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Tree-ring patterns in stems and root systems of
black spruce (Picea mariana) caused by spruce
budworms
Cornelia Krause and Hubert Morin
Abstract: Radial growth along the stems and root systems of black spruce trees (Picea mariana (Mill.) BSP) was
examined to determine the effects of spruce budworm defoliation. A mixed conifer and pure black spruce stand located
in the boreal zone of Quebec, Canada were sampled. Following defoliation, dendrochronological analyses revealed the
percent growth reduction in the ring width at different stem heights and throughout the root system. Ring widths of
black spruce were found to be reduced during the last three spruce budworm outbreaks. The reduction of the tree-ring
width after spruce budworm outbreaks started first in the crown region and was followed by reduction at the stem
base. For the whole root system, the ring-width index exhibited a decrease. The root system showed a high sensitivity
to defoliation by spruce budworm. Inside the root system, the growth reduction after a spruce budworm outbreak was
variable in each root branch. The growth decrease of the pure black spruce stand was less intensive than in the mixed
stand.
Résumé : La croissance radiale le long de la tige et du système racinaire d’épinettes noires (Picea mariana (Mill.)
BSP) a été étudiée dans le but de déterminer les effets de la défoliation par la tordeuse des bourgeons de l’épinette. Un
peuplement mixte de conifères et un peuplement pur d’épinette noire situés dans la zone de forêt boréale du Québec,
au Canada, ont été échantillonnés. Des analyses dendrochronologiques ont permis d’évaluer le pourcentage de réduction
de croissance reflétée dans la largeur des cernes annuels à différentes hauteurs sur la tige et un peu partout dans le
système racinaire après une défoliation. Lors des trois dernières épidémies de tordeuses des bourgeons de l’épinette, la
largeur des cernes a diminué. La réduction de la largeur des cernes après les épidémies de tordeuses des bourgeons de
l’épinette a débuté dans la cime et s’est poursuivie à la base de la tige. L’indice de largeur des cernes a diminué dans
l’ensemble du système racinaire. Le système racinaire montre une grande sensibilité à la défoliation par la tordeuse des
bourgeons de l’épinette. Dans le système racinaire, la réduction de croissance après une épidémie de tordeuses des
bourgeons de l’épinette varie dans chaque embranchement racinaire. La diminution de croissance a été moins
importante dans le peuplement pur d’épinette noire que dans le peuplement mélangé.
[Traduit par la Rédaction]
Krause and Morin
Introduction
In the boreal zone, fire and insect defoliation influence the
dynamics of the natural forest (MacLean 1984; Cogbill
1985; Baskerville 1986). In northeastern North America,
balsam fir (Abies balsamea (L.) Mill.), and white spruce
(Picea glauca (Moench) Voss.) have been defoliated periodically by spruce budworm (Choristoneura fumiferana (Clem.))
(Blais 1965; Hardy et al. 1983; Lynch and Witter 1985;
MacLean 1984; Baskerville 1986). Red spruce (Picea
rubens Sarg.) and black spruce (Picea mariana (Mill.) BSP)
seem to be less affected by the insect (Mattson 1985).
For balsam firs, ring-width patterns at the stem base resulting from consecutive periods of defoliation are well
known (Blais 1958, 1965, 1983; Morin and Laprise 1990;
Piene 1980, 1989). Spruce budworm outbreaks in the Lake
Received May 4, 1998. Accepted May 14, 1999.
C. Krause1 and H. Morin. Département des sciences
fondamentales, Université du Québec à Chicoutimi,
Chicoutimi, QC G7H 2B1, Canada.
1
Corresponding author.
e-mail: [email protected]
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Libéral (known under the official name of Lake Kanushemuakushkatsh) study area have been documented from 1900
and have caused significant ring-width reductions from
1909–1915, 1949–1953, and 1976–1981 in white spruce and
balsam fir (Morin and Laprise 1990). The larvae of the
spruce budworm prefer to feed on the needles of balsam fir
and white spruce, but needles of red and black spruce are
also accepted (Blais 1957; Miller 1981). Spruce budworm
infestations cause volume increment reduction and sometimes tree mortality to black spruce (Elliot 1960; Lussier
1991, 1996). The role of black spruce in spruce budworm
outbreak cycles is unknown, but this species is affected and
its percent of mortality increases with the severity of defoliation (Raske 1984; Lussier 1991, 1996).
Stem analysis is widely used for many applications, but
dendrochronological analyses of roots are rare. The radial
growth of roots is often eccentric and variable, mainly because of differences in soils and different functions of the
various root parts (Fayle 1968). Root close to the stem ensure the stability of the tree whereas the more distant roots
secure the transport of water and nutrients substances and
store the assimilates (Fayle 1968; Krause and Eckstein 1993).
The roots, despite their great variability, react similarly to
stems in years influenced by extreme external factors, such
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Table 1. Location of the two study areas in Quebec, Canada, sample size, and tree
characteristics.
Site
Lake Libéral
Lake Onatchiway
Location
Stem height, mean (m)
Date of origin (fire scars)
Number of trees
Number of stem samples
Number of root samples
Age range at soil level (years)
49°46′ N, 72°34′ W
17.8
Unknown
10
197
1518
143–223
48°53′ N, 71°01′ W
14.5
1895
20
329
659
83–132
as periods of drought (Schulman 1945; Krause 1992; Krause
and Eckstein 1993; Krause and Morin 1995b). The tree-ring
width of balsam fir roots were reduced during and following
the periods of defoliation by spruce budworm (Krause and
Morin 1995b). The highest total reduction in radial growth
caused by other insect defoliators has been measured in
roots of Scots pine (Ericsson et al. 1980). This sensitivity of
growth in woody roots to defoliation seems to be a good indicator of the impact of the spruce budworm outbreaks.
We studied the impact of defoliation on the radial growth
of black spruce roots and stems. The ring-width reduction
pattern of the two study sites was compared in relation to
the forest composition (mixed and pure black spruce forest).
This aspect was further examined in the discussion with similar data obtained from balsam fir in a prior study (Krause
and Morin 1995a, 1995b). We hypothesized that spruce
budworm starts to feed on balsam fir and black spruce during the same year, and that ring-width reduction in black
spruce and balsam fir in the same region are synchronized.
Study area
The two study areas were located in the boreal zone of Quebec
(Rowe 1972), in ecological zone 12 b (Thibault 1987). The Lake
Libéral site (49°46′ N, 72°34′ W; Table 1) is a virgin mixedsoftwood stand composed of balsam fir and black spruce interspersed with white spruce (Picea glauca (Moench) Voss.) and paper birch (Betula papyrifera Marsh.). The Lake Onatchiway site
(48°53′ N, 71°01′ W) is a pure, even-aged black spruce stand
(Table 1) with <1% balsam fir. Fire scars on surviving trees were
used to date the origin of the stand.
The growing period extends from June to August. Temperatures
below freezing are common during more than 6 months of the year
and can reach temperatures below –40°C. Precipitation events are
regular during the year reaching, and total amounts exceed 800 mm
at the Albanel climate station, Que., 130 km south of Lake Libéral
and Bagotville, Que., 50 km south of Lake Onatchiway.
For the Lake Libéral area, even in this boreal zone, a reduction
in ring width due to spruce budworm outbreaks has been described
for white spruce and balsam fir by Morin and Laprise (1990) and
for black spruce at Lake Onatchiway (Lussier et al. 1996).
Material and methods
Thirty dominant and codominant black spruce trees, defoliated
by spruce budworm outbreaks and with visible ring-width reduction, were analysed. At the Lake Libéral site, 10 black spruce were
cut from a mixed balsam fir – black spruce stand. The age of these
trees was variable, but generally older than 200 years. From the
Lake Onatchiway site, 20 black spruce were selected to evaluate
the impact of defoliation in the absence of balsam fir (Table 1).
Stem disks were taken at 0, 0.3, 0.6, 1.0, and 1.3 m (breast
height) and at every meter starting from 2 m to the top of the trees.
In addition, the root systems of 10 of the above black spruces were
excavated at each study site and the position of each root mapped.
Root disks were analysed every 10–15 cm down to a diameter of
1 cm.
After sanding (Blais 1962; Swetnam et al. 1985), ring widths of
the stem disks were measured with a precision of 0.01 mm along
four radii (cardinal points). In the case of root samples, only the radius with the largest number of visible rings was measured (Krause
1992; Krause and Eckstein 1993; Krause and Morin 1995b). Ringwidth data were plotted as time series and cross-dated on a light
table (Stokes and Smiley 1968; Fritts 1976). The program
COFECHA was used to verify the cross-dating (Holmes 1983;
Holmes et al. 1986) and corrections were made when necessary.
Root cross-dating is a more time consuming process than stem
cross dating, because of the variability of root growth patterns
(Krause 1992; Krause and Eckstein 1993). Root sections were also
cross-dated on a light table with the ring pattern at the stem base
and afterward along one root (more information in Krause and
Morin 1995b). The findings were verified using the program
COFECHA (Holmes 1983; Holmes et al. 1986) to eliminate dating
errors. A total of 201 root discs (14%) from the Lake Libéral and
138 (17%) from the Lake Onatchiway areas were excluded from
the analysis. For these time series, the correlation values were
below 0.3281, too low to be significant at the 0.01 confidence
level. In other cases, long periods of narrow ring-widths without
annual variation were measured and these lead to cross-dating failures. The excluded root sections were mainly located toward the
root tips with either a diameter of only 1–2 cm or showing abnormal growth due to root branching (C. Krause and H. Morin, in
preparation).
The total stem height of the trees was divided into three equal
sections: lower, middle and upper section. The upper section included the majority of the living crown. This enabled sequential
analysis of the impact of defoliation on ring width. Only data from
the lower and upper sections are presented, to better demonstrate
the contrast between the two opposite parts of the tree. The root
system was studied as a whole without division to give a general
view of the radial growth reduction. Final index chronologies were
developed for the different parts of the trees using the program
ARSTAN (Cook 1985; Cook and Holmes 1986). The program was
used with a double detrending; (i) a negative exponential or a
straight line and (ii) a cubic spline function with a 50% cutoff
wavelength. Standard chronologies for the two different sections of
the stem and the root system were selected. This calculation transforms ring width into dimensionless index values.
The percent reduction in the tree-ring index associated with the
spruce budworm outbreaks were calculated for the lower and upper
stem sections and the root system separately. The ring index values
with a visible reduction were compared with the mean for the
10 years preceding the outbreak (inter-outbreak years) (Schweingruber et al. 1986). The period of 10 years was judged to be a long
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Fig. 1. Mean ring width indices in different parts of black spruce trees, with reductions (black arrow) caused by spruce budworm
defoliation in Quebec, Canada. Grey arrows indicate a second wave of defoliation at Lake Onatchiway.
enough calibration period to avoid the influence of other past
spruce budworm outbreaks. The mean tree-ring index values from
1965 to 1974 were calculated and compared with each index value
for the years of the outbreak, 1974–1989, according to survey
maps. For the previous outbreak, 1947–1957, the mean value was
calculated for the 10-year period preceding the outbreak, 1940–
1949, and compared with the index values of the years of the outbreak. According to Schweingruber et al. (1986), a reduction of
more than 40% for one or more years is considered an abrupt
change in conifer growth.
Moreover, each root branch was also analysed individually. Index chronologies were calculated for each root branch using
ARSTAN as above and the percent reduction of the tree-ring index
associated with spruce budworm outbreaks was analysed. By analysing each root branch separately, the variability of the reduction
inside a root system can be observed more precisely (Fig. 2a).
Results
General growth patterns in black spruce during and
after a spruce budworm outbreak
Three declines occurred in stem and root radial growth in
the Lake Libéral stand and two declines for the Lake
Onatchiway stand, corresponding to the last three spruce
budworm outbreaks. Radial growth at the stem base of spruce
exhibited reductions at 1912–1914 for the Lake Libéral site
only, in 1950–1951 and in 1977–1979 in both regions
(Fig. 1).
The general ring-width reduction pattern during and after
insect defoliation took place first in the upper section, within
the living crown. A reduction of >40% occurred at least one
year earlier in the upper section than the lower section
(Table 2, Fig. 1). However, ring-width reduction is noted in
the upper and lower sections in the same year for the last
outbreak at the Lake Libéral site (Table 2). Following a delay of 1 or 2 years, growth reduction was also evident in the
lower part of the trunk. The decrease in the ring width was
also more pronounced in the upper section than in the lower
one (Table 2). For the root system the growth suppression of
>40% occurred in the same year as in the lower section
(Table 2).
Between the different root branches the tree-ring reduction of >40% was variable in time (example in Fig. 2a). In
45% of the root branches, the tree-ring reduction occurred in
the same year as in the lower section (Fig. 2b). Growth exhibited reduction of >40% in the root branch separately over
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Table 2. Ring width reduction of >35% compared with the mean index value of the previous
10 years within the different sections of the tree in Quebec, Canada.
Lake Libéral
Year
1911
1912
1913
1914
1949
1950
1951
1952
1953
1957
1958
1959
1976
1977
1978
1979
1985
1986
Upper stem
section
44
77
68
59
40
59
41
Lake Onatchiway
Lower stem
section
Roots
66
67
64
57
67
64
55
54
35
40
Upper stem
section
Lower stem
section
36
35
40
40
35
54
40
61
40
53
68
72
51
48
78
48
a period of 6–9 years in the two stands for the outbreaks
(Fig. 2b). For example, in the Lake Libéral site the first root
branch reduction occurred in 1944 and another root branch
registered a decrease only in 1953 (Fig. 2b).
The effects of spruce budworm defoliation on the root
systems is also apparent in the large number of absent and
discontinuous rings in the spruces during the periods of
spruce budworm outbreaks. A total of 125 discontinuous
tree-rings were noted in Lake Onatchiway and 370 in Lake
Libéral. The majority of these rings are from 1911 to 1916,
from 1949 to 1954, and from 1976 to 1982 especially (83%
for Lake Libéral and 80% for Lake Onatchiway including
the second wave of defoliation 1984–1988) (Fig. 3). The
percentages represent the total missing rings.
Growth pattern of the mixed black spruce – balsam fir
stand in the Lake Libéral site
Only the results for black spruce in the mixed stand will
be presented here. These results will be compared to the reaction of balsam fir in the discussion section. Abrupt growth
reductions corresponding to the documented spruce budworm
outbreaks occurred in the index chronologies of black spruce.
Small ring widths were measured for the periods 1912–
1914, 1950–1951, and 1976–1979 in the lower section of the
trunk (Table 2, Fig. 1). Small ring-width in the upper section
of the stem occurred in 1911 to 1914 and 1949 to 1951. For
the third outbreak, 1976–1979, a radial growth reduction of
>40% exhibited in the same year at all stem heights (Table 2).
The growth reductions in the root, lower and upper sections, were similar. The correlation coefficients (r) between
the three index chronologies were upper section with lower
section = 0.81, p = 0.01; upper section with roots = 0.84, p =
0.01; lower section with roots = 0.72, p = 0.01. For the first
outbreak, the smallest ring width in the root system occurred
between 1912 and 1914 (Table 2, Fig. 1). Radial growth
Roots
40
35
40
36
36
40
40
57
35
45
analysis in each root branch indicated decreases of >40% in
11% of root branch as early as 1907 and reaching 50% by
1912 (Fig. 2). At this time, the spruce trees averaged only
8.4 m in height. The second outbreak period induced small
index values for 1950–1952 for the root system. Radial
growth responses of each root branch were more variable.
By 1945, growth decrease occurred in 3.6% of the root
branch, whereas that number reached 50% by 1951 (Fig. 2b).
During the third outbreak, small index values for the root
system ocurred in 1977, 1978, and 1979 (Fig. 2). By 1976,
31% of the individual root branches showed a decrease in
their index values. By 1978 this changed to 15% (Fig. 2).
Growth pattern of the pure black spruce stand at the
Lake Onatchiway site
Regeneration of spruce at the Lake Onatchiway site followed a fire in 1895, and the trees were still young during
the 1909–1915 outbreak. Therefore the impact of the outbreak on ring widths of black spruce was less pronounced
(Fig. 1). Stem reductions in these trees started in the upper
sections in 1950 and 1976, respectively. With a delay of
1 year, stem reductions started in the lower sections in 1951
and 1977 (Fig. 1). Radial growth was reduced in the upper
sections from 1950 to 1952, from 1977 to 1978, and in the
lower sections from 1952 and 1978.
The index series of the root system had greater annual
variation in reductions of the ring indices for the last two
spruce budworm outbreaks (Figs. 1 and 2). Correlation coefficients (r) for the root system and the lower section chronologies with the upper section were 0.52 and 0.43
respectively. For 1950, radial growth analysis in individual
root branches indicated a decrease in 52.5% of the cases
(Fig. 2b). Five percent of the root branches had suppressed
growth in 1946; whereas others revealed 12.5% and 15.5%
suppressed growth in 1951–1952 (Fig. 2). For the third
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Fig. 2. (A) Year of first ring-width reduction in one black spruce root system with >40% compared with the mean index value of the
previous 10 years in Quebec, Canada (–, reduction less than 40%). (B) The number of root branches with a reduction of more than
40% (upper graph), and total number of roots sampled (lower graph).
outbreak, most of the root branch reduction occurred in 1977
and 1978 with 28 and 42%, respectively.
Growth reduction in spruce at the pure spruce stand of the
Lake Onathiway site was not as severe as at the mixed forest
stand of the Lake Libéral site. However at the Lake Onatchiway site, the ring widths of all the sections were reduced
during a second wave of defoliation in 1957 to 1959 and in
1983 to 1987 (Fig. 1). During this second period, the growth
reduction in the upper section and root system once again
reached more than 40% compared to the indices before
spruce budworm outbreaks (Table 2, Fig. 1).
Discussion
Feeding behaviour of the spruce budworm and its
impact on the growth
In the spring, larva of the spruce budworm feed on buds
and new foliage of balsam fir (Miller 1981). When only new
foliage is eaten, a reduction of ring widths occurs in the upper part of the stem (Solomon 1983). Once all new needles
and current shoots are gone, the insects will feed on previ-
ous year’s needles of mature firs and the new foliage of
understory trees (Miller 1981). By destroying the 3 or 4 year
old needles of mature trees, a decrease of the radial increment is caused in the lower parts of the stem (Solomon
1983). During the first year of insects feeding on the current
year’s foliage, a concomitant reduction in volume growth
occurs in young firs (Piene 1980). When defoliation occurs
over a long period and with a high intensity, the increment
decrease in the lower section of the stem will be proportional to the loss of the old needles (Solomon 1983, 1985).
The higher defoliation rate on balsam fir and white spruce
are associated with their earlier opening of the buds (Swaine
and Craighead 1924; Graham and Orr 1950; Blais 1957). In
black spruce the opening of the buds takes place 10–14 days
later than in balsam fir and white spruce, and this delay
seems to protect the black spruce trees from severe spruce
budworm damage (Blais 1957). The nutritional quality of
the foliage is comparable for the three species (Albert and
Parisella 1985).
Analysis of thirty black spruces indicated a growth decrease
beginning first in the upper section of the stem, corresponding
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Fig. 3. Percentage of missing rings (discontinuous rings) in the root section of the two study areas Lake Libéral and Lake Onatchiway
in Quebec, Canada. Periods of spruce budworm infestations are shaded.
to the first year of a severe insect infestation. This confirms
that black spruce needles were also consummed by spruce
budworm larvae.
Reaction pattern of black spruce after spruce budworm
outbreaks in comparison with balsam fir
The history of the last three spruce budworm outbreaks in
both study areas has been documented (Morin and Laprise
1990; Krause and Morin 1995b). Indeed, three periods of defoliation by the spruce budworm are shown by a reduction
of balsam fir ring widths in the 20th century in the Lake
Libéral area (Morin and Laprise 1990; Krause and Morin
1995a, 1995b) and for other regions in eastern Canada
(Swaine and Craighead 1924; Batzer 1973; MacLean 1979;
Piene 1980; Piene et al. 1981). However, the impact of the
spruce budworm on volume increment and mortality of
black spruce has not been well documented. Elliot (1960)
and Blais (1964) observed that black spruce is generally little affected by defoliation, but in some cases mortality may
result. More recently Raske and Sutton (1986) examined the
increase of rootlets mortality and Lussier et al. (1992) analysed the volume lost in the stems after a spruce budworm
outbreak.
Following a spruce budworm outbreak, the pattern of
ring-width reduction in balsam firs at the stem base and
along the stem is well known. The reduction starts in the
crown and continues with a delay of a few years at the stem
base for old trees (Mott et al. 1957; Stark and Cook 1957;
Blais 1958; Ericsson et al. 1980; Solomon 1983, 1985;
Krause and Morin 1995a, 1995b). Furthermore, after a defoliation the reaction at the crown level is more intense than at
the stem base (Mott et al. 1957; Blais 1958; Krause and
Morin 1995a, 1995b).
The same pattern occurred in black spruce, especially for
spruce in mixed forests, where the radial growth reduction
started in the same year in surviving black spruce and in balsam fir (Table 3). At the stem base, a decided reduction occurred in the same year for black spruce and balsam fir; in
the upper section, a reduction occurred in the same year for
the first outbreak only (1911), but growth reduction started
1 year earlier in balsam fir for the two other outbreaks
(Krause and Morin 1995b; Table 3).
The first growth decrease was most pronounced in the upper section for black spruce at both sites (Fig. 1). After a delay of one or two years, the reduction was also obvious in
the lower section. The black spruces in the pure stand (Lake
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Table 3. First year of >40% reduction in ring width for the upper and lower sections of the stems compared
with the mean index value of the previous 10 years for two black spruce sites in comparison with two nearby
balsam fir sites in Quebec, Canada.
Location
Outbreak
Lake Libéral
black spruce
Lake Libéral
balsam fir
Lake
Onatchiway
black spruce
Mount Valin
balsam fir
First
Second
Third
1911
1949
1976
1911
1948
1975
1950
1977
1950
1976
First
Second
Third
1912
1950
1976
1912
1950
1976
1952
1978
1914
1951
1977
Upper stem section
Lower stem section
Onatchiway) seem to be less affected, but followed the same
pattern as fir and spruce from the Lake Libéral region with
the growth redution beginning and showing the highest intensity in the upper section. Radial growth reduction started
two years later than at the Lake Libéral site (Table 3).
Radial growth of black spruce from Lake Onatchiway
with the balsam fir from the Mount Valin site (-40 km) (fir
data published in Krause and Morin 1995b) showed growth
reductions among black spruce 1 year later than the balsam
fir (Table 2). Balsam fir also had reduced radial growth 1–2
years later than the spruce at Lake Libéral. This 1- to 3-year
discrepancy in the first year of reduction between the different sites was not important and may be explained by variation among the sites. The fact that black spruce stand at
Lake Onatchiway was younger may have contributed to it
receiving less damage as reported by Boulet (1994).
Growth reduction during outbreaks (1950–1952 and
1977–1979) was less for black spruce at Lake Onatchiway.
A second wave of suppressed growth occurred, possibly
caused by additional defoliation during 1957–1959 for the
outbreak in the fifties and 1983–1987 for the outbreak in the
1980s (Fig. 1). This second wave of budworm defoliation
was more severe in the pure black spruce stand for the stem
and root systems. Furthermore, in a balsam fir stand near the
pure black spruce stand (Mount Valin), a growth reduction at
the crown level occurred in 1957 (C. Krause, unpublished
data). Maps based on defoliation surveys reported the presence of spruce budworm insects in the region of Lake
Onatchiway and Mount Valin during the period of 1957 to
1959 (Hardy et al. 1983). During the last spruce budworm
outbreak, insect infestation was present in the Lake
Onatchiway area during the period 1983–1987 (B. Boulet,
unpublished data). Two successive suppression periods in
balsam fir, black spruce, and white spruce occurred also at
the beginning of this century (Blais 1964, 1965). Blais
(1964, 1965) suggests two waves of defoliation for the outbreak in the fifties in Laurentides Park, 200 km south of
Lake Onatchiway.
Reaction pattern in the root system of black spruce
The most severely affected part of a tree, during and after
defoliation, is the root system; followed by the branches and
then the stem (Piene and Little 1990). For black spruce, a
large number of absent rings are noted during periods of
highest reduction along the stem. The number of absent
rings in root systems of black spruce was lower than in balsam fir (Krause and Morin 1995a), but tree-ring reduction
was severe in this part of the tree (Fig. 1). Roots seem to be
sensitive to defoliation. Indeed, the beginning of ring reduction is variable inside one root system, where delays of 4
years and more can be observed between different roots
(Fig. 2a). Even if the root system registered outbreaks with
high sensitivity, the variability within a root system makes
this part unsuited for dating the beginning of spruce budworm outbreaks. Analysis of the spatial and temporal root
development in relation to spruce budworm outbreaks would
be interesting to help understand the full impact of defoliation in a tree.
Conclusions
Black spruce shows a growth response pattern to spruce
budworm outbreaks that is similar to balsam fir. Growth reduction during defoliation starts in the upper section and
continues, following a delay of one to two years, in the
lower section. Similarly, the intensity of radial increment reduction was also higher in the upper section, as observed for
balsam fir (Mott et al. 1957; Solomon 1983, 1985).
At the Lake Libéral site, in the lower stem of black spruce
the first decrease in growth took place in the same year as
for balsam fir. A similar reduction took place in the same
year or one year later in the upper stem (Table 2). Therefore,
in a mixed forest, black spruce was defoliated at the same
time as balsam fir.
The pure black spruce at Lake Onatchiway had reduced
growth 1 year later than the nearest balsam fir stand at
Mount Valin. For the Lake Onatchiway stand, the impact of
the spruce budworm infestation occurred during a similar
period as for the balsam fir. The second period of reduction
(8 years later) in the pure black spruce stand in the Lake
Onatchiway requires further investigation.
The root system was severely affected by spruce budworm outbreaks, as evidenced by reduced tree-ring width
and by the high number of missing rings. The beginning of
growth reduction of each root branch is variable and can differ over a period of 9 years inside one root system.
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Acknowledgements
We sincerely thank François Gionest, Pierre-Y. Plourde,
Germain Savard, and Dominique Simard for technical assistance. We thank Judit Ozoray and especially an anonymous
reviewer for their editorial efforts and for helpful comments.
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