Journal of Biogeography (J. Biogeogr.) (2007)
ORIGINAL
ARTICLE
Rapid expansion of lichen woodlands
within the closed-crown boreal forest
zone over the last 50 years caused by
stand disturbances in eastern Canada
F. Girard1*, S. Payette1 and R. Gagnon2
1
NSERC Northern Research Chair, Centre
d’Études Nordiques and Département de
Biologie, Université Laval, Québec, G1K 7P4,
Canada, and 2Département des Sciences
Fondamentales, 555 Boulevard de l’Université,
Université du Québec à Chicoutimi,
Chicoutimi, Québec, G7H 2B1, Canada.
ABSTRACT
Aim Our two main goals are first to evaluate the resilience of the boreal forest
according to latitude across the closed-crown forest zone using the post-disturbance
distribution and cover of lichen woodlands and closed-crown forests as a metric,
and second to identify the disturbance factors responsible for the regeneration and
degradation of the closed-crown forest according to latitude since the 1950s.
Location The study area extends between 7000¢ and 7200¢ W and throughout
the closed-crown forest zone, from its southern limit near 4730¢ N to its
northern limit at the contact with the lichen woodland zone at around 5240¢ N.
Methods Recent (1972–2002) and old (1954–1956) aerial photos were used to
map the distribution of lichen woodlands across the closed-crown forest zone.
Forest disturbances such as fire, spruce budworm (Choristoneura fumiferana
(Clemens)) outbreak, and logging were recorded on each set of aerial photos.
Each lichen woodland and stand disturbance was validated by air-borne surveys
and digitized using GIS software.
Results Over the last 50 years, the area occupied by lichen woodlands has
increased according to latitude; that is, 9% of the area that was occupied by
closed-crown forests has shifted to lichen woodlands. Although logging activities
have been concentrated in the same areas during the last 50 years, the area
covered by logging has increased significantly. Outbreaks by the spruce budworm
occurred predominantly in the southern (4730¢ N to 4830¢ N) and central
(4853¢ N to 5042¢ N) parts of the study area, where balsam fir stands are
extensive. In the northern part of the study area (51–5240¢ N), extensive fires
affected the distribution and cover of closed-crown forests and lichen woodlands.
*Correspondence: François Girard, NSERC
Northern Research Chair, Centre d’Études
Nordiques and Département de Biologie,
Université Laval, Québec, G1K 7P4, Canada.
E-mail: [email protected]
Main conclusions Over the last 50 years, the area occupied by closed-crown
forests has decreased dramatically, and the ecological conditions that allow
closed-crown forests to establish and develop are currently less prevalent. Fire is
by far the main disturbance, reducing the ability of natural closed-crown forests
to self-regenerate whatever the latitude. Given the current biogeographical shift
from dense to open forests, the northern part of the closed-crown forest zone is in
a process of dramatic change towards the dominance of northern woodlands.
Keywords
Black spruce, boreal forest, closed-crown forest, disturbances, fire, lichen
woodland, logging, resilience, spruce budworm outbreak.
The boreal forest is the largest biome in North America and
extends across the continent from Newfoundland to Alaska.
The eastern part of this biome is divided into three distinct
zones corresponding to the closed-crown forest, the lichen
woodland (or taiga), and the forest-tundra (Rowe, 1972;
Payette et al., 2001). The closed-crown forest zone includes a
ª 2007 The Authors
Journal compilation ª 2007 Blackwell Publishing Ltd
www.blackwellpublishing.com/jbi
doi:10.1111/j.1365-2699.2007.01816.x
INTRODUCTION
1
F. Girard, S. Payette and R. Gagnon
2
Northern treeline
Forest-tundra
Latitude
majority of dense black spruce (Picea mariana (Mill.) B.S.P.),
jack pine (Pinus banksiana Lamb.) and balsam fir (Abies
balsamea (L.) Mill.) stands on well-drained soils. However, the
overall distribution of these stands is not homogenous, as there
are low-density or open forest stands forming lichen woodlands. The distribution of lichen woodlands may be viewed as
the opposite of the distribution of closed-crown forests:
lichen–black spruce stands are distributed predominantly on
well-drained soils, whereas dense black spruce stands grow on
poorly to well-drained soils (Hare, 1959; Hare & Ritchie, 1972;
Rowe, 1984). The northern part of the boreal forest, the foresttundra, is a large ecotone between the lichen woodland zone
and the treeless tundra. The forest-tundra is composed of
lichen woodlands and krummholz (stunted forests), and
treeless tundra-like communities on well-drained sites.
The decrease of the closed-crown forest cover is the result of
a regression process of the boreal forest under the long-lasting
influence of disturbance factors occurring during the Holocene
(Jasinski & Payette, 2005). There are currently three major
disturbances in the eastern North American boreal forest,
namely wildfire, logging, and insect outbreaks. Fire is the main
disturbance factor in several parts of the boreal forest that
inhibits or terminates ecological succession. Post-fire tree
regeneration depends on several factors, including climatic
conditions, seed quality and fire severity (Johnson, 1992;
Payette, 1992; Johnstone & Kasischke, 2005; Jayen et al., 2006).
Fires ignited by natural causes can cover large areas in the
closed-crown forest zone, the taiga zone, and the forest-tundra
(Johnson & Miyanishi, 1999; Payette et al., 2001; Bergeron
et al., 2004).
In the closed-crown forest zone, the distribution of several
lichen woodlands is the consequence of reduced post-fire
regeneration caused by successive disturbances (Payette et al.,
2000) not necessarily associated with limited climatic conditions. In contrast to the case for the taiga zone, both
anthropogenic and natural disturbances occur frequently in
the area occupied by the southernmost lichen woodlands.
Natural disturbances such as insect outbreaks followed by fire
(Payette et al., 2000) or anthropogenic disturbance such as
logging followed by fire (Payette & Delwaide, 2003) cause the
regression of the closed-crown forest. Logging is a new
disturbance in the closed-crown forest zone. Since the end of
the 19th century, the southern boreal forest has been subjected
to logging, particularly in eastern Canada. In the early 1900s,
logging activity in southern Québec was confined to areas
south of 49N, whereas it currently extends to 51N, which
corresponds to a northward displacement of 220 km.
Although the origin and dynamics of lichen woodlands at
their southern range limit in eastern Canada have recently been
documented (Payette et al., 2000; Payette & Delwaide, 2003;
Jasinski & Payette, 2005), there are no data on their
distribution and abundance across the closed-crown forest
zone. Because lichen woodlands have been found to derive
from closed-crown, spruce–moss forests unable to re-establish
after disturbance, an evaluation of the proportion of both of
these forest types across the closed-crown forest zone is
Lichen woodland
Closed-crown forest
0
50
100 %
Figure 1 Potential distribution of lichen woodland (percentage
of the total well-drained soil surface occupied by lichen woodland) across the boreal forest in eastern Canada.
necessary to document the current status and resilience of the
closed-crown forest. In a boreal forest context, the resilience
(sensu Holling, 1973) of the closed-crown forest corresponds
to its ability to self-regenerate after a disturbance without any
change in species composition and structure.
The distribution and abundance of the main forest types
across the closed-crown forest zone may be represented by a
model in which woodlands and forests coexist but in changing
proportion according to latitude (Fig. 1) (see also Timoney
et al., 1993). In the closed-crown forest zone, lichen woodland
stands are sparsely distributed and their abundance increases
to reach a maximum of occupation on well-drained soils in the
taiga zone (Fig. 1). The maximum of the curve corresponds to
a zone of about 2 of latitude in which most (> 95%) welldrained sites are occupied by lichen woodland stands. North of
the area of maximum woodland cover in the taiga zone, tundra
communities expand with latitude across the forest-tundra
zone (Fig. 1 and Payette et al., 2001). Thus the overall
distribution and coverage of the three main vegetation types,
the closed-crown forest, the lichen woodland and the tundra,
according to latitude are associated with a variable regeneration success closely linked with the most frequent disturbance
factors. In this perspective, field surveys and historical data
may be used to determine the degree of resilience of the closedcrown forest.
The main objectives of this study are to evaluate the spatial
distribution of the lichen–spruce woodland and the resilience
of the coniferous forest according to latitude across the closedcrown forest zone. It is hypothesized that the latitudinal
distribution of the lichen–spruce woodland on well-drained
soils follows a unimodal curve from a minimum cover at its
southern limit of distribution to a maximum cover near the
northern limit of the spruce–moss forest. It is also hypothesized that the coniferous forest is more resilient to disturbances
in the southern part than it is in the northern part of the
closed-crown forest zone. To meet our two objectives, we
Journal of Biogeography
ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
Expansion of lichen woodlands by stand disturbances
documented the distribution of the lichen woodland and of the
closed-crown forest as well as the main stand disturbances over
the last 50 years using aerial photos of a large area covering the
latitudinal distribution of the closed-crown forest zone in
central and southern Québec.
STUDY AREA
Lichen woodlands of the closed-crown forest zone are
dominated by black spruce and/or jack pine with a cover
generally less than 40% (Johnson & Miyanishi, 1999; Payette
et al., 2000). The ground layer is dominated by Cladina
rangiferina (L.) Nyl, Cladina stellaris (Opiz) Brodo, Cladina
mitis (Sandst.) Hustich, and several species of the genus
Cladonia. The shrub layer is composed of small shrubs such as
Rhododendron groenlandicum Retz., Kalmia angustifolia L.,
Vaccinium angustifolium and Betula glandulosa Michx. (Morneau & Payette, 1989; Riverin & Gagnon, 1996; Johnson &
Miyanishi, 1999; Payette et al., 2000; Simard & Payette, 2001).
The study area extends between 7000¢ W and 7200¢ W
throughout the closed-crown boreal forest zone from its
southern limit near 4730¢ N to its northern limit at the
contact with the lichen woodland zone around 5240¢ N. The
study area is dominated by black spruce–moss forest stands.
Balsam fir stands with paper birch (Betula papyrifera Marsh.)
and white spruce (Picea glauca (Moench) Voss) and jack pine
stands are also present. Most of the studied forest stands are on
well-drained, podzolic soils developed in glacial and fluvioglacial deposits. For logistical and sampling purposes, the study
area was subdivided into three parts. The southern part is
located 120 km north-east of Québec City (4730¢ N, 70–
72 W) in the Parc des Grands-Jardins (PGJ) (see no. 1 in
Fig. 2) and the Réserve faunique des Laurentides (RFL) (see
no. 2 in Fig. 2). The area is dominated by balsam fir–paper
birch stands and corresponds to the southernmost boreal
forest zone (Bergeron, 1996). The mean altitude ranges
between 600 m and 800 m above sea level (a.s.l.) in the PGJ,
with scattered hills above 950 m a.s.l., whereas the mean
altitude of the RFL is 900 m a.s.l. The mean annual temperature is about 0C and )0.5C in the PGJ and the RFL areas,
respectively (Boisclair, 1990). There is a strong precipitation
gradient from east to west; that is, from the PGJ
(< 800 mm year)1) to the RFL (1500 mm year)1). Balsam fir
and paper birch stands dominate at low and medium
elevations in the RFL area, whereas mixed black spruce and
balsam fir stands predominate at higher elevations. In the PGJ
area, dense black spruce stands are found in wet and mesic
sites, and lichen woodlands in mesic and dry sites. Clearcutting and spruce budworm (Choristoneura fumiferana
(Clemens)) outbreaks are the main disturbances in the RFL
area. During the 20th century, there were three major
outbreaks in the study area: 1914–1919, 1944–1951 and
1975–1985 (Blais, 1983). The spruce budworm outbreaks
recorded during the 20th century extended across the closedcrown forest zone (Morin & Laprise, 1990). In the PGJ area,
fire and spruce budworm outbreaks are the principal distur-
Figure 2 Location of the study area (rectangle) in southern and
central Québec. Vegetation zones according to Payette (1992)
are distributed as follows from south: the mixed forest zone, the
closed-crown forest zone, the lichen woodland zone (taiga), the
forest-tundra zone, the shrub tundra zone, and the herb tundra
zone.
bances, but clear-cutting has been prohibited since the creation
of the PGJ in 1981 (Dussart & Payette, 2002). The middle part
of the study area (50 N, 70–72 W) is located about 400 km
north of Québec City and corresponds to the main core of the
black spruce–moss forest within the closed-crown forest zone.
The mean annual temperature is about 1.1C, and there is
more than 850 mm of precipitation (including 3.50 m of snow
– 350 mm of precipitation) (Environnement Canada, 2003).
The northern part of study area extends to the limit of the
closed-crown forest zone and the beginning of the lichen
woodland zone (5241¢ N, 70–72 W). The mean annual
temperature is about )2C, and annual precipitation totals
800 mm of rain (including 3 m of snow) (Environnement
Canada, 2003).
MATERIALS AND METHODS
The distribution of lichen woodlands and on-site disturbances
(fire, insect epidemics and logging) were evaluated based on a
comparative analysis of aerial photos taken at an interval of
about 50 years. Two sets of aerial photos at a scale of 1/40,000
were considered, namely a recent collection (1972–2002) from
the Québec photo service and an old collection (1954–1956)
from the Canadian photo service. The only aerial photos
Journal of Biogeography
ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
3
F. Girard, S. Payette and R. Gagnon
available north of the logging zone date back to 1972,
representing a 30-year difference from the 2002 survey. Lichen
woodland cover and site disturbances are spatially dynamic, so,
in order to have representative data, air-borne surveys and
field verification were used to update the distribution of lichen
woodlands and site disturbances to the year 2002. A total of 19
transects were used to evaluate the spatial distribution of the
main vegetation types on well-drained soils and site disturbances over the 50-year period, from the southern limit of the
lichen woodland at 4730¢ N to the northern limit of the
closed-crown forest zone near 5130¢ N and the southern part
of the lichen woodland zone (5240¢ N). Each transect was
1 km wide and 140 km long and was located at exactly the
same geographic coordinates on each set of aerial photos. The
first transect was randomly selected at the limit of the southern
lichen woodlands in the PGJ area, and then the subsequent
transects were placed systematically at every 15 min of latitude
between 70 and 72 W.
The boundaries of each lichen woodland were delineated
and validated in each transect so that the total area occupied by
lichen woodlands could be calculated, and an identification of
lichen woodlands was made on each set of aerial photos so that
the difference in cover over the 50-year period could be
evaluated. Aerial photos were corrected and interpreted by the
authors. Then, air-borne surveys with field checks in each of
the 19 transects were carried out for validation of every lichen
woodland identified on the aerial photos according to methods
in Payette et al. (2001). Multiple but small-scale disturbances
were sometimes difficult to detect using photo interpretation
(e.g. spruce budworm outbreak prior to wildfire), given that
fire erases signs of previous disturbances. Only fires occurring
during short intervals are generally detected on aerial photos,
but no such fires were recorded in the study area. Logging
areas were easy to identify on aerial photos because of their
somewhat polygonal shapes and the presence of skid trails.
Burned areas were more extensive than logged areas and
showed uniform grey patterns. The density of dead stems lying
on the ground was used to evaluate stand density prior to fire.
The spatial pattern of a spruce budworm outbreak in a mature
forest resembles the pattern of a Swiss cheese; that is, circular
or crescent-shape hollows within a continuous and homogenous forest cover.
All lichen woodlands identified and validated on the two sets
of aerial photos were digitized. Ground control points were
determined with ground surveys (global positioning system)
and 1/50,000 topographic maps provided by the Québec
Department of Natural Resources. These points were taken on
visible physical features on the landscape (water bodies, etc.).
On the corresponding image, the x, y photo coordinates were
then determined for each corresponding ground control point.
Between 9 and 15 ground control points were established for
each photo. The relationship of the x, y photo coordinated to
the real world and the ground control points were then used to
determine the algorithm to orthorectify the image. Orthorectification of aerial photos was realized using MapInfo
Professional (version 7.5; MapInfo Corporation, Toronto,
4
ON, Canada) with a pixel size of 25 m2. The ratio between the
total lichen woodland (LW) cover and the total forest
(CF + LW) cover was then calculated [LW/(CF + LW)] for
each transect. All stand disturbances (fires, epidemics, logging)
were identified on the two sets of aerial photos and used for
the identification of the origin and evaluation of the dynamics
of the two dominant forest types, i.e. the lichen woodland and
the closed-crown forest.
Mean elevation was calculated using contour lines on forestsurvey maps from the Québec Department of Natural
Resources (Ministère des Ressources Naturelles de la Faune
et des Parcs 2005). The mean elevation of each lichen
woodland stand was determined from 1/50,000 topographic
maps (10-m precision) and then used to calculate the mean
elevation of lichen woodlands in each transect. Vegetation
changes across the landscape from the southern limit to the
northern limit of the closed-crown forest zone were evaluated
over the 50-year window of observation, in terms of an
increase or a decrease of the lichen woodland cover over the
study time interval. All changes in the proportion of lichen
woodland relative to the total forest cover in each transect and
in all the transects were used to calculate the resilience of
the closed-crown forest across the closed-crown forest zone.
The resilience index (RI) of the closed-crown forest is the
proportion of this forest that self-regenerated after stand
disturbances (logging and fires) over the last 50 years. The
resilience index was calculated for each transect along the
gradient of latitude.
The areas covered by lichen woodlands and also by logged,
burned and insect-damaged sites were measured using MapInfo Professional (version 7.5; MapInfo Corporation, 2005).
Then, regression analyses between lichen woodland area,
proportion of lichen woodland in the closed-crown forest
and latitude were performed using the sas/stat statistical
software package (proc reg, SAS Institute 2000) (Devore &
Peck, 1994). The evaluation of the changing vegetation cover
according to the latitudinal gradient was based on the area
covered by lichen woodlands [LW/(CF + LW)] and the total
area occupied by well-drained sites. An analysis of frequency
distribution (Wilcoxon matched pairs test) using statistica
(Statsoft, 1984–2006) was used to compare the closed-crown
forest cover of the 1950s and of 2002 (Devore & Peck, 1994).
This test was also used to compare the areas of logged, burned
and insect-damaged sites of the 1950s and in 2002.
RESULTS
Except for the southernmost transects, the mean altitude of all
study sites increases steadily from south to north (Fig. 3). The
highest altitudes are in the southern part of the study area
(RFL and PGJ), with similar values only in the northern part,
with a mean altitude of 750 m a.s.l. near the Otish Mountains
(5223¢ N) (see no. 9 in Fig. 2). The lowest elevations
correspond to the Lac-St-Jean (see no. 3 in Fig. 2) lowlands
(48 N), with a mean altitude < 350 m a.s.l. The overall
structure of the main cover types (terrestrial vegetation,
Journal of Biogeography
ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
Expansion of lichen woodlands by stand disturbances
(Fig. 4a), whereas the northern part of the study area (from
5053¢ to 5212¢ N) has never been logged. Logging disturbance has changed significantly since the 1950s (Table 1). The
RFL and PGJ areas corresponded to the heart of the logging
zone during the 1950s. At present, the expansion to the north
of logging corresponds approximately to the northern limit of
logged areas during the 1950s. In the 1950s, the middle part
of the study area (Péribonka Reservoir, 50 N) was subjected
to clear-cuts near the Péribonka river, which was used for log
floating over a distance of 250 km. Tree harvesting has
doubled with the introduction of mechanical practices since
1970 (Table 1). The northern limit of clear-cut logging is at
present located near 5053¢ N.
Of the three major spruce budworm outbreaks occurring in
eastern Canada during the last century, the aerial photos taken
between 1950 and 1955 show the immediate impact of the
1944–1951 outbreak (Table 1). The RFL was the most insectaffected area across the closed-crown forest zone. The Monts
Valin area (4923¢ N) (see no. 4 in Fig. 2) was also heavily
damaged in the 1950s. Both the RFL and Monts Valin areas are
largely dominated by balsam fir–paper birch stands, which are
more subject to attacks of the spruce budworm. Balsam fir–
paper birch stands are seldom found in the northern part of
the study area. During the 1950s, however, the forest was
damaged by the spruce budworm even at these northern
latitudes (51 N) (Fig. 4b). The recent aerial photos have not
shown such degrees of disturbance. The RFL region was again
affected by the insect during the 1975–1985 outbreak, with
signs of stand deterioration. The forests that have regenerated
over the last 20 years showed less damage by the spruce
budworm than forests that regenerated during the 1950s.
Fire occurrences were widespread in the 1950s (Fig. 4c).
Even in the humid areas of the PGJ and RFL, fires burned
nearly 20% of the land. Although fires have affected both areas
recently, the current distribution of burned sites was significantly different from that of the 1950s (Table 1). In the logging
zone corresponding to the southern and the middle parts of
the study area (4853¢ N to 5042¢ N), 1950s fires burned
approximately 30% of the forest. Currently, extensive logging
creates large firebreaks, and recently burned areas are practically absent. The northern part of the study area was less
Terrestrial vegetation
Water
Peatland
Mean altitude
Figure 3 Surfaces occupied by water bodies (lakes, reservoirs
and rivers), peatlands and forests in each transect along the
latitudinal gradient. The mean altitude of each lichen woodland
along the latitudinal gradient is also shown.
peatlands and water bodies) is fairly homogenous all along the
latitudinal gradient. Forest vegetation covers nearly 80% of the
surface in most transects. However, in transects 7, 9 and 13 (at
4912¢ N, 4942¢ N and 5042¢ N, respectively), the forest
cover is < 80% because of large hydroelectric reservoirs
(Pipmuacan, Péribonka and Manouane) (see nos 5, 6, 7,
respectively, in Fig. 2). Similarly, the reduced forest cover at
4823¢ N and 5142¢ N results from the presence of two large
lakes, Lac St-Jean and Lac Plétipi (see no. 8 in Fig. 2). The
peatland cover in the study area is fairly homogenous, except
for three areas where the peatlands extend over a much larger
surface at 4842¢ N (Lac St-Jean), 5042¢ N (Manouane
reservoir), and 5112¢ N (Lac Plétipi).
The main disturbances that have influenced the vegetation
cover over the last 50 years varied significantly along the
latitudinal gradient. The southern part of the study area has
been affected by clear-cuts since the end of the 19th century
50
(b)
(a)
(c)
40
%
30
20
10
Figure 4 Surface (percentage) occupied by
(a) logged sites, (b) insect-damaged sites
and (c) burned sites in each transect in
the 1950s and in 2002 across the closedcrown forest zone.
0
48
49
50
Journal of Biogeography
ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
51
52
48
49
50
Latitude
51
52
48
49
50
51
52
2002
1950
5
F. Girard, S. Payette and R. Gagnon
Table 1 Difference in the cover of lichen woodlands, logged areas, insect-outbreak areas and burned areas between the 1950s and 2002.
Variable
Mean
SD
Logged areas 2002
Logged areas 1950
Insect-outbreak areas 2002
Insect-outbreak areas 1950
Burned areas 2002
Burned areas 1950
*LW area 2002
LW area 1950
Proportion LW/CF + LW 2002
Proportion LW/CF + LW 1950
15.492
3.462
3.387
6.686
3.467
17.437
2448.992
1530.975
20.610
12.919
14.956
6.380
5.044
9.779
4.736
10.930
2867.005
2131.242
23.961
17.658
Diff.
SD diff.
T
d.f.
P-value
Logged areas 2002 vs. 1950
14.128
3.711
18
Insect-outbreak areas 2002 vs. 1950
6.971
)2.062
18
Burned areas 2002 vs. 1950
11.041
)5.515
18
LW area 2002 vs. 1950
1157.630
3.457
18
Proportion LW/CF + LW 2002 vs. 1950
9.833
3.410
18
12.03
)3.30
)13.97
918.02
7.69
0.002
0.053
< 0.001
0.002
0.003
*LW and CF refer to lichen woodlands and closed forests, respectively; T, Wilcoxon matched pairs statistic; d.f., degrees of freedom.
6
Lichen woodland cover (ha)
(a)
10000
2002
1950
2002
8000
1950
6000
4000
2000
0
48
49
50
Latitude
51
52
(b)
100
Ratio LW / LW+CF (%)
influenced by human activity, and the most common
disturbances were large fires. In the 1950s, fires burned nearly
40% of the total forest cover from latitude 51N to 5212¢ N.
Even in 2002, large fires occurred in this area but with a far
lower impact than in the 1950s (P < 0.001).
Both in the 1950s and in 2002, the lichen woodland cover
increased significantly with latitude (R2 = 0.73, P < 0.0001,
and R2 = 0.81, P < 0.0001, respectively) (Fig. 5a). The distribution of lichen woodlands has changed significantly over the
last 50 years in the closed-crown forest zone (Table 1). Lichen
woodlands currently cover only small areas in the southern
part of the study area and in the logging zone. The shifting
point of the respective dominance of the closed-crown forest in
the south and the lichen woodland in the north is at latitude
51o N. However, the northernmost transects near the Otish
Mountains around 52o N included a smaller lichen woodland
cover both in the 1950s and today. Globally, from south to
north, 19 500 ha of closed-crown forest has shifted into lichen
woodland following 1950s disturbances, which represents a
loss of about 9% of dense forest. The proportion of the lichen
woodland cover relative to the overall forest cover shows the
same trend (Fig. 5b & Table 1). Lichen woodland stands
increased significantly across the whole study area (2002:
R2 = 0.81, P < 0.0001; 1950: R2 = 0.61, P = 0.0002). In the
southern part, lichen woodlands covered only 1% and 2.2% of
the forest in the 1950s and in 2002, respectively, whereas in the
logging zone (4853¢ N to 5042¢ N) this cover was between
1% and 6% in the 1950s and between 5% and 11% in 2002.
Currently, the cover of lichen woodland varies between 20%
and 77% in the northern part of the study area.
A small proportion of the closed-crown forest shifted into
lichen woodland following logging (Fig. 6a). This shift represents 6% in the extensive logging zone and 7% in the PGJ area.
Several closed-crown forests shifted to lichen woodlands
following the 1950s fires. Closed-crown forests of the PGJ
and RFL were less susceptible to shifting towards lichen
woodlands than those in the central (4853¢ N to 5042¢ N)
and northern (5042¢ N to 5212¢ N) zones. About 3% of the
burned forests in the 1950s shifted to lichen woodlands in the
PGJ and RFL areas. Between 8% and 30% of the closed-crown
forests have shifted to lichen woodlands since the 1950s in the
80
60
40
20
0
48
49
50
Latitude
51
52
Figure 5 (a) Lichen woodland cover and (b) proportion (percentage) of the lichen woodland relative to the total forest cover
(LW/LW + CF) for each transect along the latitudinal gradient
in the 1950s and in 2002. LW and CF refer to lichen woodlands
and closed forests, respectively.
logging zone, and between 4% and 66% in the northern part of
the study area. A proportion of the closed-crown forest shifted
to lichen woodland following the 1950s fires, but another
proportion remained stable; that is, the closed-crown forests
self-regenerated after fire (Fig. 6b). In the RFL and PGJ areas, a
Journal of Biogeography
ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
Expansion of lichen woodlands by stand disturbances
(a) 100
1.0
0.8
60
40
20
0
48
49
50
51
52
Resilience (%)
%
80
0.6
0.4
0.2
Latitude
0.0
CF - LW shift following 1950s logging
CF - LW shift following 1950s wildfires
48
50
51
52
Latitude
(b) 100
Figure 7 Resilience (percentage) of the closed-crown forest
according to latitude. The resilience index (%) of the closed-crown
forest is the proportion of this forest that self-regenerated after
stand disturbance (fire and logging) over the last 50 years.
80
%
49
60
40
20
0
48
49
50 51
Latitude
52
Self-regenerated CF following
1950s wildfires
Self-regenerated LW following
1950s wildfires
Figure 6 (a) Proportion (percentage) of the closed-crown
forest (CF) that has shifted to lichen woodland (LW) following fire
and logging during the 1950s. (b) Stability of the closed-crown
forest and the lichen woodland following fires during the 1950s.
The percentages correspond to the proportion (percentage) of
the total area in each transect.
large proportion (98%) of the closed-crown forest has
remained stable over the last 50 years. In the logging zone,
between 67% and 100% of the closed-crown forest selfregenerated, whereas only 15% to 56% of the northernmost
closed-crown forest self-regenerated following the 1950s fires.
The northern zone had a proportion of lichen woodlands
significantly greater than those in the other zones before the
1950s fires. When these lichen woodlands burned, they selfregenerated without any shift towards closed-crown forests.
The percentage of closed-crown forests that self-regenerated
after stand disturbance over the 50-year period varies greatly
according to latitude (Fig. 7). In the southern zone, the closedcrown forest is highly resilient to disturbances, with an RI
varying from 0.8 in the RFL to 1.0 in the Lac St-Jean area. In
the central zone, the RI decreases with increasing latitude, with
higher values in areas dominated by extensive balsam fir forests
and in high-altitude forests. In the northern zone, the RI drops
dramatically to reach about 0.2; that is, in the forest areas most
susceptible to shifting to lichen woodlands following fire
disturbance. Near the Otish Mountains (52oN), the RI
increases sharply to reach a value similar to or slightly lower
than that of the logging zone.
DISCUSSION
The short- and long-term stabilities of biomes have been
estimated based on palaeoecological data (Plöchl & Cramer,
1995; Lavoie & Payette, 1996; Williams et al., 2000; Masek,
2001; Yemshanov & Perera, 2001). In this study, direct
observations based on a comparison of air photos taken at
different times have shown lichen woodland expansion across
the closed-crown forest zone since the 1950s. As hypothesized,
the area covered by lichen woodlands increased significantly
from south to north to reach a maximum at the limit of the
closed-crown forest zone. According to Hare (1959), the
northern limit of the closed-crown forest zone is at about
52 N. Our data show that the maximum increase of the lichen
woodland cover is located south of Hare’s limit (52 N) and
north of the central zone (4853¢ N to 5042¢ N) dominated by
logging. The increase of lichen woodland cover within the
closed-crown forest zone is the consequence of recurrent
disturbances such as fire and logging, and if the trend is
maintained in the long term it will result in a southward
migration of the lichen woodland zone.
The overall resilience of the closed-crown forest is a function
of stand disturbances, latitude and altitude. The nature of the
disturbance varies spatially. In the southern zone, spruce
budworm periodically affects black spruce stands (Blais, 1983;
Jasinski & Payette, 2005). In the central zone, 75% of the area
was subjected to logging, and in the northern zone, extensive
fires were common. Closed-crown forests are less resilient to
disturbances with increasing latitude. Although the reversion
of lichen woodlands to dense black spruce stands is possible,
no field evidence has yet been found across the closed-crown
Journal of Biogeography
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7
F. Girard, S. Payette and R. Gagnon
forest zone. Over the last 50 years, about 19,500 ha of the
225,000 ha of dense coniferous forest has shifted to lichen
woodland, which represents about 9% of the forest. Lichen
woodlands are expanding in the heart of the closed-crown
forest zone and are in a process of massive establishment after
fire in the northern zone.
The shift of the closed-crown forest takes place mainly in the
northern part of the study area. Except for the PGJ area, the
closed-crown forest in the southern part of the zone is more
resilient to disturbance. During the 20th century, spruce
budworm outbreaks or logging impacted on balsam fir forests
several times (Blais, 1983; Payette et al., 2000; Jasinski &
Payette, 2005). In this area where balsam fir dominates, the
shift from closed-crown forest to lichen woodland is marginal
because forest gaps are rapidly filled by balsam fir, paper birch,
eastern larch (Larix laricina (Du Roi) K. Koch) and trembling
aspen (Populus tremuloides Michx.). Indeed, the shift of the
closed-crown forest to open forest occurs where black spruce is
dominant and where fire intervals are relatively short (such as
in the PGJ area). During spruce budworm outbreaks, most
trees are killed, and survivors show reduced growth and seed
production for several years (Morin & Laprise, 1990; Simard &
Payette, 2001). If fire occurs shortly after an epidemic, the
closed-crown forest is more susceptible to a shift to lichen
woodland (Payette et al., 2000).
Closed-crown black spruce stands dominate the landscape
in the central zone (4853¢ N to 5042¢ N), with logging as
the main stand disturbance, and insect outbreaks and fires
restricted spatially. In monospecific black spruce stands,
logging can induce a shift from closed-crown forest to open
forest. As a result, degradation of the closed-crown forest and
reduction of tree density occurs when the latter species
(balsam fir, paper birch or trembling aspen) are absent.
Closed-crown forests of the central zone are less resilient to
disturbance than those of the southern zone but more
resilient than those of the northern zone. The northern zone
is dominated by extensive fires, which are currently shifting
closed-crown forests to lichen woodlands. In the northernmost closed-crown forests near the Otish Mountains, the
reduced expansion of lichen woodland stands is related to
topography and altitude, which is often above 1100 m a.s.l.
Wetter conditions in the Otish Mountains, caused by
orography, probably reduce fire ignition and frequency. With
longer fire intervals, the inception and expansion of lichen
woodlands are reduced greatly.
Lichen woodlands began to expand in the southern part of
the closed-crown forest zone around 1500 years ago (Jasinski
& Payette, 2005). This is supported by pollen diagrams of the
study area, which show a decrease of conifer pollen in the
closed-crown forest zone during the late Holocene (Garralla &
Gajewski, 1992). The major increase of lichen woodland cover
in the closed-crown forest zone as recorded in this study
appears unusual, given the short time interval of 50 years. If
this trend is maintained, closed-crown forests could disappear
within about 550 years. However, it is likely that the conversion of closed-crown forest to lichen woodlands is part of a
8
natural process of long-term biome changes associated with
changing environmental conditions during the Holocene.
ACKNOWLEDGEMENTS
For field and laboratory assistance, Vincent Beaulieu, Damien
Côté, Ann Delwaide and Mathieu Tremblay are gratefully
acknowledged. We are also grateful to Gilles Houle for his
valuable comments on the manuscript. This research was
financially supported by the Fonds Québécois de la Recherche
sur la Nature et les Technologies (FQRNT, Québec), the Fond
Forestier du Saguenay-Lac-St-Jean Région 02, The Consortium
de Recherche sur la Forêt Boréale Commerciale and the
Natural Sciences and Engineering Research Council of Canada
(NSERC).
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BIOSKETCHES
François Girard is a PhD student at the Départment de
Biologie and Centre d’Études Nordiques, Université Laval. His
dissertation focuses on the origin and dynamics of lichen
woodlands in the closed-crown forest zone.
Serge Payette is a professor of plant ecology and palaeoecology at the Département de Biologie and Centre d’Études
Nordiques, Université Laval. As Chairman of the NSERC
Northern Research Chair on disturbance ecology, he studies
the relationships between boreal and subarctic ecosystems,
climate and stand disturbances at various temporal and spatial
scales.
Réjean Gagnon is a professor of forest ecology at the
Département des Sciences Fondamentales and Chairman of
the Consortium de Recherche sur la Forêt Boréale Commerciale. He studies the ecology of closed-crown forests and the
dynamics of natural disturbances.
Editor: Glen MacDonald
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ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
9