion of individual
species, dominant stand height, total basal area, and total number of stems. The
Gini coefficient (GC) describing stand diameter diversity was also computed [20].
GC was found superior with respect to discriminating ability and capability of
providing logical ranking of different distributions [20]. Parameters
reflecting contemporary dynamics of the stands (growth, mortality, and recruitment) were
computed in terms of basal area (dm2ha−1yr−1)
for two distinct periods corresponding to the first two (1970–1975 and 1978–1980) and the
last two measurements (1990–1992 and 1998–2001). For a
given period, mortality and net recruitment were evaluated by dividing the
total basal area of dead or new stems by the time between surveys and the total
plot area sampled, respectively. Growth of living trees was also assessed in
terms of basal area. Finally, net growth (growth of living trees + net recruitment − mortality) was estimated by the difference in
the total basal area of living trees between successive surveys divided by the
time interval between them and the total sampled plot area [19].Stand age was estimated by averaging the ring count of sampled trees. Stand
age was categorized into three classes containing a more or less equivalent
number of plots; the upper limit of the first and the second classes being the
33.3% and the 66.6% percentile, respectively. Stand age varied from 22 to 59
years, 60 to 100 years, and 101 to 200 years for the first, second, and third
classes, respectively. We could not ensure that the cored trees in the last age
class belonged to the postfire cohort. We could, however, ensure that no major stand
clearing occurred since the recruitment of these trees.For each plot, normal mean annual temperature and total annual
precipitation were computed over the 1977–2001 period. Mean
seasonal (winter: Jan., Feb., Mar.; spring: Apr., May, Jun.; summer: Jul., Aug.,
Sept.; autumn: Oct., Nov., Dec.) temperature and total precipitation were also
computed over the two distinct periods studied. Soil surface area (SSA) was
estimated using the following equation [21]: (1)SSA=(0.003×Sand+0.022×Silt+0.08×Clay)×Dens×106,
where SSA is the soil specific surface area (m2m−3), Sand, Silt, and Clay variables are particle fractions (%),
and Dens is mineral soil density (g cm−3). Data for soil
density were not available. Consequently, a mineral soil density of 1 g cm−3 was used for all plots. Soil surface area was chosen as the soil parameter
because it reflects both soil water-holding capacity and soil nutrient
availability, which in part determine site productivity [22]. Temperature,
precipitation, and SSA were categorized into two classes each (cold and warm
sites; wet and dry sites; fine and coarse textured soils (from SSA)) containing
approximately the same number of stands (see Table 1). Cut level classes
were not based on biological considerations. They were set to ensure that each class
contained approximately the same number of plots. Temperature, precipitation,
SSA, and age classes result in 24 categories (2×2×2×3), each containing 6
to 7 plots. The relationship between stand age, air temperature, precipitation,
soil texture and initial stand characteristics, and stand dynamics parameters
(growth, mortality, and recruitment) in both periods were examined with
analyses of variance using the mixed model procedure (PROC MIXED) in SAS [23].
Linear and second-degree polynomial contrasts were built to assess the effect
of stand age, temperature, precipitation, and soil texture on every dependent variable.
Calculations of the linear and quadratic coefficients for unequally spaced time
intervals were made according to Robson [24]. The chronosequence approach, in
which stands of differing ages are compared to provide insight into changes
that occur over time, is a commonly used technique [12, 25]. As a “substitute
of space for time,” the proper use of a chronosequence requires that
differences among plots in terms of edaphic, meteorological, and disturbance history
must be kept to a
minimum. Our approach differed from the commonly used technique for which
stands were generally a priori categorized based on site characteristics (e.g.,
soil type, site index productivity) in order to form a chronosequence. In the approach
we followed, the relationships between stand characteristics or stand dynamics
parameters and stand age, air temperature, precipitation, and soil texture were
statistically tested.Table 1
Mean, standard error and number of plots included in air temperature, precipitation,
and soil texture classes.
Temperature class (°C)Precipitation class (mm)Soil texture class (m2m−3)Cold sitesWarm sitesDry sitesWet sitesCoarse textureFine textureMean−2.7−1.3883984608616110Standard error0.070.116.94.32852800n747374737473Comparison of the results obtained from the
analysis of initial stand characteristics and from the contemporary stand
dynamics parameters should provide insights on the change in forest dynamics
occurring over the mid term (last 100 years), while a comparison of results of
the contemporary dynamics between the two measurement periods should provide
insights into changes in forest dynamics over the short term (last 30 years). In
order to assess differences in climate conditions over the two periods studied,
mean seasonal temperature and precipitation were compared with a Wilcoxon
signed-rank test [26].


## 2.1. Dendrometric and Site Data

Permanent
sample plots were circular, covering 400m2 (radius = 11.28 m). At the
time of establishment and successive measurements, every tree having a diameter at breast height (DBH; 1.3 m above the
highest root) larger than 90 mm was numbered with paint, classified according to
its species and condition (recruit, alive, standing dead, fallen, or harvested),
and its DBH was measured using a diameter tape to the nearest millimetre. Nine dominant
trees were also sampled systematically nearby each plot with an increment borer
at breast height (at the first sampling) for tree age approximation based on tree
ring count. Their height was also measured using a clinometer or a hypsometer. Main
soil characteristics (forest floor thickness, depth of the B horizons, drainage
class) were recorded, and mineral B horizons were sampled for particle size composition
(sand, silt, and clay content), following the Bouyoucos method [17]. Visual
indications of perturbation (insect defoliation, windfall, fire, or cutting)
were recorded when more than 25% of the stand basal area were affected. Spruce budworm defoliation
was recorded in only one plot during the first period. Partial windfall was
recorded in three plots and seven plots during the first and the second period,
respectively, while a partial burn was observed in only two and three plots
during those periods.


## 2.2. Meteorological Data

Daily air temperature
and precipitation from 1977 to 2001 were computed for each permanent plot using
BioSIM, a system designed to model insect development at the landscape level on
the basis of daily weather time series [18], and subsequently used for
biophysical site index development [4]. This system performs two main functions:
(1) matching georeferenced sources of weather data to the specified locations,
and (2) adjusting data from the selected stations for differences in latitude,
longitude, elevation, slope, and aspect between the source of weather data and a
specified location. Data were available from 378 normal-generating weather
stations scattered throughout Quebec, but mainly distributed in the southern
part of the province. Data from 42 weather stations located in the northern
part of the province were used to generate seasonal climatic conditions at the
studied plots (see Figure 1).Figure 1
Location of the 147 forest
permanent sample plots (white circles) located above the 50th parallel, and
meteorological stations (black circles) used to generate climatic conditions at
each plot. Note that several plot locations are superimposed due to the map
scale. The shaded area corresponds to the black spruce-moss bioclimatic domain,
while the grey hatched line corresponds to the limit of the current managed
forest area in Quebec, Canada.


## 2.3. Data Handling and Statistical Analysis

Each record
of the permanent plot dataset was compared with previous and successive
evaluations, and logical consistency was used to check and correct the data, if
necessary, according to the validation procedure outlined in Duchesne and
Ouimet [19]. We selected all plots that were forested at the time of the first
survey and for which information on vegetation, stand age, and soil texture was available (organic
soils were not considered). The final database contained 147 plots with over 6800
trees for the purpose of this study (see Figure 1).For each plot, parameters reflecting initial stand structure and
composition were computed, including basal area proportion of individual
species, dominant stand height, total basal area, and total number of stems. The
Gini coefficient (GC) describing stand diameter diversity was also computed [20].
GC was found superior with respect to discriminating ability and capability of
providing logical ranking of different distributions [20]. Parameters
reflecting contemporary dynamics of the stands (growth, mortality, and recruitment) were
computed in terms of basal area (dm2ha−1yr−1)
for two distinct periods corresponding to the first two (1970–1975 and 1978–1980) and the
last two measurements (1990–1992 and 1998–2001). For a
given period, mortality and net recruitment were evaluated by dividing the
total basal area of dead or new stems by the time between surveys and the total
plot area sampled, respectively. Growth of living trees was also assessed in
terms of basal area. Finally, net growth (growth of living trees + net recruitment − mortality) was estimated by the difference in
the total basal area of living trees between successive surveys divided by the
time interval between them and the total sampled plot area [19].Stand age was estimated by averaging the ring count of sampled trees. Stand
age was categorized into three classes containing a more or less equivalent
number of plots; the upper limit of the first and the second classes being the
33.3% and the 66.6% percentile, respectively. Stand age varied from 22 to 59
years, 60 to 100 years, and 101 to 200 years for the first, second, and third
classes, respectively. We could not ensure that the cored trees in the last age
class belonged to the postfire cohort. We could, however, ensure that no major stand
clearing occurred since the recruitment of these trees.For each plot, normal mean annual temperature and total annual
precipitation were computed over the 1977–2001 period. Mean
seasonal (winter: Jan., Feb., Mar.; spring: Apr., May, Jun.; summer: Jul., Aug.,
Sept.; autumn: Oct., Nov., Dec.) temperature and total precipitation were also
computed over the two distinct periods studied. Soil surface area (SSA) was
estimated using the following equation [21]: (1)SSA=(0.003×Sand+0.022×Silt+0.08×Clay)×Dens×106,
where SSA is the soil specific surface area (m2m−3), Sand, Silt, and Clay variables are particle fractions (%),
and Dens is mineral soil density (g cm−3). Data for soil
density were not available. Consequently, a mineral soil density of 1 g cm−3 was used for all plots. Soil surface area was chosen as the soil parameter
because it reflects both soil water-holding capacity and soil nutrient
availability, which in part determine site productivity [22]. Temperature,
precipitation, and SSA were categorized into two classes each (cold and warm
sites; wet and dry sites; fine and coarse textured soils (from SSA)) containing
approximately the same number of stands (see Table 1). Cut level classes
were not based on biological considerations. They were set to ensure that each class
contained approximately the same number of plots. Temperature, precipitation,
SSA, and age classes result in 24 categories (2×2×2×3), each containing 6
to 7 plots. The relationship between stand age, air temperature, precipitation,
soil texture and initial stand characteristics, and stand dynamics parameters
(growth, mortality, and recruitment) in both periods were examined with
analyses of variance using the mixed model procedure (PROC MIXED) in SAS [23].
Linear and second-degree polynomial contrasts were built to assess the effect
of stand age, temperature, precipitation, and soil texture on every dependent variable.
Calculations of the linear and quadratic coefficients for unequally spaced time
intervals were made according to Robson [24]. The chronosequence approach, in
which stands of differing ages are compared to provide insight into changes
that occur over time, is a commonly used technique [12, 25]. As a “substitute
of space for time,” the proper use of a chronosequence requires that
differences among plots in terms of edaphic, meteorological, and disturbance history
must be kept to a
minimum. Our approach differed from the commonly used technique for which
stands were generally a priori categorized based on site characteristics (e.g.,
soil type, site index productivity) in order to form a chronosequence. In the approach
we followed, the relationships between stand characteristics or stand dynamics
parameters and stand age, air temperature, precipitation, and soil texture were
statistically tested.Table 1
Mean, standard error and number of plots included in air temperature, precipitation,
and soil texture classes.
Temperature class (°C)Precipitation class (mm)Soil texture class (m2m−3)Cold sitesWarm sitesDry sitesWet sitesCoarse textureFine textureMean−2.7−1.3883984608616110Standard error0.070.116.94.32852800n747374737473Comparison of the results obtained from the
analysis of initial stand characteristics and from the contemporary stand
dynamics parameters should provide insights on the change in forest dynamics
occurring over the mid term (last 100 years), while a comparison of results of
the contemporary dynamics between the two measurement periods should provide
insights into changes in forest dynamics over the short term (last 30 years). In
order to assess differences in climate conditions over the two periods studied,
mean seasonal temperature and precipitation were compared with a Wilcoxon
signed-rank test [26].


## 3. Results

### 3.1. Stand Age, Temperature, Precipitation, and Soil Texture Class Distribution

Stand age and soil texture classes were distributed throughout the study
area with no specific regional pattern (see Figure2). Conversely, temperature
and precipitation regimes showed distinct patterns over the territory. Indeed,
most plots in the western part of the territory were among the warmer temperature
and the lower precipitation classes, while most plots from the eastern sector
exhibited a lower temperature and a higher precipitation regime. Some plots were
exceptions within those regions.Distribution of stand age, soil texture, temperature, and precipitation
classes throughout the study area.
(a)
Age class(b)
Soil texture class(c)
Temperature class(d)
Precipition class


### 3.2. Stand Composition

There was a significant quadratic increasing trend in the proportion ofP.
mariana with stand age class (P=.025; see Figure 3). On average, P.
mariana represented 45.8% of stand basal area of stands in the 40-year age class,
while the proportion reached 88.2% for the 120-year age class. On average, the
proportion of P. mariana was 9.6% greater on fine textured soils as
compared to coarse textured soils, but the difference was only marginally
significant (see Figure 3; P=.054). The proportion of Abies
balsamea also increased with stand age, but the relation was linear (P=.010). On average, A. balsamea proportions increased from 1.6% to 9.0% when the stand age class passed from 40
to 120 years, respectively (see Figure 3). There was no difference in the
proportion of this species among soil texture classes (P=.754). The proportion of A. balsamea was greater on sites
receiving more precipitation (average 9.6%) and sites subjected to colder
temperatures (average 10.6%) than on drier (average 3.6%) and warmer sites (average
2.6%), respectively (P≤.011). However, no significant interactions between
temperature and precipitation regimes were detected. Contrary to P. mariana and A. balsamea, the proportion of Pinus banksiana decreased in a
quadratic fashion as stand age class increased (P≤.001). On average, P.
banksiana represented 46.1% of stand basal area in stands in the 40-year
age class, while this proportion dropped to 0.5% for stands in the 120-year age
class. A significant interaction with soil texture and precipitation regime was
detected (P=.028). On coarse-textured soils, proportions of P.
banksiana were greater on sites receiving lower amounts of precipitation (mean
proportion 30.9%) as compared to sites receiving higher amounts (mean
proportion 10.6%); however, this difference was not perceptible on fine-textured
soils (see Figure 3). Contrary to other species, the proportion of Betula papyrifera did not vary significantly with stand age. The proportion of this species was, however,
associated with the precipitation regime (P=.019), and this relationship
differed slightly between the two soil texture classes (see Figure 3; P=.054). On coarse-textured soils, the proportion of B. papyrifera was
estimated at 0.7% for sites receiving less precipitation, as compared to 11.0%
for stands receiving higher amounts. On average, fine-textured soils had a
lower proportion of B. papyrifera than did coarse-textured soils, but
the effect of the precipitation regime was not perceptible on the fine-textured
soils.Relationships between stand composition and stand age, air temperature, precipitation,
and soil texture. Data from the first inventory (1970–1975).


### 3.3. Stand Structure

Stand basal area, density, height, and GC increased with stand age class
(P≤.001). The relationships were linear, with the exception of GC, which
exhibited a quadratic trend. On average, stand basal area, density, and height were
greater on warmer sites (15.1 m2ha−1, 961 stems ha−1,
14.8 m) than on colder sites (10.3 m2ha−1, 578 stems ha−1, 13.6 m) (see Figure 4; P≤.002). The trend of increasing basal
area and density with stand age class also differed slightly between precipitation
regimes (P≤.056). On sites receiving higher levels of precipitation,
basal area and density increased from the first to the second age class, and
then decreased for the 120-year age class. In contrast, on sites receiving less
precipitation, basal area increased linearly with stand age class. Finally,
stands in the warmer temperature class were less structurally diversified than those
in the colder temperature class, as expressed by the lower values of GC for the
warmer sites (P=.005).Relationships between stand structure and stand age, air temperature, precipitation,
and soil texture. Data from the first inventory (1970–1975).


### 3.4. Stand Dynamics

Growth of living trees, recruitment, and net growth decreased as
mortality increased linearly with stand age class (see Figure5, P≤.0004).
Trends were linear, with the exception of recruitment, which exhibited a
quadratic trend. On average, growth of living trees was greater on warmer (11.1 dm2ha−1yr−1) than on colder sites (8.0 dm2ha−1yr−1) (P=.003; see Figure 5). Growth of
living trees was also greater on sites receiving higher precipitation (11.2 dm2ha−1yr−1) than on drier sites (8.0 dm2ha−1yr−1) (P=.005). Recruitment was also greater on sites
receiving higher amounts of precipitation (7.8 dm2ha−1yr−1) than on sites receiving lower amounts (5.9 dm2ha−1yr−1) (P=.013). A significant interaction was detected with
temperature, precipitation, and age class for recruitment (P=.005). On
sites receiving lower precipitation, recruitment was more or less stable over the
various age classes. In contrast, on sites receiving greater precipitation,
warmer sites exhibited a sharp decrease from the 40- (16.0 dm2ha−1yr−1) to the 80- (4.0 dm2ha−1yr−1)
year age class, while on colder sites recruitment was roughly stable for the
first two age classes, and decreased at the 120-year age class (5.4 dm2ha−1yr−1). On average, mortality was also higher on warmer
sites (23.5 dm2ha−1yr−1) than on colder
sites (16.4 dm2ha−1yr−1) (P=.027; see
Figure 6), but there was no interaction with stand age class. Net growth
generally declined with stand age, but trends differed both between temperature
regimes and soil texture classes (P≤.029). These negative trends observed
with increasing stand age class were more pronounced on warmer than on colder sites,
and on fine-textured than on coarse-textured soils.Relationships between parameters reflecting contemporary dynamics
and stand age, air temperature, precipitation, and soil texture. Data
from the two measurement periods (1970–1975 to 1978–1980 and 1990–1992
to 1998–2001).Tree growth and stand recruitment, mortality and net growth during two time
periods: (1) 1970–1975 to 1978–1980 and (2) 1990–1992 to 1998–2001.The only differences observed between the two periods (1970–1975 to 1978–1980 and 1990–1992 to 1998–2001) were the
following: growth of living trees and stand net recruitment increased by 68%
and 40% from the first to the second period studied, respectively (P≤.003;
see Figure 6), regardless of stand age, temperature, precipitation, or soil texture
classes; mortality was also higher during the second period (22.8 dm2ha−1yr−1) than the first one (17.0 dm2ha−1yr−1), but the difference was marginally significant (P=.073). Consequently, net growth of the studied stands was on average negative
(−3.4±2.3dm2ha−1y−1), with no significant
differences between the first and the second periods (P=.681).


### 3.5. Climate Variations

Mean seasonal
temperature and total precipitation for the two periods studied are shown in Table2. On average, mean temperatures in summer and autumn were higher during the
second period as compared to the first period studied (P≤.001).
Marginally significant differences were also observed in spring temperatures,
which were higher by 0.3°C during the second period as compared to the first
period (P=.052). The difference was more pronounced in autumn, during which
temperature was 1.3°C higher during the second period as compared to the first.
Concurrently, total winter and spring precipitation was 11.1% and 12.2% lower, respectively,
while summer and autumn precipitation was 9.5% and 3.2% higher during the second
period, respectively, as compared to the first. Snow precipitation was also lower
during the second period.Table 2
Comparison of seasonal temperature, precipitation, and cumulative degree days
during the two periods studied. Note: n.e. = not estimated.
ParametersTime period 1Time period 2Change (%)P-valueTemperature (°C)Winter−17.5±0.09−17.8±0.11−1.7P=.077Spring4.9±0.075.2±0.085.9P=.052Summer11.5±0.0712.1±0.075.3P≤.001Autumn−8.3±0.11−7.0±0.1015.6P≤.001Total precipitation (mm)Winter162 ± 2.1144 ± 1.3−11.1P≤.001Spring254 ± 2.3223 ± 2.3−12.2P≤.001Summer334 ± 3.1366 ± 2.29.5P≤.001Autumn212 ± 2.2219 ± 1.83.2P=.018Snow precipitation (mm)Winter150 ± 2.1129 ± 1.2−14.2P≤.001Spring35 ± 0.726 ± 0.4−26.3P≤.001Summern.e.n.e.n.e.n.e.Autumn118 ± 2.1104 ± 1.0−11.9P≤.001


## 3.1. Stand Age, Temperature, Precipitation, and Soil Texture Class Distribution

Stand age and soil texture classes were distributed throughout the study
area with no specific regional pattern (see Figure2). Conversely, temperature
and precipitation regimes showed distinct patterns over the territory. Indeed,
most plots in the western part of the territory were among the warmer temperature
and the lower precipitation classes, while most plots from the eastern sector
exhibited a lower temperature and a higher precipitation regime. Some plots were
exceptions within those regions.Distribution of stand age, soil texture, temperature, and precipitation
classes throughout the study area.
(a)
Age class(b)
Soil texture class(c)
Temperature class(d)
Precipition class


## 3.2. Stand Composition

There was a significant quadratic increasing trend in the proportion ofP.
mariana with stand age class (P=.025; see Figure 3). On average, P.
mariana represented 45.8% of stand basal area of stands in the 40-year age class,
while the proportion reached 88.2% for the 120-year age class. On average, the
proportion of P. mariana was 9.6% greater on fine textured soils as
compared to coarse textured soils, but the difference was only marginally
significant (see Figure 3; P=.054). The proportion of Abies
balsamea also increased with stand age, but the relation was linear (P=.010). On average, A. balsamea proportions increased from 1.6% to 9.0% when the stand age class passed from 40
to 120 years, respectively (see Figure 3). There was no difference in the
proportion of this species among soil texture classes (P=.754). The proportion of A. balsamea was greater on sites
receiving more precipitation (average 9.6%) and sites subjected to colder
temperatures (average 10.6%) than on drier (average 3.6%) and warmer sites (average
2.6%), respectively (P≤.011). However, no significant interactions between
temperature and precipitation regimes were detected. Contrary to P. mariana and A. balsamea, the proportion of Pinus banksiana decreased in a
quadratic fashion as stand age class increased (P≤.001). On average, P.
banksiana represented 46.1% of stand basal area in stands in the 40-year
age class, while this proportion dropped to 0.5% for stands in the 120-year age
class. A significant interaction with soil texture and precipitation regime was
detected (P=.028). On coarse-textured soils, proportions of P.
banksiana were greater on sites receiving lower amounts of precipitation (mean
proportion 30.9%) as compared to sites receiving higher amounts (mean
proportion 10.6%); however, this difference was not perceptible on fine-textured
soils (see Figure 3). Contrary to other species, the proportion of Betula papyrifera did not vary significantly with stand age. The proportion of this species was, however,
associated with the precipitation regime (P=.019), and this relationship
differed slightly between the two soil texture classes (see Figure 3; P=.054). On coarse-textured soils, the proportion of B. papyrifera was
estimated at 0.7% for sites receiving less precipitation, as compared to 11.0%
for stands receiving higher amounts. On average, fine-textured soils had a
lower proportion of B. papyrifera than did coarse-textured soils, but
the effect of the precipitation regime was not perceptible on the fine-textured
soils.Relationships between stand composition and stand age, air temperature, precipitation,
and soil texture. Data from the first inventory (1970–1975).


## 3.3. Stand Structure

Stand basal area, density, height, and GC increased with stand age class
(P≤.001). The relationships were linear, with the exception of GC, which
exhibited a quadratic trend. On average, stand basal area, density, and height were
greater on warmer sites (15.1 m2ha−1, 961 stems ha−1,
14.8 m) than on colder sites (10.3 m2ha−1, 578 stems ha−1, 13.6 m) (see Figure 4; P≤.002). The trend of increasing basal
area and density with stand age class also differed slightly between precipitation
regimes (P≤.056). On sites receiving higher levels of precipitation,
basal area and density increased from the first to the second age class, and
then decreased for the 120-year age class. In contrast, on sites receiving less
precipitation, basal area increased linearly with stand age class. Finally,
stands in the warmer temperature class were less structurally diversified than those
in the colder temperature class, as expressed by the lower values of GC for the
warmer sites (P=.005).Relationships between stand structure and stand age, air temperature, precipitation,
and soil texture. Data from the first inventory (1970–1975).


## 3.4. Stand Dynamics

Growth of living trees, recruitment, and net growth decreased as
mortality increased linearly with stand age class (see Figure5, P≤.0004).
Trends were linear, with the exception of recruitment, which exhibited a
quadratic trend. On average, growth of living trees was greater on warmer (11.1 dm2ha−1yr−1) than on colder sites (8.0 dm2ha−1yr−1) (P=.003; see Figure 5). Growth of
living trees was also greater on sites receiving higher precipitation (11.2 dm2ha−1yr−1) than on drier sites (8.0 dm2ha−1yr−1) (P=.005). Recruitment was also greater on sites
receiving higher amounts of precipitation (7.8 dm2ha−1yr−1) than on sites receiving lower amounts (5.9 dm2ha−1yr−1) (P=.013). A significant interaction was detected with
temperature, precipitation, and age class for recruitment (P=.005). On
sites receiving lower precipitation, recruitment was more or less stable over the
various age classes. In contrast, on sites receiving greater precipitation,
warmer sites exhibited a sharp decrease from the 40- (16.0 dm2ha−1yr−1) to the 80- (4.0 dm2ha−1yr−1)
year age class, while on colder sites recruitment was roughly stable for the
first two age classes, and decreased at the 120-year age class (5.4 dm2ha−1yr−1). On average, mortality was also higher on warmer
sites (23.5 dm2ha−1yr−1) than on colder
sites (16.4 dm2ha−1yr−1) (P=.027; see
Figure 6), but there was no interaction with stand age class. Net growth
generally declined with stand age, but trends differed both between temperature
regimes and soil texture classes (P≤.029). These negative trends observed
with increasing stand age class were more pronounced on warmer than on colder sites,
and on fine-textured than on coarse-textured soils.Relationships between parameters reflecting contemporary dynamics
and stand age, air temperature, precipitation, and soil texture. Data
from the two measurement periods (1970–1975 to 1978–1980 and 1990–1992
to 1998–2001).Tree growth and stand recruitment, mortality and net growth during two time
periods: (1) 1970–1975 to 1978–1980 and (2) 1990–1992 to 1998–2001.The only differences observed between the two periods (1970–1975 to 1978–1980 and 1990–1992 to 1998–2001) were the
following: growth of living trees and stand net recruitment increased by 68%
and 40% from the first to the second period studied, respectively (P≤.003;
see Figure 6), regardless of stand age, temperature, precipitation, or soil texture
classes; mortality was also higher during the second period (22.8 dm2ha−1yr−1) than the first one (17.0 dm2ha−1yr−1), but the difference was marginally significant (P=.073). Consequently, net growth of the studied stands was on average negative
(−3.4±2.3dm2ha−1y−1), with no significant
differences between the first and the second periods (P=.681).


## 3.5. Climate Variations

Mean seasonal
temperature and total precipitation for the two periods studied are shown in Table2. On average, mean temperatures in summer and autumn were higher during the
second period as compared to the first period studied (P≤.001).
Marginally significant differences were also observed in spring temperatures,
which were higher by 0.3°C during the second period as compared to the first
period (P=.052). The difference was more pronounced in autumn, during which
temperature was 1.3°C higher during the second period as compared to the first.
Concurrently, total winter and spring precipitation was 11.1% and 12.2% lower, respectively,
while summer and autumn precipitation was 9.5% and 3.2% higher during the second
period, respectively, as compared to the first. Snow precipitation was also lower
during the second period.Table 2
Comparison of seasonal temperature, precipitation, and cumulative degree days
during the two periods studied. Note: n.e. = not estimated.
ParametersTime period 1Time period 2Change (%)P-valueTemperature (°C)Winter−17.5±0.09−17.8±0.11−1.7P=.077Spring4.9±0.075.2±0.085.9P=.052Summer11.5±0.0712.1±0.075.3P≤.001Autumn−8.3±0.11−7.0±0.1015.6P≤.001Total precipitation (mm)Winter162 ± 2.1144 ± 1.3−11.1P≤.001Spring254 ± 2.3223 ± 2.3−12.2P≤.001Summer334 ± 3.1366 ± 2.29.5P≤.001Autumn212 ± 2.2219 ± 1.83.2P=.018Snow precipitation (mm)Winter150 ± 2.1129 ± 1.2−14.2P≤.001Spring35 ± 0.726 ± 0.4−26.3P≤.001Summern.e.n.e.n.e.n.e.Autumn118 ± 2.1104 ± 1.0−11.9P≤.001


## 4. Discussion

### 4.1. Stand Composition

In the northern boreal forest, postfire forest composition is characterized by
even-aged pure or mixed stands dominated by pioneer species. At
maturity, this first cohort of trees is replaced by a second one dominated by
shade-tolerant conifers [8]. In old-growth stands, the dynamics driven by gaps
or insect epidemics permit
the self-maintenance of heterogeneous uneven-aged stands of conifers [27]. In the
present study, P. mariana and A. balsamea represented on average 88.2%
and 9.0% of total basal area in older stands, respectively, while they represented
only 45.8% and 1.6% in the younger stands, respectively. An inverse trend was
observed for the most abundant pioneer species, namely, Pinus banksiana (see
Figure 3). The observed decrease in the proportion of pioneer species over time
may be related to canopy succession from postfire pioneer species towards
dominance of P. mariana and, to a lesser extent, A. balsamea.It has been suggested that postfire species composition and dynamics may
differ among soil types [11–13]. In our study, we also noted that stand composition
was associated with soil texture and also with temperature and precipitation regimes.
In the two younger age class stands, the proportion of P. mariana was greater
on fine-textured than on coarse-textured soils (see Figure 3), while P.
banksiana was particularly favored on coarse-textured soils receiving less precipitation.
Seedbed type is an important factor affecting seed germination and seedling
development. Germination and survival of P. banksiana is generally favored
by mineral seedbeds and hindered by organic ones, while the opposite is observed
for P. mariana [28, 29]. Among P. mariana companion species, A.
balsamea was more abundant on colder sites or sites receiving higher amounts
of precipitation, while B. papyrifera was more abundant on coarse-textured
sites receiving higher precipitation (see Figure 3). Germination and early
survival of B. papyrifera are often best on
mineral soils; however, the small seeds of this species are also sensitive to
soil moisture [30]. It has been demonstrated that excessively drained soils,
insufficient rainfall, and unshaded and undisturbed seedbeds deterred its establishment
[30]. Meanwhile, A.
balsamea has the ability to be reproduced by layering in swamps and mossy areas [31]. Our
findings regarding tree species composition in the northern boreal forest are in
accordance with the reproductive ecology of these species.


### 4.2. Stand Structure

Both stand structure and structural changes over time were related to
temperature and precipitation regimes. It is well known that air temperature
controls the rate of plant metabolism, which in turn determines the amount of
photosynthesis that can take place [32]. Temperature also influences forest
soil and site productivity through the decomposition rate of organic matter [33].
The depth of the surface organic layer in P. mariana forests, which determines
seedbed quality, is also influenced by fire frequency and severity, which are
closely associated with the air temperature and precipitation regime [34]. On
average, stand basal area, density, and height were positively related to the
annual air temperature, while stand structural diversity was negatively related.
Air temperature regime appears to have mainly influenced site stocking without,
however, greatly influencing structural development trends over time. Stand
density is mainly determined by regeneration conditions, including seed input
and seedbed quality, soon after fire [35]. Also, the percentage of filled seeds
and germination of P. mariana are associated with the regional variation
in air temperature [35].In opposition to the air temperature regime, site stocking capacity was
not related to the precipitation regime, but structural development over time
was. On sites receiving high amounts of precipitation, basal area and density
increased from the first to the second age class, and then decreased for the
120-year age class, while basal area increased linearly with stand age on sites
receiving low amounts; the maximum are reaching similar values for the 120-year age class in both
precipitation classes (see Figure4). It has been documented that when cohort
stands are not destroyed by fire or other catastrophic disturbances, they
continue to grow past peak maturity into a senescing or lowered vigour state [36, 37]. From a combination of fire
reconstruction maps and an ecological forest data base, Harper et al. [11] documented
the stand-level structure along a postfire chronosequence at the landscape
scale in the Abitibi region of Quebec, Canada. They observed that stand height
and cover reached their maximum between 75–150 and 50–100 years following
fire, respectively. Following this maximum, stand cover and height were reduced
during the understory reinitiating phase, to reach a relative equilibrium state
at the old-growth stage [12]. Furthermore, the rate of succession has been
found to be greater on fine-textured soils, on soils showing higher nitrogen
and moisture content, and in stands with higher productivity [12, 25, 38]. This
greater dynamics may be explained by a higher growth rate for richer stands
that likely induces earlier senescence [25]. Our results also revealed that the
stand breakpoint, in terms of basal area and stand density, occurred sooner on
sites receiving higher amounts of precipitation than on sites receiving less
precipitation, without distinguishing between soil texture classes (see Figure 4).


### 4.3. Stand Dynamics

Stand dynamics analysis also revealed the influence of air temperature
on the growth of living trees, and on stand mortality, recruitment, and net
growth (see Figure5). Overall, growth conditions appeared to be favored by a warmer
temperature regime. The positive effect of air temperature regime on tree
growth appears to be translated into a more abrupt
senescing phase at a time when the pioneer cohort reaches maturity earlier on
warmer sites than on colder ones. Over the last 30 years, negative net growth
was clearly observed in older stands on sites in the warmer air temperature
class. Contrary to results from the analysis of stand structure and composition
(see Figure 4), and in agreement with Harper  et al. [12], the analysis of the
remeasured permanent plots revealed a significant relationship between soil
texture and the timing of senescence (see Figure 5), apparently more rapid on fine-textured
than on coarse-textured soils.


### 4.4. Change in Stand Dynamics over Time

It has been reported that the shift in composition related to stand
dynamics occurs generally sooner on more fertile sites because of faster growth
and subsequent mortality of pioneer species [38]. Consequently, the duration of
the transition phase is reduced on such sites. Improved tree growth over time
may have reduced the stand transition period as compared to historical
dynamics. The analysis of initial stand characteristics revealed that stand
basal area increased with stand age, and rapidly reached a plateau, but on
average, no decline in basal area was observed for the 120-year-old stands (see
Figure 4). These observations suggest that most of these stands had not yet
peaked at this age class in terms of basal area. However, the contemporary
dynamics observed from remeasurement data revealed a different portrait. Negative
net growth was observed for the oldest age class (see Figure 5). In the
meantime, the growth of living trees and stand recruitment increased by 68% and
40%, respectively, from the first to the second period studied, independent of
stand age, soil texture, air temperature, or precipitation regime (see Figure 6).
These observations suggest that the shift in composition and the transition
phase occurring in boreal forest ecosystems has accelerated under new
environmental conditions in the last 30 years, as compared to the dynamics of
the boreal forest in the last 100 years.Generally, variation in tree growth and
recruitment over time can be a result of changes in the intensity or frequency
of partial perturbations such as insect defoliation, partial windfall, or
partial burn. In the boreal coniferous forests of Canada, defoliation caused by spruce
budworm (Choristoneura fumiferana [Clem.]) outbreaks has been one
of the major natural disturbances leading to important tree mortality in balsam
fir and spruce stands [39]. During the last spruce budworm
outbreak (1975–1985) in Southern
Quebec, tree growth and recruitment in balsam fir and black spruce stands were
23% and 51% lower, respectively, and mortality was 2.6 times higher than in the
postoutbreak period [19]. Following the outbreak, mortality decreased and tree
growth and recruitment increased. However, the present analyses do not indicate
similar trends, because tree growth, recruitment, and mortality increased in
synchronicity during the second period as compared to the first one.
Differences between periods were greater for tree growth and recruitment than
for mortality, while perturbations by insect outbreaks should affect primarily tree
mortality, which is followed by an increase in growth of remnant trees and in recruitment.
Moreover, among the plots studied, there was only one record of this
perturbation in the survey plots. Partial windfall and partial burn were also rarely
observed in the survey plots. Consequently, perturbation regimes do not appear
to explain the observed trends over time.In the
present study, growth of living trees was 39% higher in the warmer site class
(mean temperature −1.3°C) as compared to the colder site class (mean
temperature −2.7°C). Growth of living trees was also 40% higher on sites
receiving higher precipitation (mean precipitation 984 mm) as compared to drier
sites (mean precipitation 883 mm). Air temperature was considerably
higher during the second period (mean temperature −1.9°C) as compared to the
first one (mean temperature −2.4°C) (see Table 2). Summer precipitation
and autumn precipitation (total precipitation 585 mm) were also higher during
the second period than in the first one (total precipitation 546 mm). Since
there were differences in growth among temperature and precipitation classes, differences
in climatic conditions among the two time periods considered may explain the
improved tree growth over time. At northern latitudes of the boreal forest in Quebec,
it has been suggested that stem height growth has increased in response to the
recent climate warming [40–43]. Snow precipitation was also lower in
the second period as compared to the first period studied. At alpine
timberline, it has
been suggested that wind-driven snow and ice particles at the snow-air
interface cause needle loss and the death of branches [44, 45], while cuticle
erosion leads to desiccation or even needle loss and death [46, 47]. Such
mechanisms may also play a role in the trend observed in the continuous forests
studied herein. These observations suggest that
climatic conditions were more favorable in the second study period than in the
first one, and that climate was probably one of the main drivers for the improvement
in tree growth from the first to the second period. Consequently, comparison of the results from the initial stand
structure, the contemporary dynamics (growth, recruitment, and mortality), and
the two periods considered suggests that changes in climatic conditions
contributed to accelerate the successional dynamics of these ecosystems through
their favorable effect on tree growth.A direct
effect of climate change on longevity of pioneer species is essential to drive
the accelerated successional response herein proposed. For many species, tree
longevity is positively correlated to wood density and negatively correlated
with growth rates [48]. For example, pioneer species exhibit rapid growth and
short longevity, while subcanopy species have slow growth and high longevity. Although
mechanisms behind those relations are not fully understood, it has been
suggested that slow growth of the trees results in a much greater mechanical
strength in the wood that minimizes maintenance and repair costs while maximizing
durability and strength [49].


## 4.1. Stand Composition

In the northern boreal forest, postfire forest composition is characterized by
even-aged pure or mixed stands dominated by pioneer species. At
maturity, this first cohort of trees is replaced by a second one dominated by
shade-tolerant conifers [8]. In old-growth stands, the dynamics driven by gaps
or insect epidemics permit
the self-maintenance of heterogeneous uneven-aged stands of conifers [27]. In the
present study, P. mariana and A. balsamea represented on average 88.2%
and 9.0% of total basal area in older stands, respectively, while they represented
only 45.8% and 1.6% in the younger stands, respectively. An inverse trend was
observed for the most abundant pioneer species, namely, Pinus banksiana (see
Figure 3). The observed decrease in the proportion of pioneer species over time
may be related to canopy succession from postfire pioneer species towards
dominance of P. mariana and, to a lesser extent, A. balsamea.It has been suggested that postfire species composition and dynamics may
differ among soil types [11–13]. In our study, we also noted that stand composition
was associated with soil texture and also with temperature and precipitation regimes.
In the two younger age class stands, the proportion of P. mariana was greater
on fine-textured than on coarse-textured soils (see Figure 3), while P.
banksiana was particularly favored on coarse-textured soils receiving less precipitation.
Seedbed type is an important factor affecting seed germination and seedling
development. Germination and survival of P. banksiana is generally favored
by mineral seedbeds and hindered by organic ones, while the opposite is observed
for P. mariana [28, 29]. Among P. mariana companion species, A.
balsamea was more abundant on colder sites or sites receiving higher amounts
of precipitation, while B. papyrifera was more abundant on coarse-textured
sites receiving higher precipitation (see Figure 3). Germination and early
survival of B. papyrifera are often best on
mineral soils; however, the small seeds of this species are also sensitive to
soil moisture [30]. It has been demonstrated that excessively drained soils,
insufficient rainfall, and unshaded and undisturbed seedbeds deterred its establishment
[30]. Meanwhile, A.
balsamea has the ability to be reproduced by layering in swamps and mossy areas [31]. Our
findings regarding tree species composition in the northern boreal forest are in
accordance with the reproductive ecology of these species.


## 4.2. Stand Structure

Both stand structure and structural changes over time were related to
temperature and precipitation regimes. It is well known that air temperature
controls the rate of plant metabolism, which in turn determines the amount of
photosynthesis that can take place [32]. Temperature also influences forest
soil and site productivity through the decomposition rate of organic matter [33].
The depth of the surface organic layer in P. mariana forests, which determines
seedbed quality, is also influenced by fire frequency and severity, which are
closely associated with the air temperature and precipitation regime [34]. On
average, stand basal area, density, and height were positively related to the
annual air temperature, while stand structural diversity was negatively related.
Air temperature regime appears to have mainly influenced site stocking without,
however, greatly influencing structural development trends over time. Stand
density is mainly determined by regeneration conditions, including seed input
and seedbed quality, soon after fire [35]. Also, the percentage of filled seeds
and germination of P. mariana are associated with the regional variation
in air temperature [35].In opposition to the air temperature regime, site stocking capacity was
not related to the precipitation regime, but structural development over time
was. On sites receiving high amounts of precipitation, basal area and density
increased from the first to the second age class, and then decreased for the
120-year age class, while basal area increased linearly with stand age on sites
receiving low amounts; the maximum are reaching similar values for the 120-year age class in both
precipitation classes (see Figure4). It has been documented that when cohort
stands are not destroyed by fire or other catastrophic disturbances, they
continue to grow past peak maturity into a senescing or lowered vigour state [36, 37]. From a combination of fire
reconstruction maps and an ecological forest data base, Harper et al. [11] documented
the stand-level structure along a postfire chronosequence at the landscape
scale in the Abitibi region of Quebec, Canada. They observed that stand height
and cover reached their maximum between 75–150 and 50–100 years following
fire, respectively. Following this maximum, stand cover and height were reduced
during the understory reinitiating phase, to reach a relative equilibrium state
at the old-growth stage [12]. Furthermore, the rate of succession has been
found to be greater on fine-textured soils, on soils showing higher nitrogen
and moisture content, and in stands with higher productivity [12, 25, 38]. This
greater dynamics may be explained by a higher growth rate for richer stands
that likely induces earlier senescence [25]. Our results also revealed that the
stand breakpoint, in terms of basal area and stand density, occurred sooner on
sites receiving higher amounts of precipitation than on sites receiving less
precipitation, without distinguishing between soil texture classes (see Figure 4).


## 4.3. Stand Dynamics

Stand dynamics analysis also revealed the influence of air temperature
on the growth of living trees, and on stand mortality, recruitment, and net
growth (see Figure5). Overall, growth conditions appeared to be favored by a warmer
temperature regime. The positive effect of air temperature regime on tree
growth appears to be translated into a more abrupt
senescing phase at a time when the pioneer cohort reaches maturity earlier on
warmer sites than on colder ones. Over the last 30 years, negative net growth
was clearly observed in older stands on sites in the warmer air temperature
class. Contrary to results from the analysis of stand structure and composition
(see Figure 4), and in agreement with Harper  et al. [12], the analysis of the
remeasured permanent plots revealed a significant relationship between soil
texture and the timing of senescence (see Figure 5), apparently more rapid on fine-textured
than on coarse-textured soils.


## 4.4. Change in Stand Dynamics over Time

It has been reported that the shift in composition related to stand
dynamics occurs generally sooner on more fertile sites because of faster growth
and subsequent mortality of pioneer species [38]. Consequently, the duration of
the transition phase is reduced on such sites. Improved tree growth over time
may have reduced the stand transition period as compared to historical
dynamics. The analysis of initial stand characteristics revealed that stand
basal area increased with stand age, and rapidly reached a plateau, but on
average, no decline in basal area was observed for the 120-year-old stands (see
Figure 4). These observations suggest that most of these stands had not yet
peaked at this age class in terms of basal area. However, the contemporary
dynamics observed from remeasurement data revealed a different portrait. Negative
net growth was observed for the oldest age class (see Figure 5). In the
meantime, the growth of living trees and stand recruitment increased by 68% and
40%, respectively, from the first to the second period studied, independent of
stand age, soil texture, air temperature, or precipitation regime (see Figure 6).
These observations suggest that the shift in composition and the transition
phase occurring in boreal forest ecosystems has accelerated under new
environmental conditions in the last 30 years, as compared to the dynamics of
the boreal forest in the last 100 years.Generally, variation in tree growth and
recruitment over time can be a result of changes in the intensity or frequency
of partial perturbations such as insect defoliation, partial windfall, or
partial burn. In the boreal coniferous forests of Canada, defoliation caused by spruce
budworm (Choristoneura fumiferana [Clem.]) outbreaks has been one
of the major natural disturbances leading to important tree mortality in balsam
fir and spruce stands [39]. During the last spruce budworm
outbreak (1975–1985) in Southern
Quebec, tree growth and recruitment in balsam fir and black spruce stands were
23% and 51% lower, respectively, and mortality was 2.6 times higher than in the
postoutbreak period [19]. Following the outbreak, mortality decreased and tree
growth and recruitment increased. However, the present analyses do not indicate
similar trends, because tree growth, recruitment, and mortality increased in
synchronicity during the second period as compared to the first one.
Differences between periods were greater for tree growth and recruitment than
for mortality, while perturbations by insect outbreaks should affect primarily tree
mortality, which is followed by an increase in growth of remnant trees and in recruitment.
Moreover, among the plots studied, there was only one record of this
perturbation in the survey plots. Partial windfall and partial burn were also rarely
observed in the survey plots. Consequently, perturbation regimes do not appear
to explain the observed trends over time.In the
present study, growth of living trees was 39% higher in the warmer site class
(mean temperature −1.3°C) as compared to the colder site class (mean
temperature −2.7°C). Growth of living trees was also 40% higher on sites
receiving higher precipitation (mean precipitation 984 mm) as compared to drier
sites (mean precipitation 883 mm). Air temperature was considerably
higher during the second period (mean temperature −1.9°C) as compared to the
first one (mean temperature −2.4°C) (see Table 2). Summer precipitation
and autumn precipitation (total precipitation 585 mm) were also higher during
the second period than in the first one (total precipitation 546 mm). Since
there were differences in growth among temperature and precipitation classes, differences
in climatic conditions among the two time periods considered may explain the
improved tree growth over time. At northern latitudes of the boreal forest in Quebec,
it has been suggested that stem height growth has increased in response to the
recent climate warming [40–43]. Snow precipitation was also lower in
the second period as compared to the first period studied. At alpine
timberline, it has
been suggested that wind-driven snow and ice particles at the snow-air
interface cause needle loss and the death of branches [44, 45], while cuticle
erosion leads to desiccation or even needle loss and death [46, 47]. Such
mechanisms may also play a role in the trend observed in the continuous forests
studied herein. These observations suggest that
climatic conditions were more favorable in the second study period than in the
first one, and that climate was probably one of the main drivers for the improvement
in tree growth from the first to the second period. Consequently, comparison of the results from the initial stand
structure, the contemporary dynamics (growth, recruitment, and mortality), and
the two periods considered suggests that changes in climatic conditions
contributed to accelerate the successional dynamics of these ecosystems through
their favorable effect on tree growth.A direct
effect of climate change on longevity of pioneer species is essential to drive
the accelerated successional response herein proposed. For many species, tree
longevity is positively correlated to wood density and negatively correlated
with growth rates [48]. For example, pioneer species exhibit rapid growth and
short longevity, while subcanopy species have slow growth and high longevity. Although
mechanisms behind those relations are not fully understood, it has been
suggested that slow growth of the trees results in a much greater mechanical
strength in the wood that minimizes maintenance and repair costs while maximizing
durability and strength [49].


## 5. Conclusion

Results of this study show that tree species composition of stands is
associated with stand age, soil texture, air temperature, and precipitation
regime. After establishment of the pioneer cohort, temperature and
precipitation were associated with the postsuccessional dynamics probably by
their influence on tree growth. Our results support the hypothesis that shifts
in composition related to stand dynamics, and the subsequent senescing phase,
generally occur sooner and more quickly on more fertile sites due to faster
growth and the subsequent earlier mortality of pioneer species. This study
suggests that climate warming will accelerate the successional dynamics of
these ecosystems through its positive influence on tree growth.



---

*Source: 398389-2009-01-28.xml*