Bark Beetle and Fire Interactions in Western Coniferous Forests: Research Findings
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Bark Beetle and Fire
Interactions in Western
Coniferous Forests:
Research Findings
Christopher J. Fettig, Sharon M. Hood, Justin B. Runyon, and Chris M. Stalling
ative bark beetles and wildfires Figure 1—Tree mortality following a bark beetle outbreak in the Sierra Nevada in California.
N are important disturbances
in western coniferous forests.
Bark beetles can colonize and kill trees
California experienced a severe drought from 2012 to 2015, stimulating a large bark beetle
outbreak in the central and southern Sierra Nevada. Most tree mortality was caused by western
pine beetle (Dendroctonus brevicomis), which readily colonizes drought-stressed ponderosa
of all species, ages, and sizes, but each pine (Pinus ponderosa), but other tree and shrub species were also affected. About 89 percent
species exhibits unique host preferences of the ponderosa pines in the three largest diameter classes were killed (Fettig and others 2019),
and impacts. Some bark beetles cause representing the loss of an important structural component of these forests. Mortality of
extensive levels of tree mortality (table sugar pine (Pinus lambertiana), caused primarily by mountain pine beetle (Dendroctonus
1), as demonstrated by mountain ponderosae), was also substantial (48 percent). In total, 49 percent of the trees died between
pine beetle (Dendroctonus ponderosae) 2014 and 2017. Photo: C. Fettig, USDA Forest Service.
in several pines, western pine beetle
(Dendroctonus brevicomis) in ponderosa
pine (Pinus ponderosa), Douglas-fir beetle In general, bark beetles require
(Dendroctonus pseudotsugae) in Douglas- living phloem (the layer of cells
fir (Pseudotsuga menziesii), and spruce Christopher Fettig is a research entomologist
within the inner bark that transports for the Forest Service, Pacific Southwest
beetle (Dendroctonus rufipennis) in several photosynthates (sugars) within the
spruces. Other bark beetles are secondary Research Station, Davis, CA; Sharon Hood is
tree) to reproduce. When bark beetle a research ecologist for the Forest Service, Rocky
agents that colonize stressed, dead, populations are low, the beetles create
or dying trees. The impacts of these Mountain Research Station, Missoula, MT;
small gaps in the forest canopy by Justin Runyon is a research entomologist for
secondary agents often go unnoticed, colonizing and killing trees stressed by
while the former occasionally drive the Forest Service, Rocky Mountain Research
age or other factors. During bark beetle Station, Bozeman, MT; and Chris Stalling is a
headlines in large newspapers. outbreaks, large numbers of trees can be biologist for the Forest Service, Rocky Mountain
Research Station, Missoula, MT.
Fire Management Today 14 JANUARY 2021 • VOL. 79 • NO. 1
Table 1—Bark beetles recognized as causing substantial levels of tree mortality during outbreaks in the Western United States.
Current knowledge of effects on
Common name Scientific name Common host(s)
wildfire behavior and severitya
Lodgepole pine, sugar pine,
California fivespined ips Ips paraconfusus Low
ponderosa pine
Douglas-fir beetle Dendroctonus pseudotsugae Douglas-fir Moderate
White fir, grand fir, California
Fir engraver Scolytus ventralis Low
red fir
Jeffrey pine beetle Dendroctonus jeffreyi Jeffrey pine Low
Whitebark pine, lodgepole pine,
Mountain pine beetle Dendroctonus ponderosae limber pine, sugar pine, western High
white pine, ponderosa pine
Northern spruce engraver Ips perturbatus White spruce, Lutz spruce Low
Lodgepole pine, Jeffrey pine,
Pine engraver Ips pini Low
sugar pine, ponderosa pine
Pinyon ips Ips confusus Pinyon pine(s) Low
Engelmann spruce, white
Spruce beetle Dendroctonus rufipennis Moderate
spruce, Lutz spruce
Western balsam bark beetle Dryocoetes confusus Subalpine fir Low
Western pine beetle Dendroctonus brevicomis Ponderosa pine, Coulter pine Low
a. Level (low, moderate, or high) defined in relation to knowledge of the effects imposed by mountain pine beetle outbreaks, which have been most intensively studied.
killed over extensive areas (fig. 1), often to repel beetle attack (Kolb and others In this article, we consider two common
adversely affecting timber and wood 2016). Accordingly, recent bark beetle interactions between bark beetles and
fiber production, water quality and outbreaks have been correlated with wildland fires:
quantity, fish and wildlife populations, shifts in temperature and precipitation
opportunities for outdoor recreation, caused by climate change. In some 1. The effects of fuel reduction
and biodiversity and carbon storage, forests, increases in tree density have treatments (prescribed fire and
among other ecological goods and exacerbated the effect by providing an mechanical thinning) and wildfires
services (Morris and others 2018). abundance of hosts and by increasing on bark beetles; and
competition among hosts for limited 2. The effects of bark beetle outbreaks
HISTORIC OUTBREAK LEVELS resources, making trees more vulnerable and associated levels of tree
The amount of tree mortality caused to beetle attacks. mortality on fuels and wildfire
by bark beetles in the Western United behavior and severity.
Wildfires have sculpted many western
States has exceeded that caused by
forests for millennia, reducing the We briefly describe the current state
wildfires in the last 3 decades (Hicke
quantity and continuity of fuels, of knowledge and identify gaps in
and others 2016), and several recent
discouraging establishment of fire- knowledge needed to make informed
outbreaks are considered the most
intolerant tree species, and influencing management decisions.
severe in history. Since 2000, for
the susceptibility of forests to bark
example, about 25.5 million acres (10.3
beetle outbreaks and other disturbances. EFFECTS OF FUEL
million ha) in the Western United
Climate change is increasing the REDUCTION TREATMENTS
States have been affected by mountain
number of large wildfires (fires greater
pine beetle. Activity peaked in 2009, ON BARK BEETLES
than 1,000 acres (400 ha) in size), the
with 8,842,698 acres (3,578,513 ha) Tens of millions of acres of forest in the
frequency of wildfires, the length of
affected in that year alone. Western United States are classified as
the wildfire season (by up to 90 days
Bark beetles are cold-blooded in some locations), and the cumulative having moderate to high fire hazards.
organisms highly sensitive to changes area burned (Vose and others 2018). Efforts to lower hazards focus on
in temperature, which influence their Suppression costs and risks to homes reducing surface fuels, increasing
survival and population growth (Bentz and other infrastructure are also the height to live crowns, decreasing
and others 2010). Drought stress increasing (Flannigan and others 2006). crown bulk density, and retaining large
adversely affects the ability of conifers trees of fire-resistant species such as
Fire Management Today 15 JANUARY 2021 • VOL. 79 • NO. 1
Fettig and McKelvey (2014) monitored
A common management concern is that bark beetles the effects of fuel reduction treatments
on levels of tree mortality at Blacks
might colonize and kill trees injured by prescribed fire. Mountain Experimental Forest in
California over a 10-year period. Twelve
experimental plots (ranging from
190 to 356 acres (76–142 ha)) were
ponderosa pine (Agee and Skinner and others (2006) showed that chipping established to create two distinct forest
2005). When applied under prescription, submerchantable and unmerchantable structural types: midseral stage (with
planned ignitions and their mechanical ponderosa pines and depositing the low structural diversity) and late-seral
surrogates (such as thinning from below) chips back into treated stands increases stage (with high structural diversity).
are generally effective in meeting fuel the risk of infestation by several species Following harvesting, half of each plot
reduction goals (McIver and others of bark beetles in the Southwestern was treated with prescribed fire.
2013; Stephens and others 2012). For United States. The effect was due
example, the effectiveness of prescribed to large amounts of monoterpenes A total of 16,473 trees (9 percent of all
fire for treating surface and ladder being released during chipping, which trees) died. Mortality was concentrated:
fuels to reduce the incidence of passive enhanced attraction to bark beetles.
z On plots with high structural diversity
crown fire (that is, the torching of small Impacts were greater from chipping
(64 percent);
groups of trees) is well supported by in spring (April–May) than in late
modeling of predicted fire behaviors summer (August–September) because z On burned-split plots (61 percent);
(Stephens and others 2009) and by spring is the time of peak flight activity z Within the two smallest diameter
empirical research (Ritchie and others for several species of bark beetles in classes (87 percent); and
2007). Furthermore, results from the the Southwestern United States as
z During the second sample period (3 to
National Fire and Fire Surrogate Study, they search for new hosts. If possible,
5 years after prescribed burns).
the largest study of its kind (www. chipping should be conducted in fall to
frames.gov/ffs/about), indicate that the minimize tree losses to bark beetles if Most mortality was caused by bark
incidence of active crown fire is best the chips will remain onsite. beetles (65 percent), notably fir
reduced by combining prescribed fire engraver (Scolytus ventralis) in white
with mechanical fuel treatments (McIver PRESCRIBED FIRE fir (Abies concolor) and mountain pine
and others 2013). Following fire, tree mortality can be beetle, western pine beetle, and pine
immediate due to consumption of engraver (Ips pini) in ponderosa pine.
The type of fuel reduction treatments The authors concluded that this level
living tissue or heating of critical plant
and their manner of implementation of tree mortality did not interfere
tissues; or it can be delayed, occurring
have different effects on the fuel matrix, with management objectives aimed at
over the course of a few years as a
which can influence the susceptibility of increasing overall forest resilience.
result of fire injuries to the crown, bole,
forests to bark beetles in different ways
or roots (Hood and others 2018a). Similarly, Douglas-fir beetle, pine
(Fettig and others 2007). For example,
Levels of delayed tree mortality caused engraver, and western pine beetle caused
prescribed fire can affect the health
by bark beetles depend on numerous some tree mortality following prescribed
and vigor of residual trees; the size,
factors, including tree species; tree size; fires in western Montana. Mortality
distribution, and abundance of preferred
tree phenology; degree of fire-caused occurred shortly after prescribed fires,
bark beetle hosts; and the physical
injuries; initial and postfire levels of tree and unburned plots were unaffected.
environment within forests. Associated
vigor; the postfire environment; and However, following a regional mountain
reductions in tree density can alter
the scale, severity, and composition of pine beetle outbreak that started about 5
microclimates, affecting beetle fecundity
bark beetle populations and other tree years after treatments were completed,
(the ability to produce offspring) and
mortality agents in the area. 50 percent of ponderosa pines in control
fitness as well as the phenology (timing
of life cycle events) and voltinism A common management concern is that (untreated) plots and 39 percent in
(number of generations per unit of time) bark beetles might colonize and kill trees prescribe-burned plots were colonized
of bark beetles and their predators, that were injured by prescribed fire and
parasites, and competitors. Tree density otherwise would have survived. These
reductions can also disrupt pheromone trees may then serve as a source of It is reasonable to
plumes that attract bark beetles to a tree beetles and attractive semiochemicals as
during initial colonization. host volatiles are released by the boring assume that bark
activity of bark beetles. In addition to beetles and wildfires will
Volatiles (volatile organic compounds)
host volatiles, the pheromones produced increasingly interact to
released from trees are known to
by bark beetles might attract other shape western forests.
influence the behavior of many bark
beetles and result in additional levels of
beetles (Seybold and others 2006). Fettig
tree mortality over time.
Fire Management Today 16 JANUARY 2021 • VOL. 79 • NO. 1
and killed. Almost no trees were killed that unburned areas do not benefit from 2006). Six and others (2002) showed that
by mountain pine beetle in thinned the positive effects of prescribed fire pine engravers are unable to colonize
plots and thinned-and-prescribe-burned (such as increased growing space due to and reproduce in chips. Moreover,
plots (Hood and others 2016). Thinning reduced tree density), which affect tree slash can be managed to minimize
treatments, with or without prescribed vigor and susceptibility to colonization colonization of residual trees by bark
fire, dramatically increased tree growth by bark beetles. Notable infestations in beetles (DeGomez and others 2015).
rates and production of resin ducts adjacent unburned areas are uncommon
(a measure of conifer defense against but can occur and should be watched EFFECTS OF WILDFIRES ON
bark beetles) relative to the control and for in case additional management is BARK BEETLES
prescribed fire treatments. warranted to limit tree losses (Fettig and Factors that influence tree mortality
Hilszczański 2015). caused by bark beetles are the same
In some cases, concerns about
maintaining large-diameter trees after wildfires as after prescribed fires.
MECHANICAL FUEL Our distinction is based not on ignition
following prescribed fire have been
justified. For example, Fettig and
TREATMENTS type but largely on differences in fire
McKelvey (2014) reported that most Factors such as stand density, host intensity and fire severity: most wildfires
tree mortality (78 percent) in the largest density, and average tree diameter are are higher in intensity and severity than
diameter class occurred during the first strong predictors of the severity of bark prescribed fires (though not always).
5 years after prescribed fire and that beetle infestations in the Western United Low-severity wildfires can induce tree
66 percent was caused by bark beetles. States. High levels of beetle-caused defenses against bark beetles (Hood and
Tree protection treatments (such as tree mortality (for example, greater others 2015). Resin-duct-related defenses
insecticides and semiochemicals) than 20 percent) should be expected take about 1 year after wildfire to form;
can be selectively used to protect following fuel reduction treatments during this time, fire-injured trees can be
individual trees from colonization by
bark beetles (Fettig and Hilszczański
2015). Furthermore, methods such as Firefighters should anticipate the potential for unusual
raking litter and duff from the bases of fire behavior in beetle-affected forests and the unique
large-diameter trees have been shown suppression challenges that can result.
to reduce prescribed fire severity and
levels of tree mortality (Fowler and
others 2010; Hood 2010). Additional
research is needed to determine under that retain high residual stand densities, more susceptible to colonization by bark
what conditions large-diameter trees are regardless of treatment effects. Although beetles, which might help explain some
most susceptible to delayed mortality thinning has long been advocated as a of the near-term increases in levels of
following prescribed fire and when tree measure to reduce beetle-caused tree beetle-caused tree mortality after some
protection treatments are warranted. mortality (Fettig and others 2007), wildfires and prescribed fires (Hood and
thinning prescriptions for fuel reduction others 2015, 2016). The level of tree
The limited number of studies on the differ from prescriptions for reducing injury influences bark beetle attraction,
effects of season of burn (spring versus susceptibility to bark beetles. In the with moderately injured trees being
fall) on levels of tree mortality caused by latter, crown or selection thinning most susceptible to colonization by bark
bark beetles show mixed results. Some (that is, removal of larger trees in the beetles (see, for example, Hood and
studies show increases in certain bark dominant and codominant crown Bentz 2007; Lerch and others 2016; and
beetle species following fall treatments classes) is typically required to achieve Powell and others 2012).
(that is, when fuels are drier and burns target threshold densities and residual
are more intense); see, for example, tree spacing as well as significant High-severity wildfires generally reduce
Fettig and others (2010). Other studies reductions in the abundance of preferred susceptibility to bark beetles by killing
show stronger effects following early- hosts. Nevertheless, thinning from large numbers of host trees. For example,
season burns (that is, when bark beetles below (for fuels reduction) does release research in subalpine forests in Colorado
are more active); see, for example, growing space, reducing a stand’s shows that spruce beetle outbreaks are
Schwilk and others (2006). More susceptibility to bark beetles. reduced for decades after high-severity
research is needed to fully define these wildfires (Bebi and others 2003), the
relationships in different forest types. A common concern following dominant fire regime in these forests. As
mechanical fuel treatments is that bark with prescribed fires, bark beetles routinely
Although most of the tree mortality beetles could breed in logging residues cause additional levels of tree mortality
caused by bark beetles following (chips and/or slash) and emerge to after wildfires, but infestations in adjacent
prescribed fire occurs during the first few colonize residual trees. However, most unburned areas are uncommon (Davis
years, this pattern might differ in adjacent studies indicate that this is uncommon and others 2012; Lerch and others 2016;
untreated areas. The reason, in part, is (see, for example, Fettig and others Powell and others 2012).
Fire Management Today 17 JANUARY 2021 • VOL. 79 • NO. 1EFFECTS OF BARK BEETLE
OUTBREAKS ON FUELS
AND FIRE BEHAVIOR AND
SEVERITY
Although fuel reduction treatments
and wildfires can affect bark beetles,
the reverse is also true: bark beetles
can alter wildfire behavior by changing
fuel conditions. Of the bark beetle–
host systems to consider, the effects
of mountain pine beetle in lodgepole
pine (Pinus contorta) have been most
intensively studied (table 1), and for
good reason: mountain pine beetle alone
is responsible for almost half of the
total area affected by bark beetles in the
Western United States. All other bark
beetle–host systems have received less
attention, some little or none (table 1).
Figure 2—Bark beetles can cause dramatic changes to forest canopies in coniferous forests.
Fire behavior in beetle-affected forests For example, during and after mountain pine beetle (Dendroctonus ponderosae) outbreaks,
largely depends on the severity of lodgepole pine (Pinus contorta) canopies transition from the green-infested stage (A) to the
the outbreak (the proportion of trees red stage (B) in about 1 to 3 years and then to the gray stage (C), which lasts from about 3 to
colonized and killed) and the amount 25 years following the outbreak. The likelihood of crown and spot fires is greater during the
of time since the outbreak occurred. red stage and possibly reduced during the gray stage. Bark beetle activity frequently results in
Jenkins and others (2008, 2014) use the a mosaic of green, red, and gray trees (D) as trees are attacked and killed over several years
term “bark beetle rotation” to describe (usually 2 to 6 years for mountain pine beetle), which can complicate fire behavior prediction in
the period from the start of a bark beetle these forests. Photos: J. Runyon, USDA Forest Service.
outbreak to the next outbreak within
susceptible forests. In the “endemic trees turn red (fig. 2B). The final stage is energy required for fuels to ignite and
phase,” beetle-caused tree mortality the “gray” stage, when needles fall off burn. Trees colonized and killed by
is limited (for example, to less than 2 the trees (fig. 2C). The timing of needle bark beetles rapidly dry out and lose
trees per acre per year) and generally fade and color change varies considerably most of their water content by the first
isolated to stressed hosts. In the by tree species, bark beetle species, and summer following attack as needles
“epidemic phase,” beetles colonize and geographic location. For example, in transition from green to red (fig. 2A,
kill large numbers of susceptible hosts. the mountain pine beetle–lodgepole 2B). For example, the twigs and needles
In the “post-epidemic phase,” beetle pine system, the green-infested stage of lodgepole pine killed by mountain
populations subside and most beetle- lasts for about 1 year; the yellow and red pine beetle lose 80 to 90 percent of
killed trees fall to the forest floor. The stages last for about 1 to 3 years; and the their water content within 1 year of
endemic and post-epidemic phases can gray stage lasts for about 3 to 25 years attack (Jolly and others 2012a; Page
last for decades to centuries, whereas the (Klutsch and others 2009). It is important and others 2012). The loss of moisture
epidemic phase usually lasts from 2 to to note that, during an outbreak, trees increases flammability by shortening
10 years. are attacked and killed over several years, time to ignition, lowering temperature
Recently attacked trees are referred to and cumulative levels of tree mortality at ignition, and raising heat yields when
as “green-infested” (fig. 2A). As needles vary considerably even within the same burned (Jolly and others 2012a; Page
fade, trees enter the “yellow” stage. In bark beetle–host system. Therefore, a and others 2012). Reduction in moisture
the “red” stage, needles on beetle-killed forest often contains trees and stands content explains nearly 80 percent of the
in multiple stages and phases of a bark increase in needle flammability (Page
beetle rotation at the same time (fig. 2D). and others 2012). Similar reductions in
foliage moisture content and increases
Not surprisingly, beetle- CROWN AND CANOPY FUELS in flammability have been documented
induced changes to foliar Not surprisingly, beetle-induced changes in Engelmann spruce (Picea engelmannii)
to foliar moisture have the greatest killed by spruce beetle (Page and others
moisture have the greatest effects on flammability. This is because 2014) and Douglas-fir killed by Douglas-
effects on flammability. water is a heat sink and moisture in fir beetle (Giunta 2016).
plant tissues increases the amount of
Fire Management Today 18 JANUARY 2021 • VOL. 79 • NO. 1the ground (Hoffman and others 2015).
Increased light and moisture availability
resulting from mortality of dominant
and codominant trees release smaller
surviving trees that were unsuitable
hosts for bark beetles, increasing ladder
fuels during the post-epidemic phase.
LITTER, FINE FUELS, AND
COARSE WOODY FUELS
Although dying and dead foliage is more
flammable than live foliage, it remains in
the canopy for only a short time (usually
3 years or less). The accumulation rate
of canopy materials (foliage and twigs)
on the forest floor can be of greater
importance, influencing surface fuel
loadings and associated fire hazards.
Stalling and others (2017) evaluated
the effects of mountain pine beetle
and Douglas-fir beetle outbreaks on
fuel conditions in Montana and Idaho.
Foliage deposition occurred mostly
during the first 2 years of the epidemic
Figure 3—Coarse woody fuel accumulations following a bark beetle outbreak in the Sierra phase. Unlike foliage, fine woody
Nevada in California. Based on a survey of 180 plots across 4 national forests, 31 percent of the and nonwoody material tended to
dead trees fell to the forest floor within 4 years of being killed, which is much faster than reported come down irregularly, deposited at
elsewhere in the literature for other locations and bark beetle–host systems (Fettig and others 2019). essentially the same rate from year to
Stephens and others (2018) concluded that a greater potential for “mass fires” exists for this region, year. Tree fall and accumulations of
driven by the amount, size, and continuity of dry combustible woody fuels, which could produce coarse woody surface fuels were limited
large, severe, and uncontrollable wildfires. Mass fires or “firestorms” (Finney and McAllister 2011) over the 10-year period of study. These
can occur when large areas burn simultaneously for long periods at high intensities, generating their findings suggest that fire hazards in these
own weather conditions. The science on mass fires (such as Countryman 1965) is limited and the forests were influenced less during the
risks are poorly understood. Photo: L. Mortenson, USDA Forest Service. late epidemic and early post-epidemic
phases than previously thought. Highly
Bark beetles alter foliage flammability in 2011; Courty and others 2012) but has flammable dead foliage does not stay in
other ways as well. The proportions of not yet been studied in the Western the canopy for long, and accumulations
fat, fiber, lignin and cellulose, starches, United States. Firefighters have reported of canopy materials on the forest floor
and sugars change in needles after observations that support the existence do not exceed the annual rate of fuel
beetle colonization, and each factor can of terpene clouds in western forests decomposition (Stalling and others 2017).
affect flammability (Jolly and others during the epidemic phase; additional
2012a; Page and others 2012, 2014). research is warranted on terpene Ultimately, all of the woody fuel from
Conifers also contain large amounts clouds and their potential effects on fire beetle-killed trees is transferred to the
of flammable terpenes. Beetle attack behavior in the Western United States. forest floor during the post-epidemic
generally increases the emission of phase. This period varies considerably
terpenes and terpene concentrations Outbreaks also alter canopy fuel by site and tree species, among other
within needles (Giunta and others 2016; arrangement (Hicke and others 2012; factors, with half-lives (the time required
Page and others 2012, 2014), which Jenkins and others 2008). As trees move for half of the dead trees to fall to
can shorten time to ignition, lower from the red to the gray stage (fig. 2B, the forest floor) ranging from years
temperature at ignition, and increase 2C), canopy fuels decrease. As dead (such as for ponderosa pines killed by
the maximum rate of mass loss (an trees deteriorate and fall to the forest western pine beetle in the central and
indication of burning rate) (Page and floor (fig. 3), canopy bulk density and southern Sierra Nevada (fig. 3)) to
others 2012, 2014). Moreover, the canopy cover decline over time. The decades (such as for lodgepole pines
emission of terpene “clouds” from decrease in canopy fuel continuity also killed by mountain pine beetle in
plants has been linked to eruptive fire reduces the sheltering effect of the northeast Oregon (Harvey 1986)). Over
behavior in Europe (Barboni and others forest, causing higher wind speeds near time, medium and coarse woody fuels
Fire Management Today 19 JANUARY 2021 • VOL. 79 • NO. 1gradually accumulate, increasing fuel Physics-based models suggest that bark Using a physics-based model, Sieg and
bed depth. For example, Jenkins and beetle outbreaks increase wildfire rates of others (2017) found higher fire severity
others (2014) reported that medium spread, with spread rates peaking during (that is, tree mortality) in ponderosa-
and coarse woody fuels increased by a the red stage. Rates of spread remain pine-dominated forests during the red
factor of about 2.5 to 8 in forests of the higher than in the endemic phase even stage than in the endemic phase but
Intermountain West following mountain though canopy fuels decrease (Hoffman unchanged or even lower fire severity
pine beetle outbreaks 20 years earlier. and others 2015). In an experiment on during the gray stage. The observed
needle flammability, red needles from increases in fire severity attributed to the
FIRE BEHAVIOR beetle-attacked trees ignited faster than bark beetle outbreak declined under high
AND SEVERITY green needles, which could lead to wind conditions (Sieg and others 2017).
Modeling fire behavior in beetle-affected increased crown fire potential (Jolly and
others 2012b). However, it is unknown Fire severity, measured as change
forests is challenging, largely because fire in vegetation, decreased over time
behavior model assumptions are violated whether the increase in flammability
scales up to entire canopies and results in following mountain pine beetle
(Hood and others 2018b; Jenkins and outbreaks in Washington and Oregon
others 2012, 2014; Page and others 2014). higher intensity crown fires (Alexander
and Cruz 2013b). After a crown fire (Meigs and others 2016). By contrast,
The limited ability to use empirical data Prichard and Kennedy (2014) reported
to evaluate model predictions (Alexander begins, fire behavior in lodgepole pine
forests under dry, windy conditions is higher fire severity during the red
and Cruz 2013a) has fed controversy over stage following mountain pine beetle
whether epidemic and/or post-epidemic likely to be similar, regardless of bark
beetle activity (Schoennagel and others outbreaks in Washington.
phases have more fire behavior hazards
than the endemic phase (Jolly and others 2012). Under most circumstances, severe Harvey and others (2014) reported
2012a; Simard and others 2011, 2012). fire weather conditions trump beetle- similar levels of fire severity in red-
induced changes in fuel conditions. stage and gray-stage lodgepole pine
forests following mountain pine beetle
outbreaks in the Northern Rockies.
They evaluated several metrics of fire
severity and found that only extreme
fire weather conditions increased fire
severity in terms of deep charring (that
is, charring into the wood along tree
boles and into crowns, with less than 5
percent of branches remaining) (Harvey
and others 2014).
After forests enter the gray stage,
evidence suggests that fire severity—
and, presumably, the potential for
heightened fire behavior—diminish for
some time (Hicke and others 2012) (fig.
4). However, more research is needed to
fully understand the potential for crown
fires in the gray stage.
Although bark beetle outbreaks can
influence fire behavior and severity, they
have little effect on the extent of the area
burned (see, for example, Hart and others
2015) or the likelihood or frequency of
wildfire occurrence (Bebi and others
2003; Meigs and others 2015).
Figure 4—Gray-stage coniferous forest following a bark beetle outbreak. Fire behavior in The effects on fire behavior and severity
beetle-affected forests largely depends on the proportion of trees killed and the amount of time have important implications for fire
since the bark beetle outbreak (that is, the “bark beetle rotation”). In the gray stage, fire severity— management and firefighter safety. For
and, presumably, the potential for heightened fire behavior—is reduced. However, high-severity example, the likelihood of torching,
crown fires have been reported in some gray-stage forests (Agne and others 2016), especially after crowning, and spotting can be greater in
significant accumulations of coarse woody fuels. The science on fire behavior in gray-stage forests is forests containing an abundance of red
limited. Photo: S. Hood, USDA Forest Service.
Fire Management Today 20 JANUARY 2021 • VOL. 79 • NO. 1crowns (Schoennagel and others 2012; fire interactions in western coniferous small trees or big trees are killed and
Jenkins and others 2014). Moreover, forests in the last 15 years. Much of whether some trees or nearly all trees
firefighters have observed “surprising” this information has been synthesized are killed);
fire behavior in some forests during the in notable publications (such as Hicke z The dominant fire regime of the forest
epidemic and post-epidemic phases and others 2012; Jenkins and others type;
(Moriarty and others 2019; Page and 2008, 2012, 2014; Kane and others
others 2013). Rapid shifts from surface 2017; and Stephens and others 2018). z The spatial scale; and
fires to crown fires pose safety risks We encourage the reader interested in z Limitations in the prediction of fire
to firefighters caused by increases in delving deeper into this topic to consult behavior in beetle-affected forests.
fireline intensities and spotting, which these publications. Furthermore, most studies are
retrospective (rather than controlled
experiments) and inherently have a lot
of variability.
Since 2000, about 25.5 million acres in the Western United
States have been affected by mountain pine beetle. Given the implications for firefighter
safety and fire suppression activities,
work is needed to more accurately
quantify relationships between bark
beetle outbreaks and fire behavior and
might necessitate larger safety zones The scientific community now has severity, especially in understudied
(Butler and Cohen 1998). Snags are an a pretty solid understanding of the forest types (table 1). Creating and
important safety concern, particularly effects of fuel reduction treatments maintaining forest structures that
during the post-epidemic phase (fig. on bark beetles, which tend to lead are more resilient to bark beetles and
3). Accordingly, firefighters should to fairly consistent responses among wildfires will go a long way toward
anticipate the potential for unusual fire different forest types in the Western addressing concerns regarding increases
behavior (even during the green-infested United States. Nevertheless, knowledge in fire behavior and severity following
stage) in beetle-affected forests and the gaps exist and need to be addressed. bark beetle outbreaks.
unique suppression challenges that can Many initial fears concerning long-
result (such as increased difficulties in term increases in levels of delayed ACKNOWLEDGMENTS
fireline construction and establishment tree mortality caused by bark beetles The genesis of this article was the
of access, egress, and escape routes). were unfounded. Accordingly, one symposium “Bark Beetle and Fire
might view the associated increases in Interactions in Western North America:
LOOKING FORWARD tree mortality as “short-term losses” The Current State of Knowledge
suffered for “long-term gains” (Fettig and Implications for Forest and Fire
A key finding of the recently published
and McKelvey 2014). This is especially Managers,” chaired by Christopher
U.S. Fourth National Climate
true when considering that rates of J. Fettig and Justin B. Runyon at the
Assessment is that it is “very likely
tree mortality caused by bark beetles 7th International Fire Congress held
that more frequent extreme weather
are generally low (less than 5 percent) in Orlando, FL, in November 2017.
events will increase the frequency
and concentrated in small-diameter We acknowledge the support of—and
and magnitude of severe ecological
trees (that is, ladder fuels) and in fire- the thoughtful debate and dialogue
disturbances, driving rapid (months to
intolerant tree species (such as white fir). conducted by—numerous colleagues at
years) and often persistent changes in
forest structure and function across large Although the scientific literature shows that symposium and elsewhere, which
landscapes” (Vose and others 2018). As mixed results, most suggests that bark helped shaped the content of this article
such, it is reasonable to assume that bark beetle outbreaks can significantly and furthered our understanding of bark
beetles and wildfires will increasingly change fuel profiles and fire behavior beetle and fire interactions. We thank Dr.
interact to shape western forests. and severity during the epidemic and Daniel Dey (USDA Forest Service) for
post-epidemic phases. Nevertheless, the invitation to contribute to this special
Recognizing this, the fire science and issue and Dr. Dey and Hutch Brown
discussions of the effects of bark beetle
forest health communities have largely (USDA Forest Service) for their reviews.
outbreaks on fuels and fire behavior get
bridged cultural and communication Much of the research cited herein was
unnecessarily complex and occasionally
divides (Jenkins and others 2009) and funded, in part, by the National Fire and
contentious if they ignore:
have strengthened relationships with Fire Surrogate Study, the Forest Health
managers in hopes of delivering science z The amount of time that has occurred Monitoring-Evaluation Monitoring
of utmost relevance to managers’ since the outbreak (that is, the bark Program, and the USDI-USDA Joint
concerns (Kocher and others 2012). beetle rotation); Fire Sciences Program.
A tremendous amount of knowledge
z The type and severity of the outbreak
has been developed on bark beetle and
(including such factors as whether
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