Dendroecological Reconstruction of
Mountain Pine Beetle Outbreaks in the
Chilcotin Plateau of British Columbia
René I. Alfaro1, Rochelle Campbell1, Paula Vera2, Brad Hawkes1, and Terry L. Shore1
1
Canadian Forest Service, Pacific Forestry Centre, 506 W. Burnside Rd., Victoria, BC, V8Z 1M5
2
B.A. Blackwell & Associates Ltd. 3087 Hoskins Rd, North Vancouver, BC V7J 3B5
Abstract
The mountain pine beetle (Dendroctonus ponderosae Hopk.) (Coleoptera: Scolytidae) is an
aggressive bark beetle that periodically increases to outbreak levels killing thousands of trees.
It is considered one of the major natural disturbance agents in North America. In British
Columbia, the main host species is lodgepole pine (Pinus contorta var. latifolia Engelm.), but
western white pine (Pinus monticola Dougl.), ponderosa pine (Pinus ponderosa Laws.), whitebark
pine (Pinus albicaulis Engelm.), and limber pine (Pinus flexilis James) are also attacked. We used
dendrochronology to establish the history of canopy disturbances indicative of potential
past beetle outbreaks. For this we relied on the fact that beetle outbreaks do not normally
kill all the trees in a stand and that trees that survive outbreaks, experience extended periods
of increased growth, visible in tree ring series as prolonged periods of release. Increased
growth is thus used as a proxy for canopy disturbance. Fifteen chronologies studied in the
south central area of British Columbia showed three fairly synchronous large-scale release
periods which are proposed as three large outbreaks: 1890s, 1940s and the 1980s. The three
releases averaged 13.8 years (Min=5, Max=23 years) in duration and recurred every 42
years (Min=28, Max=53 years), counted from the start of the release.
Introduction
The mountain pine beetle (Dendroctonus ponderosae Hopk.) (Coleoptera: Scolytidae) is an aggressive bark
beetle whose populations periodically increase to outbreak levels in infestations that kill thousands of trees.
It is considered one of the major natural disturbance agents in North America (Furniss and Carolin 1977).
In British Columbia (BC), the main host species is lodgepole pine (Pinus contorta var. latifolia Engelm.), but
western white pine (Pinus monticola Dougl.), ponderosa pine (Pinus ponderosa Laws), whitebark pine (Pinus
albicaulis Engelm.), and limber pine (Pinus flexilis James) are also attacked (Furniss and Carolin 1977).
Occasionally, non-host trees such as Engelmann spruce (Picea engelmannii Parry) are attacked, but beetle
populations do not persist in these occasional hosts (Unger 1993). Mountain pine beetles generally attack
stands that are more than 80 years old, containing many trees of large diameter (Safranyik et al. 1974).
Mountain Pine Beetle Symposium: Challenges and Solutions. October 30-31, 2003, Kelowna, British Columbia.
T.L. Shore, J.E. Brooks, and J.E. Stone (editors). Natural Resources Canada, Canadian Forest Service, Pacific
Forestry Centre, Information Report BC-X-399, Victoria, BC. 298 p.
245
Although mountain pine beetles can attack younger trees, outbreaks have not been reported in stands
younger than 60 years (Safranyik et al. 1974).
The mountain pine beetle occurs from northern Mexico (latitude 30oN), north to central BC (latitude
o
56 N) and from the Pacific Ocean in the west, to North Dakota (Safranyik 2001). Mountain pine beetle is
distributed throughout most lodgepole pine stands in BC, with infestations being the greatest in the southcentral and southeastern part of the province (Safranyik et al. 1974).
The life cycle of the mountain pine beetle varies considerably (Furniss and Carolin 1977). The normal
cycle takes one year to complete; however, during warmer than average summers, adult parents may reemerge and establish a second brood in the same year. In cooler summers or at higher elevations, broods
may require two years to mature. Beetle flights normally occur throughout July and into August. After
locating a suitable host, females bore through the bark to the phloem and cambium region where the egg
gallery is constructed. The first beetles attacking a tree use aggregating pheromones to attract additional
beetles to mass attack and overcome the tree’s resistance. Fungi, which are introduced by the beetle, cause
blue stain in the sapwood. As the fungi become established in the phloem and xylem, they interrupt the
flow of water to the tree crown and reduce the tree’s ability to produce resin, which is its main defence
mechanism against beetle attack. The combined action of the beetle and fungi kills the tree.
Peterman (1978) described the post-outbreak dynamics in climax lodgepole pine stands and indicated
that beetle attack thins the stand and promotes increased growth among the remaining pines and
other vegetation in the stand, allowing regeneration in the understory. During an outbreak, the beetles
preferentially kill trees of the largest diameter (McGregor and Cole 1985). Cole and Amman (1980)
investigated the characteristics of residual stands (>100 years old) in Wyoming and Idaho to determine
the effect of past beetle outbreaks on stand structure. Increment cores from understory fir and spruce
indicated a growth release following the death of overstorey pine trees killed by mountain pine beetle.
Roe and Amman (1970) compared the stand structure of lodgepole pine forests that had gone
through beetle epidemics with those that had not. They found that by removing the largest trees in the
stand, the beetle promoted succession to spruce and fir. In the absence of fire, consecutive mountain pine
beetle attacks in the stand contributed to the conversion of an even-aged stand to an uneven aged stand.
Similarly, Heath and Alfaro (1990) found that mountain pine beetle-attacked forests in the Cariboo Region
of BC shifted in species composition from lodgepole pine-Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco),
to predominantly Douglas-fir.
Tree rings maintain a record of the canopy disturbance history for a locality, and are therefore useful
as indicators of ecosystem function (Becker et al. 1988; Alfaro 2001), and have been used to determine
past outbreaks of bark beetles (Stuart et al. 1989; Heath and Alfaro 1990; Veblen et al. 1991a, b; Zhang
et al. 1999; Eisenhart and Veblen 2000) and defoliating insects (Zhang and Alfaro 2002, 2003). The
identification of growth release periods in surviving host and non-host trees, synchronous with the
mortality of host trees, is the most common method of historical beetle outbreak detection in tree ring
series. The release is not precisely simultaneous because not all hosts are attacked nor die in the same year
(Eisenhart and Veblen 2000).
In spite of its prevalence as a disturbance agent of BC forests, studies to understand the impacts
of mountain pine beetle on stand dynamics are few. Heath and Alfaro (1990) measured stand structure
and growth of surviving trees after an infestation, which occurred from 1971 to 1975 at Bull Mountain,
near Williams Lake, BC. In addition, in 1987, the Pacific Forestry Centre established 30 research plots to
measure ecological changes induced by beetle in lodgepole pine forests in south-central BC (Shore and
Safranyik 1996). In 2001, a comprehensive study of beetle impacts on stand dynamics was launched in
response to increased outbreaks in BC. As part of that study, in the summer of 2001, we re-measured 15
of the plots established by Shore and Safranyik (1996) (Fig. 1) in order to determine their condition in
2001, i.e., 14 years after plot establishment. The plots are located in the Chilcotin Plateau of the Cariboo
Region of BC and are henceforth referred to as the Cariboo plots.
246
The objective of the work presented here was to determine the long-term history of mountain pine
beetle outbreaks in these 15 plots. These plots cover a substantial portion of the range of mountain pine
beetle in BC. For this, we used dendrochronological methods to identify release periods attributable to
beetle outbreaks in increment cores collected in 2001.
Brief history of mountain pine beetle in the Chilcotin Plateau
area of British Columbia
The following information on the history of mountain pine beetle outbreaks in central BC was summarized
from Wood and Unger (1996) and is based on available reports, and on ground and aerial observations by
the Forest Insect and Disease Survey (FIDS) of the Canadian Forest Service, conducted annually from the
1960s until 1995. Since then, with the discontinuation of FIDS, records have been less consistent.
An outbreak of mountain pine beetle was reported in the Cariboo Region of BC from 1930 to
1936 in the Tatla Lake area, when 60% to 90% of infested lodgepole pine was killed over 650,000 ha.
In the 1940s beetle-killed trees were reported in the Alexis Creek area (Personal communication, Dr.
Les Safranyik, Canadian Forest Service, Victoria). A series of mountain pine beetle outbreaks occurred
throughout the 1970s in the Cariboo Region. In 1974, the Klinaklini River drainage had infestations,
which by 1975 had spread over most of the West Chilcotin. In 1981, mountain pine beetle killed over
9 million trees on 72,800 ha of the Chilcotin Plateau.
Dendroecological Methods
In the summer of 2001, increment cores were collected and analyzed from each of 15 locations in the
Cariboo Region of BC (Fig. 1).
BRITISH COLUMBIA
113
124
126
129 118
119 125 130
128
121
107
Williams Lake
116
104
105
359
103
ALBERTA
163
Invermere
Kamloops
Nelson
Vancouver
Kilometers
0
25 50
100
150
200
250
USA
Figure 1. Map of the location of plots used to study the recurrence of mountain pine beetle
infestations in lodgepole pine, in Central BC. Shaded area represents the enlarged map.
247
Stands were located in the Sub-boreal Pine Spruce (SBPS) and Interior Douglas-fir (IDF)
biogeoclimatic zones (Meidinger and Pojar 1991) (Table 1). The SBPS zone is characterized by cold, dry
winters and cool, dry summers. Mean annual precipitation ranges from 335 to 580 mm (Steen and Coupe
1997). Lodgepole pine is the climax tree species in this zone and is the most common species regenerating
in the understory (Steen and Coupe 1997). The IDF zone is characterized by warm, dry summers and
cool, dry winters (Meidinger and Pojar 1991). Climax vegetation on sites in the IDF zone is a Douglas-fir
forest, often with intermixed lodgepole pine in the forest canopy (Steen and Coupe 1997).
Table 1. Summary data for lodgepole pine (host) stand chronologies used to study recurrence of
mountain pine beetle disturbance in the Chilcotin Plateau area of BC.
No. of
Stand
BGC1
BGC subChronology Year with
Mean Serial
Location
cores crossNo.
zone
zone
period
>5 cores2
Correlation3
dated
Cariboo
103
21
IDF
dk4
1890-2001
1897
0.618
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
Cariboo
104
105
107
113
116
118
119
121
124
125
126
128
129
130
21
16
9
14
19
14
14
13
16
17
14
16
18
18
IDF
IDF
SBPS
SBPS
IDF
SBPS
SBPS
IDF
SBPS
SBPS
IDF
SBPS
SBPS
SBPS
dk4
dk4
xc
xc
dk4
xc
xc
dk4
xc
xc
dk4
xc
xc
xc
1849-2000
1865-2000
1886-2000
1758-2000
1849-2001
1853-2000
1912-2000
1901-2000
1887-2000
1886-2000
1864-2000
1865-2000
1860-2000
1895-2000
1890
1869
1915
1809
1889
1867
1951
1931
1915
1905
1915
1941
1891
1906
0.590
0.569
0.493
0.448
0.558
0.456
0.544
0.403
0.430
0.454
0.496
0.457
0.495
0.493
1
Biogeoclimatic zone
Starting year when the chronology is based on 5 or more trees
3
Describes the amount of common signal within the chronology (Fritts 1976)
2
Increment core sample collection and preparation
Increment cores were collected from lodgepole pine as well as from non-host (these are trees not normally
attacked by mountain pine beetle) Douglas-fir and interior spruce trees. In total, we collected 259
increment cores: 240 from lodgepole pine and 19 from non-host Douglas-fir and spruce. The cores (one
per tree) were extracted at breast height with an increment borer parallel to the slope contour. In the
field, each core was labelled with stand and plot number, tree number and species. Collected cores were
transported to the Pacific Forestry Centre, Canadian Forest Service, Victoria, BC, for storage and analysis.
Cores were glued and mounted in slotted mounting boards, which were labelled with tree identifiers.
The surface of the cores was sanded with progressively finer sand paper (grits 220 to 600) to enhance the
boundaries between annual rings.
Sample measurement and chronology development
Ring-width measurement was conducted in the Tree-Ring Laboratory of the Pacific Forestry Centre
using a WindendroTM tree-ring measuring system and a Measu-Chron incremental measuring system.
The precision of the measurement was 0.01 mm. The measured ring-width sequences were plotted and
the patterns of wide and narrow rings were cross-dated among trees. The cross-dating was aided by
248
the presence of distinctive narrow rings, and the quality of cross-dating was examined by the program
COFECHA (Holmes 1983). COFECHA (Holmes 1983) detects measurement and cross-dating errors by
computing correlation coefficients between overlapping 50-year segments from individual series (Eisenhart
and Veblen 2000).
We standardized all cross-dated series by dividing each ring width by the mean series ring width
(Eisenhart and Veblen 2000). Standardizing series by their mean preserved the long-term growth trend
necessary to identify canopy disturbances (Veblen et al. 1991a). Each chronology was visually inspected
for growth releases that might indicate a mountain pine beetle outbreak. After trying different methods
to remove subjectivity from the process of identifying the release periods, we settled for a purely visual
method, in which a release was called a mountain pine beetle release if it was abrupt and sustained over
several years (Fig. 2).
Stand 128
Ring-width
4
3
1940's release
1980's release
2
1
0
20
15
N
10
5
0
1860
1880
1900
1920
1940
1960
1980
2000
Figure 2. Example of tree ring chronology (top) and sample size for the chronology (bottom). Ring
width indices for this stand (#128, Cariboo Region) clearly show two release periods attributable
to canopy disturbances caused by outbreak of the mountain pine beetle (1940s and 1980s).
We defined the start of a growth release as a year that exhibited a 50% increase with respect to
the mean ring width of the previous 5 years. The end of a release was defined by the year when rings
returned to pre-release levels. Thus, the start and end of the release was compared only with the treering indices that directly preceded the release and not to the whole chronology. Releases that lasted less
than 5 years were ignored as we expected that canopy openings caused by beetle thinning would cause
release periods that would last until full canopy closure was re-established. Although no data exists on the
length of this process, we expected that, for severe outbreaks, it would last more than 5 years. Veblen et
al. (1991a, b) used a similar method for detecting release in Engelmann spruce trees following spruce bark
beetle outbreaks in Colorado.
Lodgepole pine (host) chronologies were developed for each of the 15 stands. In the initial decades
of long tree-ring chronologies, when sample size is inevitably small, identification of releases is unreliable
(Eisenhart and Veblen 2000). Therefore, interpretation of chronologies was limited to where the sample
size was at least five trees per stand.
249
It was difficult to find sufficient non-host trees in the study area to build reliable chronologies for
species other than pine. However, we succeeded in building two non-host chronologies: one spruce
chronology for stand 113 and a Douglas-fir chronology for stand 116 in the Cariboo region. Non-host
chronologies were examined for periods of release and compared to host chronologies to determine if
periods of release in non-host species were synchronous with periods of release in lodgepole pine.
Searching for spatial outbreak patterns
To study the spatial synchrony of mountain pine beetle outbreaks, the entire chronologies were visually
compared and the release periods attributable to beetle-induced thinning were tabulated and plotted for
each sampled stand. The average start and end year of the release and the interval between initial dates of
release were calculated.
Results
Outbreak history based on tree rings
Over 90% of lodgepole pine cores were successfully cross-dated and included in the tree-ring analysis.
The number of cores included in the stand chronologies ranged from 9 to 21 (Table 1). Although one
chronology (stand 113) contained one tree dating to 1758 (243 years old at breast height, Table 1), for
most chronologies the oldest date when the sample size was at least five trees was in the1880s. Therefore
our results can be applied with confidence only to the period after this date, i.e., we provide a beetle
history for the last 120 years.
On average, the 15 chronologies studied showed three fairly synchronous release periods: 1890s, 1940s
and the 1980s (Tables 2 and 3, Figs. 3 and 4). The three releases averaged 13.8 years (Min=5, Max=23
years) in duration and recurred every 42 years (Min=28, Max=53 years), counted from the start of one
release to the start of the next release (Table 2).
The first release (1890s) appears in only 5 of the 12 stands that were old enough to register this release
(Figs. 3,4). The median of the initial release date for these five stands was 1893, but ranged from 1887 to
1898. The average duration of this release was 13.2 years. Examination of fire and beetle scars in discs
from these areas indicates possible activity of these two disturbances simultaneously (Fig. 3). Without
additional sampling and lacking written records, the causes of this release are uncertain.
The second release (Figs. 3, 4) appeared with relative synchrony in 13 of the 15 stands sampled and had
an initial median date of 1935 (Min=1926, Max= 1959). The average duration of this release was 13.6
years (Min= 5, Max= 23 years). The start of the second release occurred, on average, 40.8 years after the
start of the first release. Cross-section samples collected by Hawkes et al. (2004) showed many beetle scans
in this period (Fig. 3).
The third release was evident in 12 of the 15 stands sampled and also appeared with relative synchrony
(Figs. 3, 4). This release had a median initial date of 1982 (Min=1975, Max= 1989) and lasted, on average
14.3 years, and in some stands it still continued in 2000. This release occurred, on average, 42.9 years
after the start of the second release. Cross-section samples also show many beetle scans dating in this
period (Fig. 3).
Non-host. In the two stands that had both host and non-host chronologies constructed (Tables 4 and 5,
Fig. 5), both species responded to canopy disturbance approximately at the same time as lodgepole pine.
Similarly to lodgepole pine, release periods were evident starting in the 1890s, 1930s and 1980s. Release
durations were 8, 25 and 15 years for the first, second and third releases.
250
Table 2. Dates of growth releases attributable to mountain pine beetle
thinning of lodgepole pine stands, duration of release, and interval between
releases, in the Chilcotin Plateau area of BC. Dashed line indicates that there
was no interval.
Stand No.
103
104
105
107
113
116
118
119
121
124
125
126
128
129
130
Overall
Mean
Release Dates
1939-1950
1989-2000
1895-1903
1938-1950
1975-1985
1939-1950
1932-1944
1898-1904
1926-1947
1887-1902
1933-1944
1986-1998
1895-1910
1980-2000
1941-1946
1975-1993
1932-1955
1980-1987
1959-1968
1988-1993
1935-1944
1980-1997
1934-1951
1975-1996
1939-1956
1982-1998
1890-1912
1936-1951
1981-1998
1935-1953
1982-2000
Duration of
release (Years)
11
11
8
12
10
11
12
6
21
15
11
12
15
20
5
18
23
7
9
5
9
17
17
21
17
16
22
15
17
18
18
Interval between adjacent1
releases (Years)
--50
--43
37
------28
--46
53
------34
--48
--29
--45
--41
--43
--46
45
--47
13.8
42.3
1
Three release periods were found: 1890s, 1940s and 1980s. Intervals are between
consecutive release periods.
251
Table 3. Characteristics of lodgepole pine growth releases attributable to stand thinning by
mountain pine beetle outbreaks in the Chilcotin Plateau area of the Cariboo Region.
First release
Second release
Third release
Initial year
End year
Initial year
End year
Initial year
End year
5
5
14
14
12
12
Mean
1893
1906
1937
1951
1981
1995
Median
1895
1904
1935
1950
1982
1997
Range
1887-1898
1902-1912
1926-1959
1944-1968
1975-1989
1985-2000
No. stands
Stand 103
Stand 104
Stand 105
Stand 107
Stand 113
Stand 116
Stand 118
Stand 119
Stand 121
Stand 124
Stand 125
Stand 126
Stand 128
Stand 129
Stand 130
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
YEAR
Figure 3. Release periods attributable to mountain pine beetle outbreaks in Chilcotin Plateau, BC,
inferred from growth-release periods using tree-ring chronologies. Fire (circle with cross in middle) and
mountain pine beetle (star shaped symbol) scar dates are given for each stand. For details of fire and
beetle scars, please see Hawkes et al. (2004). Asterisk indicates start year for the tree-ring chronology.
252
2000
Releases attributable to mountain pine beetle
13
12
11
10
No stands
9
8
7
6
5
4
3
2
1
0
1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990
Decade of release
Figure 4. Histogram of the initial growth release year for 15 lodgepole pine stands in the Chilcotin
Plateau area of BC. Releases are attributable to stand thinning caused by beetle outbreaks occurring
in the late 1890s, 1930s and 1970s. Years indicate interval during which release occurred.
Table 4. Summary data for non-host stand chronologies used to study recurrence of mountain pine beetle
disturbances in the Chilcotin Plateau area of BC.
Location
Stand No.
Species
No. of cores
cross-dated
Chronology
period
Mean Serial
Correlation
Cariboo
113
Spruce
10
1894-2000
0.379
Cariboo
116
Douglas-fir
9
1901-2001
0.764
Table 5. Characteristics of growth releases in non-host trees attributable to mountain pine beetle thinning in the
Chilcotin Plateau area of BC.
First release
Second release
Third release
Initial
End year
year
Location
Stand No.
Species
Cariboo
113
Spruce
1896
1903
1925
1946
1975
1993
Cariboo
116
Douglas-fir
1911
1920
1932
1955
1984
1997
Mean
1904
1912
1928
1951
1980
1995
Initial year End year Initial year End year
253
Discussion
We identified dates of releases caused by potential mountain pine beetle outbreaks using tree ring release
as a proxy for canopy disturbance (Table 2, Fig. 3). Because lodgepole stands do not grow to be very old,
we were able only to examine the disturbance history from the late 19th century forward. Because of a
delayed response in tree growth response to thinning, the initial date of release is not necessarily the year
when mountain pine beetles began to thin the stands. Heath and Alfaro (1990) indicated that the thinning
response of lodgepole pine, expressed as significant increases in ring growth, began 2 to 6 years after the
start of a severe beetle outbreak and peaked 5 to 9 years after. Therefore, the potential mountain pine beetle
outbreaks dates would have started 2 to 6 years prior to the initial release dates indicated in this paper.
There is some uncertainty in the dendrochronological approach when establishing mountain pine
beetle disturbance history, because dendrochronology is unable to distinguish between growth releases
induced by beetle thinning from above-normal periods of growth caused by better than normal climatic
conditions, e.g., above-normal precipitation. In the case of dating defoliating insect outbreaks, the
dendrochronology method makes it possible to separate the climatic signal from defoliator-induced growth
reduction by adjusting the signal of the host tree by that of the non-host tree, as both types of trees
have opposite reactions to defoliation (Swetnam and Lynch 1993; Zhang and Alfaro 2002). Separation
of climatic release from beetle-induced thinning is not possible as both beetle host and non-host trees
respond equally to the thinning action of the beetle (Heath and Alfaro 1990). However, we can be
increasingly re-assured that the 1940s and 1980s releases are beetle-induced because the records indicate
widespread infestations in the 1940s in the Chilcotin area and the 1980s plots were established in areas
with ongoing beetle infestations. Also many cros-section samples from these areas contain beetle attack
scars dating to the 1940’s and 1980’s (Hawkes et al. 2004). For complete certainty, we need samples from
control areas, i.e., from areas where we know beetle outbreaks did not occur. This is impossible for the
early outbreaks (1890s and 1940s), which are not well documented. In the 1980s the outbreak was very
large; therefore, potential control sites occurred only in very different ecosystems, which would make
comparisons inaccurate.
There is some uncertainty as to the cause of the 1890s release, as records are non-existent for this
period. In addition, fire scars in four stands in the Chilcotin date to this period, suggesting that ground
fires also played a role. Apart from beetle, ground fire is the only large-scale canopy disturbance capable
of thinning a lodgepole pine stand. However, comparing the tree ring patterns for trees that originate from
fully documented outbreaks (Heath and Alfaro 1990; Veblen et al. 1991a, b) and with the tree ring signals
in this study, strongly suggests that the 1890s release also represent responses to beetle thinning.
Several of the stands did not record a release in response to the last outbreak. This could be attributed
to the fact that many of the cores were sampled from trees that are old, fire scarred, infested with
mistletoe, and stem and root diseases, and have been previously unsuccessfully attacked by mountain pine
beetle. These trees may not have the resources (i.e., foliar biomass, live cambium, and fine root biomass)
to respond to canopy disturbance in a manner that, using the criteria of this study, would be detected as a
growth release.
The average interval between the first (1890s) disturbance and the second (1940s) was 41 years, and
between the second and third (1980s) disturbance was 43 years. This points to a strong cyclical nature of
beetle outbreaks. The cycle, recorded in the tree rings, consists of thinning of the stand by beetles which
creates a strong and sustained increase in ring-width growth, followed by a gradual decline in ring width
as the stand returns to full site occupancy by lodgepole pine and other species. The average length of the
growth release was 13.2 (1890s), 13.6 (1940s) and 14.3 years (1980s, still ongoing in some stands).
What causes the cycle?
We hypothesize that lodgepole pine stands alternate between a susceptible state and a resistant state,
on average every 42 years, with some variability between locations. Stands in the susceptible state are
254
overstocked, mature stands, usually older than 80 years and with many trees of large diameter. Under
these conditions, trees are stressed and unable to fend off beetle attack (Safranyik et al. 1974). When
conditions such as climate and proximity to active infestations (Shore and Safranyik 1992) are suitable for
population increase, outbreaks develop, which gradually, over the course of an infestation, thin the stand.
Surviving trees benefit from the additional space and resources made available through tree mortality, and
gradually become resistant to beetle invasion. This causes the outbreak to decrease and eventually cease.
Without beetle thinning, stocking increases, as trees accelerate growth and regeneration is recruited into
the overstorey. Thus, gradually, over a process that may last on average 42 years, the stand again becomes
susceptible to outbreaks.
We hope that the recurrence rates established here will assist in forecasting potential outbreaks and in
planning the timber supply of BC.
René I. Alfaro is a research scientist with the Canadian Forest Service, Pacific Forestry Centre.
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