Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects

Abstract

Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.

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Climate Change and Bark Beetles
of the Western United States and
Canada: Direct and Indirect Effects
BARBARA J. BENTZ, JACQUES RÉGNIÈRE, CHRISTOPHER J. FETTIG, E. MATTHEW HANSEN, JANE L. HAYES,
JEFFREY A. HICKE, RICK G. KELSEY, JOSE F. NEGRÓN, AND STEVEN J. SEYBOLD
Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis
of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in
temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among
and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community
associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and
climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive
to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable
spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and
elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.
Keywords: cold tolerance, mountain pine beetle, seasonality, spruce beetle, temperature
climate change components (Dale et al. 2001). Although there
are many possible avenues for atmospheric changes to influ-
ence phytophagous insect outbreaks, because of the direct link
between insect population success and seasonal temperature
(Danks 1987), outbreaks are predicted to be affected dra-
matically by global warming (Bale et al. 2002). Rapid genetic
adaptation of insects to seasonal changes in temperature has
already been documented (Balanyá et al. 2006, Bradshaw and
Holzapfel 2006), and range expansion has occurred in many
cases as species move into new niches created by increasing
temperature (Battistia et al. 2006, Nealis and Peter 2009).
Native bark beetles (Coleoptera: Curculionidae, Scolyti-
nae), which evolved within the coniferous forest ecosystems
of western North America, are key agents of change in these
systems. Beetle outbreaks raise tree mortality rates and can
result in subsequent replacement by other tree species and
plant associations (Veblen et al. 1991). In recent decades,
billions of coniferous trees across millions of hectares have
been killed by native bark beetles in forests ranging from
Mexico to Alaska, and several of the current outbreaks
are among the largest and most severe in recorded history
(Bentz et al. 2009). Although gaps exist in our understand-
ing of the processes, it is clear that mechanisms contributing
to widespread bark beetle outbreaks are complex and
During the 21st century, mean annual global temperature
is expected to increase between 1.8 and 4.0 degrees Cel-
sius (C) as a result of growing atmospheric greenhouse gas
concentrations created by human activities. Across North
America, the rise in temperatures is projected to exceed
global mean increases, particularly at high latitudes and
elevations, and more frequent extreme weather events are
expected (IPCC 2007). Associated changes in precipitation
patterns may result in earlier and longer dry seasons across
the western United States, with a greater frequency and
duration of droughts (Seager et al. 2007). These changes in
climatic conditions over the next century will significantly
affect the condition, composition, distribution, and produc-
tivity of multiple ecosystems (Easterling et al. 2000).
Coniferous forests, which provide essential ecosystem ser-
vices and host a vast array of plant and animal species, are
expected to be significantly affected by shifts in temperature,
precipitation, and atmospheric greenhouse gas concentrations
(McNulty and Aber 2001). By the end of the century, about
48% of the western US landscape is predicted to experience
climate profiles with no contemporary analog to the current
coniferous vegetation (Rehfeldt et al. 2006). Population irrup-
tions of phytophagous insects—disturbance events important
to forest ecosystem functioning—are also directly sensitive to
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include density-independent factors, in addition to spatial
and temporal dependencies at multiple scales (Aukema
et al. 2008, Raffa et al. 2008). Large areas of suitable host
trees of susceptible vigor, age, and density are required for
an outbreak to develop (Fettig et al. 2007). Because bark
beetle population survival and growth are highly sensitive to
thermal conditions, and water stress can influence host-tree
vigor, outbreaks have been correlated with shifts in tempera-
ture (Powell and Logan 2005) and precipitation (Berg et al.
2006). However, a comprehensive synthesis of the direct and
indirect effects of climate change on the population dynam-
ics of bark beetles is lacking.
In this article we assess and synthesize the state of knowl-
edge regarding effects of climate change on bark beetles
that cause extensive conifer mortality in the western United
States and Canada (table 1). We discuss potential direct and
indirect impacts of climate change on multiple aspects of
host trees, and bark beetle community ecology and popu-
lation dynamics. We present two case studies to illustrate
the potential impacts of global warming on the population
outbreak dynamics of eruptive bark beetles.
Direct effects of climate change on bark beetles
Of the hundreds of native bark beetle species in the western
United States and Canada, few species (< 1%) attack and
reproduce in live trees. Frequently referred to as aggressive
bark beetles, these species can kill healthy trees and have the
capacity to cause landscape-scale tree mortality (table 1).
Host selection and colonization behavior by bark beetles are
complex processes that involve both long- and short-range
behavioral components (Graves et al. 2008) with multiple
thresholds and rapid feedback (Raffa et al. 2008). Bark beetle
adults have sophisticated chemoreceptors and behaviors that
allow them to recognize host tree species in addition to the po-
tential defensive capacity of the host. Once a host is selected,
colonization requires overcoming constitutive and inducible
tree defenses, which include anatomical, physical, and chemi-
cal components (Franceschi et al. 2005). These defenses are
overcome only when a critical minimum number of beetles
are recruited to the host tree. This number varies with changes
in host vigor, and therefore will be affected indirectly as host
trees respond to a changing climate (see below). In most
cases, recruitment is facilitated by aggregation pheromones
that attract other colonizing adults (Raffa 2001). Following
aggregation and subsequent mating, adults lay eggs in the
phloem, and larvae excavate feeding tunnels in this tissue or
in the outer bark, depending on the beetle species—a process
that eventually results in the mortality of the host tree. Mature
adult beetles of the next generation tunnel outward through
the bark and initiate flight in search of a new host tree.
Synchronous adult emergence and life-cycle timing, criti-
cal strategies used by many bark beetle species to kill trees,
are in large part dictated by several temperature-dependent
physiological processes. Direct effects of climate change
on bark beetle population dynamics will therefore occur
predominantly through the influence of temperature on life-
history strategies that (a) maintain adaptive developmen-
tal timing leading to synchronized population emergence
and life-cycle timing, and (b) facilitate cold tolerance and
avoidance of low-temperature-induced mortality.
Table 1. Bark beetle species that have the capacity to cause landscape-scale tree mortality in the western United States
and Canada.
Common name Scientific name Major host species
Arizona fivespined ips Ips lecontei Pinus ponderosa, and others
California fivespined ips Ips paraconfusus Pinus attenuata, Pinus contorta, Pinus coulteri, Pinus jeffreyi, Pinus lambertiana,
P. ponderosa, Pinus radiata, Pinus torreyana, and others
Douglas-fir beetle Dendroctonus
pseudotsugae
Pseudotsuga menziesii
Eastern larch beetle Dendroctonus simplex Larix laricina
Fir engraver Scolytus ventralis Abies concolor, Abies grandis, Abies magnifica
Jeffrey pine beetle Dendroctonus jeffreyi P. jeffreyi
Mountain pine beetle Dendroctonus ponderosae Pinus albicaulis, Pinus aristata, Pinus balfouriana, P. contorta, Pinus flexilis,
P. lambertiana, Pinus monticola, P. ponderosa, and others
Northern spruce engraver Ips perturbatus Picea engelmannii, Picea glauca, Picea × lutzii, Picea mariana, Picea sitchensis
Pine engraver Ips pini P. contorta, P. jeffreyi, P. ponderosa, and others
Piñon ips Ips confusus Pinus edulis, Pinus monophylla
Roundheaded pine beetle Dendroctonus adjunctus Pinus arizonica, Pinus engelmannii, P. flexilis, Pinus leiophylla, P. ponderosa,
Pinus strobiformis
Southern pine beetle Dendroctonus frontalis Pinus engelmannii, P. leiophylla, P. ponderosa
Spruce beetle Dendroctonus rufipennis Picea engelmannii, Pi. glauca, Pi. sitchensis
Western balsam bark beetle Dryocoetes confusus Abies lasiocarpa, and others
Western pine beetle Dendroctonus brevicomis P. coulteri, P. ponderosa

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Developmental timing. The time required to complete a gen-
eration varies among bark beetle genera, species within a
genus, populations within a species, and individuals within
a population. Some species, such as the western pine beetle,
Dendroctonus brevicomis LeConte, and the piñon ips, Ips con-
fusus LeConte, produce more than one generation per year.
Others, including the spruce beetle, Dendroctonus rufipennis
[Kirby], and mountain pine beetle, Dendroctonus ponderosae
Hopkins, require one, two, or even three years to produce
a single generation, depending on the temperature profile
at a particular locale within their large geographic ranges.
Moreover, as a result of the evolution of narrow thermal
windows that minimize excess physiological costs (Pörtner
and Farrell 2008), the adaptation of life-cycle timing to a local
climate can result in a genetically predetermined variability in
temperature response across the range of a single bark beetle
species (Bentz et al. 2001). Although little is known about spe-
cific temperature-dependent developmental processes of many
bark beetle species, research suggests that at least two predomi-
nant strategies, diapause and direct temperature control, have
evolved to maintain appropriate life-cycle timing. Each strat-
egy may be differentially affected by climate change.
Diapause is a dynamic, endocrine-mediated and envi-
ronmentally driven dormancy that occurs at a specific life
stage. Diapause not only offers a mechanism to keep insects
synchronized with their environment and food availability
but also provides tolerance to environmental extremes (Tau-
ber et al. 1986). Although not all species have been inves-
tigated, diapause has been demonstrated or suggested to
exist in five bark beetle species indigenous to western North
America: (1) fir engraver, Scolytus ventralis LeConte (Scott
and Berryman 1972); (2) spruce beetle (Hansen et al. 2001);
(3) Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins
(Ryan 1959); (4) eastern larch beetle, Dendroctonus simplex
LeConte (Langor and Raske 1987); and (5) pine engraver,
Ips pini (Say) (Birch 1974). The effect of climate change on
these species will depend on the life stage in which diapause
occurs. For example, high summer temperatures prevent
facultative prepupal diapause of the spruce beetle, resulting
in beetles that complete their life cycles in a single year, com-
pared with two years when the diapause is invoked, potentially
leading to exponential population growth (Hansen and Bentz
2003). Conversely, an obligatory adult diapause initiated by
low temperatures, as in spruce beetles and Douglas-fir beetles,
could be disrupted by higher minimum temperatures.
Direct temperature control also maintains appropriate
seasonality in bark beetles (sensu Danks 1987). In the moun-
tain pine beetle, for example, life stage–specific develop-
mental thresholds aid in synchronizing adult emergence at
appropriate times of the year (Powell and Logan 2005). Later
life stages (e.g., fourth instar larvae) have higher temperature
threshold requirements for development than earlier life
stages. The higher temperature thresholds serve to synchro-
nize individuals during autumn, as temperatures decrease,
and also prevent development to the cold-intolerant pupal
life stage (Logan and Bentz 1999). Temperature pattern
throughout a life cycle is therefore critical to appropriate
seasonality, and predicted rises in temperature could affect
both developmental thresholds and rates in multiple life
stages. Additional fitness parameters potentially affected
include higher adult longevity and prolonged adult emer-
gence and flight. Warming trends have been associated with
shifts in generation duration for populations of spruce beetle
in Alaska, Utah, and Colorado (Hansen et al. 2001, Werner
et al. 2006), and mountain pine beetle in high-elevation
forests (Bentz and Schen-Langenheim 2007).
Little is known about developmental strategies of the many
bark beetle species indigenous to the southwestern United
States and Mexico. These species are significant because of
their potential to move northward with climate change by
following range expansion of current hosts or by adapting
to novel hosts. The roundheaded pine beetle, Dendroctonus
adjunctus Blandford, for example, colonizes pines from
southern Utah and Colorado south into Guatemala, and
its life-cycle timing differs depending on geographical loca-
tion (Chansler 1967). Although it appears that the northern
extent of this bark beetle species, and others restricted to the
southwestern United States and Mexico, is currently limited
by climate and not host-tree availability (Salinas-Moreno et
al. 2004), information is lacking on temperature-dependent
physiological aspects of its life history that may be range
limiting. The polyphagy exhibited by the roundheaded pine
beetle and other bark beetles currently found in Mexico sug-
gests they may do well in pine species they would encoun-
ter in a northward range expansion, potentially invading
niches vacated by beetle species whose population success
is disrupted by climate change. Novel species assemblages
could be created, as exemplified by recent documentation of
the Mexican pine beetle, Dendroctonus mexicanus Hopkins,
formerly thought to be limited to northern Mexico, and the
southern pine beetle, Dendroctonus frontalis Zimmermann,
attacking the same individual pine trees in the Chiricahua
Mountains of Arizona. This incidence is potentially a result
of an increase in climate suitability and the number of beetle
generations per year (Waring et al. 2009).
Cold tolerance. In addition to climate controls on adaptive
developmental timing, mortality from cold exposure is con-
sidered a key temperature-related factor in bark beetle pop-
ulation dynamics, although there are few data for most bark
beetle species. Cold hardening is the dynamic acquisition of
cold tolerance through biochemical and physiological pro-
cesses, and is most often triggered by cold temperatures (Lee
1989). Similar to the adaptations in temperature-dependent
developmental timing described above, cold tolerance will
undoubtedly vary among genera and within populations
of the same bark beetle species because of the temperature-
dependent nature of cold hardening and the metabolic
costs involved (Régnière and Bentz 2007). Spruce beetles
and mountain pine beetles accumulate cryoprotectant com-
pounds such as glycerol as temperatures decline during
autumn (Miller and Werner 1987, Bentz and Mullins 1999).

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Cold tolerance for these insects is therefore greatest during
the winter months, and lowest during periods of glycerol
synthesis and catabolism in autumn and spring, respectively,
suggesting a direct correlation between increasing minimum
temperatures associated with climate change and a reduc-
tion in cold-induced beetle mortality.
Indirect effects of climate change on bark beetles
Bark beetle population success will be influenced indirectly
by the effects of climate on community associates, host-tree
vigor, and host abundance.
Community associates. Upon colonizing a tree, bark beetles
introduce an array of fungi, bacteria, nematodes, and mites
that can significantly influence their fitness (Hofstetter
et al. 2006, Cardoza et al. 2008). The relationship between
bark beetle species and their associates is often described as
symbiotic, as many bark beetles have evolved morphological
adaptations to assist in the transport of specific associates,
derive nutritional and defensive benefits from them, or both
(Klepzig and Six 2004). For example, spruce beetles have
specialized body structures to carry associated nematodes
(Cardoza et al. 2008), and mountain pine beetles and west-
ern pine beetles have structures to transport symbiotic fungi
(Klepzig and Six 2004). Developing mountain pine beetle
larvae acquire vital nutrients not found in plant tissue by
feeding on two fungi, Grosmannia clavigera and Ophios-
toma montium, the hyphae of which spread throughout the
phloem and sapwood following inoculation into the tree by
attacking beetles. Although both fungi are important, evi-
dence suggests that one species (G. clavigera) supports faster
brood development, larger body size, and higher brood
production than does the other (Bleiker and Six 2007). Each
fungus possesses different thermal ranges for optimal growth
(Rice et al. 2008), and seasonal temperature dictates which
fungal species is ultimately vectored by dispersing beetles
(Six and Bentz 2007). Because benefits to the mountain pine
beetle are not the same for each fungal species, shifts in tem-
perature and precipitation associated with climate change
could indirectly affect mountain pine beetle population suc-
cess through direct effects on fungal symbionts.
Other community associates and trophic interactions,
including avian predators and insect parasitoids and preda-
tors (McCambridge and Knight 1972, Boone et al. 2008),
undoubtedly will also be influenced by abiotic factors associ-
ated with climate change. The ecological roles and tempera-
ture dependencies of the majority of bark beetle community
associates are not well understood, which hampers full com-
prehension of the consequences of climate change on bark
beetle population dynamics.
Host-tree physiology. Climate change will influence bark
beetle–host interactions in complex or nonlinear ways.
Although plants tend to thrive in carbon-enriched atmo-
spheres, mature wildland conifer species are not necessarily
carbon limited and therefore may not express large growth
increases in response to increased carbon dioxide (CO2;
Millard et al. 2007). However, when grown in the presence
of elevated CO2, plants accumulate carbon and the carbon-
to-nitrogen ratio increases (Zvereva and Kozlov 2006),
resulting in reduced nutrition (low nitrogen content) for
insect herbivores (Mattson 1980). To compensate, insects
consume more but grow more slowly, a trade-off with the
potential to disrupt phenological synchrony important to
bark beetle survival, in addition to prolonging exposure to
mortality agents. The indirect negative effects of enhanced
CO2 on bark beetle growth and survival are, at least in part,
outweighed by other climate-change-induced effects on host
trees, including reduced defenses.
An important consequence of climate change is higher
frequency and severity of droughts (Seager et al. 2007). In
addition to directly affecting tree death through carbon
starvation and cavitation of water columns within the
xylem, climatic water stress can have profound effects on
tree susceptibility to bark beetle attack. To avoid drought-
induced hydraulic failure, plant stomates close to restrict
transpiration. However, stomatal closing also limits car-
bon assimilation, which can result in carbon starvation
(McDowell et al. 2008). Changes in carbon assimilation will
also alter within-plant allocation of carbohydrates available
for growth, defense, and tissue repair, affecting the produc-
tion of constitutive or inducible chemical defenses (Herms
and Mattson 1992) and thus a tree’s ability to respond to
bark beetle invasions. Hydraulic failure may be further
amplified when water transport is interrupted by symbiotic
fungi inoculated into trees during the bark beetle attack pro-
cess (McDowell et al. 2008). Drought-induced alterations to
tree defensive capacity ultimately reduce the threshold num-
ber of beetles necessary for a successful mass attack, thereby
relaxing the constraints on critical thresholds that must be
surpassed for bark beetle outbreaks to occur.
Although the mechanisms connecting drought stress to bark
beetle outbreaks are not well understood in western North
American ecosystems, it is clear that effects will vary regionally
and by bark beetle–host species complex because of differ-
ences in critical feedbacks driving beetle population dynamics,
as well as physiological differences among tree species. For
example, the regional-scale piñon ips epidemic associated with
severe drought in the southwestern United States (Breshears
et al. 2005) ended as the supply of drought-stressed trees was
exhausted. In contrast, although drought stress facilitated pro-
gression from an incipient to epidemic mountain pine beetle
population in British Columbia, a significant correlation with
precipitation was no longer found after the beetle popula-
tion became self-amplifying (Raffa et al. 2008). In both cases,
elevated temperatures, which directly influence bark beetle
population success in ways described above, were associated
with drought conditions that affected tree stress.
Host-tree distribution. The distribution of coniferous vegeta-
tion across western North America resulted from climatic
shifts dating back millions of years (Brunsfeld et al. 2001), in

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addition to more recent recolonization of deglaciated lands
from multiple refugial populations (Godbout et al. 2008). In
response to an approximately 2C increase in temperature,
significant changes in community composition occurred
over the past several thousand years, including the forma-
tion of new communities, many of which no longer exist
today (Shugart 2003). As tree species ranges shifted, so did
the ranges of phytophagous insects such as bark beetles
as they tracked environmental changes and followed host
tree species (Seybold et al. 1995). These historical patterns
foreshadow large modifications to current forest ecosystem
dynamics as climate change accelerates.
In a rapidly changing environment, coniferous tree spe-
cies will persist through migration or adaptation to new
conditions, or they will go locally extinct. The fate of any
individual species will depend on multiple factors, includ-
ing phenotypic variation, fecundity, and biotic interactions
(Aitken et al. 2008). On the basis of the best existing data
for 130 tree species in North America and associated climate
information, and assuming no limitations to individual tree
growth, McKenney and colleagues (2007) predicted that the
average range for a given tree species will decrease in size by
12% and will shift northward by 700 kilometers (km) dur-
ing this century. Under a scenario in which survival occurs
only in areas where anticipated climatic conditions overlap
with current climatic conditions, niches for tree survival will
decrease in size by 58% and shift north by 330 km. More
specific to this synthesis, range predictions for several hosts
of notable bark beetle species provide striking comparisons.
Relative to contemporary distributions, by 2060 the range
of Engelmann spruce, Picea engelmanni Parry ex Engelm.,
a principal host for spruce beetle, is projected to decrease by
47% within the contiguous western United States. During
the same period, the areas inhabited by ponderosa pine, Pinus
ponderosa Laws., and Douglas-fir, Pseudotsuga menziesii
Franco, hosts of several bark beetle species, are projected to
increase by 11% and 7%, respectively (Rehfeldt et al. 2006).
If we assume that current tree distributions are driven by
nonclimatic biotic factors such as bark beetles in addition
to climate, models used to forecast specific tree distributions
may already include an inherent measure of the influence of
bark beetles on tree species distributions.
Case studies
Despite uncertainties, changes in temperature predicted by
general circulation models can be an important basis for
estimating biological response to changing conditions (Millar
et al. 2007). When used in conjunction with quantitative
models that are based on a mechanistic understanding of
biological responses to temperature, results can provide
insight into ecosystem responses to climate change. We
explore the potential effects of changing climate on bark
beetle outbreak dynamics using two case studies: (1) spruce
beetle and (2) mountain pine beetle. We chose these species
for analysis because mechanistic models for predicting tem-
perature effects on population success have been developed.
Because models incorporating the direct effects of climatic
changes on conifers and their subsequent response to bark
beetle attacks are not currently available, our quantitative
assessments are based solely on bark beetle population suc-
cess. Our assessment assumes no change in current tree distri-
butions, and that thermal conditions conducive to bark beetle
population success result in increased levels of tree mortality,
although we do not explicitly model the impacts to forests.
Weather data and bark beetle model projections. Simulated
past and future climates (1961–2100) were obtained from
the Canadian Regional Climate Model (CRCM) version
4.2.0 runs ADJ and ADL (Music and Caya 2007). We used
the Intergovernmental Panel on Climate Change A2 emis-
sions scenario, which results in relatively high projected
warming in 2100 among all scenarios (IPCC 2007), but
which has been realistic thus far given emissions estimates
in the last 20 years (Raupach et al. 2007). The simulations
provided a 201 × 193 gridded database of daily maximum
and minimum air temperatures and precipitation over
North America with a horizontal resolution of 45 km (true
at 60N). From these data, 30-year normals were computed
for each decade in the interval between 1961 and 2100, and
the “delta” method (differences between modeled decadal
normals and the reference period 1961–1990) was used
to generate unbiased decadal sets of 30-year normals into
the future. Observed (1961–1990) and predicted (2001–
2030, 2071–2100) normals were used to generate stochastic
daily minimum and maximum temperatures (Régnière and
St-Amant 2007) for input to the spruce beetle and mountain
pine beetle models.
The spruce beetle and mountain pine beetle models
(described below) are driven by hourly temperature (interpo-
lated between the minimum and maximum temperatures on
successive days) and were integrated with weather and topog-
raphy using BioSIM (http://cfs.nrcan.gc.ca/factsheets/biosim).
We ran models for 25,000 simulation points across North
America, north of Mexico, with an emphasis on mountain-
ous and forested areas. Elevations were obtained from digital
elevation models (DEM) at 30-as resolution (http://eros.usgs.
gov/products/elevation/gtopo30/gtopo30.html). Because weather
inputs are stochastic and responses are nonlinear, we replicated
each model run 30 times, each with a stochastically different
one-year temperature time series for each simulation point
and normals period. We averaged model output over repli-
cates. From these averaged outputs, maps were generated by
universal kriging, with elevation provided by the input DEM as
external drift variable (Hudson and Wackernagel 1994). Prob-
ability values were linearized by logistic transformation before
interpolation and before the maps were back transformed.
Maps depict a continuous measure of bark beetle population
success (as defined by each insect model) for the three climate
periods. Model output was masked using polygons that esti-
mate the 20th-century locations of spruce (for spruce beetle)
and pine (for mountain pine beetle) habitat in the United
States and Canada (Little 1971).

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Spruce beetle. The spruce beetle is distrib-
uted in spruce forests throughout western
North America, across the boreal forest of
Canada, and into the northeastern United
States. Extensive spruce beetle outbreaks have
occurred throughout the contiguous western
United States, Alaska, and western Canada
during the past decade, affecting more than 3
million hectares (ha) of forest (USDA Forest
Service, Forest Health Protection, and Natural
Resources Canada, Canadian Forest Service).
In Alaska alone, the spruce beetle affected ap-
proximately 2 million ha during a prolonged
outbreak in the 1980s and 1990s (Werner et al.
2006). High summer temperatures are cor-
related with a rising proportion of beetles that
complete a generation in a single year rather
than in two years, contributing significantly to
population growth (Hansen and Bentz 2003)
and to greater levels of spruce-beetle-caused
tree mortality (Berg et al. 2006). To examine the
direct effects of higher temperature on spruce
beetle population success and growth, we used
an empirical model that predicts spruce beetle
life-cycle duration as a function of hourly air
temperature (Hansen et al. 2001). A higher
probability of one-year beetles translates to a
higher probability of a population outbreak.
Spruce beetle model results. Model predic-
tions suggest that during the historical period
1961–1990, the majority of spruce forests in
Alaska, and those at high elevations in the con-
tiguous western United States and northern
latitudes of Canada, would have a moderate to
low probability of spruce beetle populations
developing in a single year (figure 1a, 1d). In
the period 2001–2030 and again from 2071 to
2100, we would expect substantial increases
in spruce forest area with high probability of
spruce beetle offspring produced annually
rather than semiannually (figure 1b, 1c, 1e, 1f).
By the end of the century, the change in tem-
peratures across the boreal forests of central
Canada may cause markedly higher probabil-
ity of spruce beetle outbreak potential, based
on developmental timing alone. A model for
predicting the cold tolerance of this insect is not available. In
addition to favorable weather, large expanses of mature spruce
forest are required for a widespread outbreak. Therefore,
although spruce beetle outbreak potential will be enhanced
by higher temperatures throughout the century, reductions in
the range of Engelmann spruce in the western United States,
also as a result of climate change (Rehfeldt et al. 2006), could
cause an overall reduction in long-term spruce beetle impacts
in that region.
Mountain pine beetle. The mountain pine beetle ranges
throughout southern British Columbia, portions of eastern
Alberta, and most of the western United States. The geo-
graphic distribution of the beetle generally reflects the range
of its primary hosts (table 1), although the range of lodgepole
pine extends further to the north and ponderosa and other
pine species further to the south than where mountain pine
beetles are currently found. In the past decade, widespread
mountain pine beetle outbreaks in British Columbia and
Figure 1. Predicted probability of spruce beetle offspring developing
in a single year in spruce forests across the range of this insect in
North America during three climate normals periods: (a) 1961–1990,
(b) 2001–2030, and (c) 2071–2100, and only in the western United
States in (d) 1961–1990, (e) 2001–2030, and (f) 2071–2100. Higher
probability of one-year life-cycle duration translates to higher probability
of population outbreak and increased levels of spruce-beetle-caused tree
mortality. Model results are shown only for areas estimated to be
20th-century spruce habitat (from Little 1971).

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accumulation and loss influencing mountain pine beetle
supercooling capacity. We ran the model on the same series of
30-year weather data for each simulation point and set of cli-
matic normals, and we calculated the location average prob-
ability of survival. A higher probability of low-temperature
survival translates to a higher probability of mountain pine
beetle population success.
the northern and central US Rocky Mountains have been
severe and long lasting, affecting more than 25 million ha
(USDA Forest Service, Forest Health Protection, and Natu-
ral Resources Canada, Canadian Forest Service). Population
outbreaks are also occurring in areas outside the recorded
historical range, including lodgepole pine forests in central
British Columbia and lodgepole pine and jack pine hybrids,
P.banksiana Lamb., in western Alberta
(Nealis and Peter 2009). Significant tree
mortality caused by mountain pine bee-
tles, relative to historical records, has also
recently occurred in high-elevation pine
forests across the western United States
(Gibson et al. 2008). We analyzed the in-
fluence of future temperature patterns on
mountain pine beetle population success
within its current range in the western
United States and Canada using models
describing the insect’s seasonality and tol-
erance to cold. Because pine hosts extend
beyond the current mountain pine beetle
range, and because the beetle is a known
polyphage, we also provide projections
of the potential for expansion into pine
forests of northern, central, and eastern
Canada and the eastern United States.
The seasonality model used here was
described by Logan and Bentz (1999),
analyzed by Powell and Logan (2005),
and used by Hicke and colleagues (2006).
In addition to predicting the probability
of beetles developing in a single year, as
described above for the spruce beetle,
a constraint on the timing of adult emer-
gence is included in the model to further
describe the adaptive nature of a particu-
lar temperature regime to the mass attack
process and subsequent population sur-
vival. In the model, if annual adult emer-
gence occurs before 1 June or after 30
September for three or more consecutive
years, that temperature time series is con-
sidered maladaptive. A higher number of
years (out of the 30-model-run replicates
for each stochastically different one-year
temperature time series) that were not
part of a maladaptive series translates to
a higher probability of mountain pine
beetle success, and hence a higher risk of
associated tree mortality. We also used
the cold tolerance model developed by
Régnière and Bentz (2007) to predict the
probability of annual survival given a
one-year temperature regime. This model
describes the dynamic temperature-
dependent process of polyhydric alcohol
Figure 2. Predicted probability of mountain pine beetle adaptive seasonality
(a–c) and cold survival (d–f ) in pine forests of the western United States during
three climate normals periods: (a) and (d) 1961–1990; (b) and (e) 2001–2030;
and (c) and (f) 2071–2100. High probability of adaptive seasonality and cold
survival suggests increased population success and increased levels of mountain
pine beetle-caused tree mortality. Model results are only shown for areas
estimated to be 20th-century pine habitat (from Little 1971).

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Articles
the seasonality model, these results suggest that substantial
warming would disrupt the emergence timing and generation
duration required for population success. Across pine habitats
in the central and eastern United States, adaptive seasonality
remains low throughout the century. An increase in low-
temperature survival is predicted for spatially isolated areas
in Canada, including west-central Alberta, where mountain
pine beetle has recently been found attacking lodgepole/
jack pine hybrids (Nealis and Peter 2009). By the end of
Mountain pine beetle model results. Temperature data from the
historical period 1961–1990 show that the majority of the area
within the current range of the mountain pine beetle had low
predicted probability of adaptive seasonality, although scat-
tered areas throughout the area had moderately high probabil-
ity (figures 2a, 3a). During this same period, low temperature
survival would have been high in coastal regions and other
low-elevation forests across the current range, although quite
low in high-elevation areas of the United States and Canadian
Rocky Mountains (figures 2d, 3d). As
temperatures rise throughout this cen-
tury, the area suitable for both adaptive
seasonality and low-temperature survival
is predicted to grow, although results are
highly spatially variable (figures 2b, 2e,
3b, 3e). Notably, both models predict
greater probability of population suc-
cess in portions of the current range that
have experienced significant increases in
tree mortality caused by mountain pine
beetles during the past decade, includ-
ing high-elevation forests of the western
United States and Canadian Rocky Moun-
tains, and lodgepole pine forests in central
British Columbia, Colorado, and central
Idaho. We note that this insect’s flexibility
in life-history strategies appears greater
than previously anticipated (Bentz and
Schen-Langenheim 2007), and our work-
ing definition of adaptive seasonality and
associated rules that drive the seasonality
model may be too restrictive. Moreover,
observed genetic variability in response to
temperature (Bentz et al. 2001) is not cur-
rently incorporated, and the development
model used for this analysis was derived
using data from populations in north-
ern Utah and central Idaho. Predictions
for mountain pine beetle in the south-
western United States, in particular, may
differ as new developmental parameters
are incorporated. An updated modeling
framework that addresses these concerns
is being developed.
Our modeling results provide some
insight into concerns expressed about the
potential for mountain pine beetle range
expansion across the boreal pine forests
of central Canada and into pine forests
of central and eastern United States. The
potential for adaptive seasonality in central
Canada decreases dramatically from the
historical period to the end of this cen-
tury, with high probability of population
success restricted to northern provinces
(figure 3a, 3b, 3c). Given assumptions of
Figure 3. Predicted probability of mountain pine beetle adaptive seasonality
(a–c) and cold survival (d–f ) across the range of pine species in the United
States and Canada during three climate normals periods: (a) and (d) 1961–
1990; (b) and (e) 2001–2030; and (c) and (f) 2071–2100. Mountain pine beetle
outbreak populations are currently restricted to pine forests in the western
United States, central British Columbia and west-central Alberta. Based
solely on the modeled response of mountain pine beetle to temperature, results
suggest that by the end of this century probability of range expansion across
Canada and into central and eastern US forests will be low to moderate.

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developmental timing, and vice versa. Although detailed in-
formation on temperature-dependent physiological processes
is not available for the majority of bark beetle species in the
southwestern United States and Mexico, many of these species
are currently limited by climate rather than host availability,
suggesting a high potential for range expansion northward.
Developmental life-history strategies have evolved to maintain
appropriate seasonality, and higher temperatures may not
always translate into population growth or range expansion
without adaptation to rapidly changing environmental con-
ditions, a phenomenon documented in several insect species
(Bradshaw and Holzapfel 2006). Because the extreme dif-
ference in generation times between bark beetles and their
coniferous hosts dictates a faster relative rate of adaptation
for beetles, this is a critical missing component in our predic-
tions. Models that incorporate genetic variability in bark beetle
temperature-dependent parameters are also needed.
Despite uncertainty in forecasts of future climate param-
eters that have been downscaled to a forest landscape,
predictions of trends in bark beetle population success
as a function of the direct effects of temperature will be
instrumental in development and application of strate-
gies for management of future forests. There is clearly
a need, however, for a better understanding and more
refined models that integrate indirect effects of climate
change on host trees with bark beetle population success,
as well as interactions among bark beetle outbreaks and
other forest disturbances. For example, drought and other
the century, cold-survival probability substantially increases
across Canada, although in areas key to mountain pine
beetle migration in central Canada, the probability for low-
temperature survival remains low. Our model results suggest
that without adaptation to increasing temperature, the prob-
ability of mountain pine beetle range expansion across jack
pine forests and into eastern US pine forests will remain low
to moderate throughout this century (figure 3c, 3f).
Conclusions
Bark beetle response to climate change can be characterized by
a high degree of complexity and uncertainty, as populations
are influenced directly by shifts in temperature and indi-
rectly through climatic effects on community associates and
host trees. Because changes in climate will not be uniformly
distributed across years, and not all temperature-dependent
processes will be equally affected, a mechanistic understand-
ing is imperative for making predictions of direct effects of
climate change on future population trends. On the basis of
temperature projections from the CRCM and mechanistic
models developed for spruce beetle and mountain pine beetle,
we expect positive changes in thermal regimes conducive to
population success of both species throughout this century.
Significant temporal and spatial variability in thermal suit-
ability is predicted, however, emphasizing the complexity in
both the thermal habitat and temperature-based physiological
processes of these insects. Temperature profiles that promote
cold-temperature survival may not also result in appropriate
Figure 4. Mountain pine beetle–killed whitebark pine on the Bridger Teton National Forest, Wyoming. The photograph was
taken on 13 July 2009 and shows trees attacked and killed over several years. Following mountain pine beetle attack, tree
foliage turns yellow, orange, then red over a one- to two-year period. Eventually the needles drop to the forest floor, leaving a
grey canopy. Photograph: Courtesy of Wally Macfarlane.

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Barbara J. Bentz (bbentz@fs.fed.us) and E. Matthew Hansen are with the US
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