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Effects of climate change on the relationship between pine bark
beetles and forests in Western North America, and impacts on the
ability of these forests to store carbon
Hannah Johnson
December 14, 2012
During this century, the average global temperature is expected to increase between
1.8 and 4 degrees Celsius due to global warming, according to the IPCC. Climate change will
have countless effects across North America, including consequences for bark beetles and
their tree hosts. Bark beetles are already responsible for the deaths of billions of coniferous
trees across millions of acres of forests in the Western United States and Canada (Bentz et
al, 2010). In the coming years, bark beetle populations will become increasingly more
active due to warmer temperatures, while forests become weakened by drought. This
dangerous combination will inhibit forests’ ability to slow global warming by sequestering
carbon.
Bark beetles are small wood boring insects in the family Scolytidae. There are over
600 species in North America, several of which are invasive species. Bark beetles can be
found in forests ranging from Mexico to Alaska (Figure 1), and can live at elevations from
sea level to 11,000 feet. They colonize beneath the bark of trees. Hundreds or thousands of
bark beetles may attack a broken, stressed, or diseased tree and lay eggs beneath the bark
(Seybold, 2008). When the larvae hatch, they feed on the tree and it dies. The mature adult
beetles emerge and embark on a search for the next host tree (Bentz et al, 2010). Figure 2
shows a landscape transformed by bark beetles; the orange trees are characteristic of a
bark beetle attack.
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Figure 1: Bark beetle range in North America (Source: National Park Service)
Figure 2: This photo shows the northern Williams Range Mountains in Colorado where beetles
have killed more than 80 percent of mature lodgepole pine over many square kilometers.
(Source: USGS Central Colorado Assessment Project)
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Two main factors help to fuel the severity and frequency of outbreaks. One is that
human management of forests increases the percentage of weak and aging trees that would
normally get wiped out by fires, and these trees are perfect prey for beetles (Nikiforuk,
2011). The other important factor is climate change.
Climate is the most important factor that influences bark beetle outbreaks (Gan,
2004). Recently, outbreaks have been more severe than ever due to climate change (Figure
3), for several reasons. Cold temperatures during winter months can kill beetle eggs and
larvae wintering under a tree’s bark (“Forest…”). Because of climate change, some winters
never reach cold enough temperatures to kill the beetles and act as a control on the
population size. Moreover, shifts in temperature have been correlated to outbreaks
because bark beetle population survival and growth are highly sensitive to temperature
(Powell and Logan, 2005). In a warmer climate, the season of attack is longer and beetles
go through more life cycles per year, compared with locations where the temperature is
cooler (Seybold, 2008). An area with an average temperature increase of 2 or 3 degrees
Celsius can allow some beetle species to double their reproduction and tree consumption
rates (Nikiforuk, 2011). Reproduction rates can double by beetles reproducing annually
rather than once every two years, as is common today, and this is predicted to happen with
spruce beetles (Bentz et al, 2010).
Furthermore, global warming results in more dead and stressed trees, which
provide resources and suitable habitats for beetles (Choi, 2011). In addition to
temperature, outbreaks have also been correlated to changes in precipitation because
during drought, trees are stressed and become susceptible hosts for beetles. Bark beetles
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already have a large area of suitable habitat under current climate conditions (Evangelista
et al, 2011), and it will probably get worse.
Figure 3: Map of recent outbreaks of three species and graph of total forest area affected by
one species (Source: mountainpinebeetletreatment.com)
Models from current climate change trends have predicted that beetle outbreaks
will continue, and beetle ranges will increase with time. One climate model predicted a 27
percent increase in habitat area for the western pine beetle by 2050. When the beetle
ranges shift, they may put new host species at risk (Evangelista et al, 2011). Global
warming will continue to increase the intensity and extent of pine beetle outbreaks. For
example, it is predicted that climate change due to doubling atmospheric CO2 would
increase the southern pine beetle infestation rate by 2.5 to 5 times (Gan, 2004).
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As bark beetle outbreaks become worse, the damage to forests will become
increasingly significant. Trees are dying, and forests are being lost. This affects forests’ role
as carbon sinks. A moderate level of disturbance can actually increase carbon sequestration
because it gets rid of weak and dying trees, which allows for regrowth and regeneration,
and an optimal density. However, the increase in bark beetles that will occur with global
warming is so great that it will cross this threshold and decrease the potential for carbon
storage (Seidl et al, 2008).
The result of decreased carbon storage in forests due to bark beetles is important
because forests are one of the world’s most important carbon sinks for all the carbon
emitted into the atmosphere through natural and anthropogenic processes. Forests cover
about 30 percent of the world’s land surface and account for 50 percent of plant
productivity. Forests contain almost half of all carbon stored on land (Carlowicz, 2012).
Western North America is home to temperate and boreal forests. Temperate forest is the
biome with the fifth highest carbon-storing capability, storing an average of 68 tons per
acre in vegetation and soil. Boreal forest is the second highest, at 182 tons of carbon per
acre (Gorte, 2009). These are significant amounts of carbon, and the disturbance of these
forests poses threats to the carbon cycle.
In simulations, climate change led to a decrease in the amount of carbon
sequestered in forests, both aboveground and belowground. Increasing disturbance
activity, such as by bark beetles, leads to decreased carbon storage potential. Increases in
insect disturbances have even been shown to result in net carbon emissions for forests
(Seidl et al, 2008). This is because during bark beetle outbreaks, widespread tree mortality
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reduces carbon uptake and increases carbon emissions from decaying dead trees (Kurz et
al, 2008).
An example is the current outbreak in British Columbia that is larger in size and
severity than any other known outbreak in history. The cumulative impact from 2000 to
2020 is estimated at 270 megatons of carbon being released into the atmosphere from the
forest. The forest became a large net carbon source. In the worst year, the impacts were
equivalent to 75 percent of the average annual forest fire direct emissions from all of
Canada during a forty-year period (1959 to 1999). This unprecedented outbreak was
influenced by climate change, and shows how beetle outbreaks like it are an important
mechanism by which climate change can hinder the ability of northern forests to take up
and store atmospheric carbon. (Kurz et al, 2008).
Bark beetles have significant short-term impacts on carbon sequestration. One
landscape-scale study found that 39 percent of forest cover was killed by bark beetles, and
between 40 percent and 50 percent of above-ground carbon was in the beetle-killed trees.
The beetles attack the trees that are sequestering the most carbon, so although a small
percentage of the area was severely impacted, there were large impacts to aboveground
carbon stocks. Percent aboveground carbon in killed trees was greater in areas of higher
aboveground carbon content for three possible reasons. Bark beetles prefer to attack larger
trees, which hold more carbon. Older trees hold more carbon, and can also be weaker and
more susceptible to successful attack. Finally, densely growing trees can lead to
vulnerability and successful attack (Bright et al, 2012).
Another study on mountain pine beetles and lodgepole pines found similar results
that beetles prefer larger trees. Mountain pine beetles killed up to 52 percent of the trees in
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the study area, with more large trees killed than smaller trees. Furthermore, the study
indicated serious short-term and long-term effects on carbon storage. Carbon stocks in
lodgepole pine trees were reduced by between 31 and 83 percent immediately following
the outbreak. To recover, forests take between 7 and 25 years to return to their previous
biomass. However, even after forests recover, the rate at which they remove carbon dioxide
from the atmosphere is reduced by at least 12 percent and up to 51 percent. Forests may
take over a century to return to the previous rate at which they accumulated carbon before
the outbreak. The study estimated that it would take forests that had experienced beetle
outbreaks between 56 and 185 years to catch up to the carbon balance that had
accumulated in unaffected stands. Carbon stocks in four of the modeled plots would never
catch up at all. (Pfeifer et al, 2011).
Evidently, bark beetles have dramatic and long-lasting impacts on forests across
large areas of land. Climate change is exacerbating an already drastic issue and will
continue to in the coming years. Warmer temperatures allow beetles to thrive through
increased reproduction and growth rates, and expanded ranges, while drought weakens
trees and provides perfect habitats for beetles. Several studies referenced have shown that
beetle outbreaks kill trees that subsequently release their carbon stores into the
atmosphere as they decompose, as well as hinder the forest’s ability to sequester carbon
indefinitely following the outbreak. Unfortunately, the loss of forests as carbon sinks is a
positive feedback. Trees die, stop absorbing and storing carbon, and instead release carbon
as they decompose. Atmospheric CO2 increases, which contributes to global warming, and
in turn helps the bark beetles thrive. Bark beetles and their relationship to Western North
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American forests will play a significant role in the future, and should be taken into account
in future climate change predictions.
Citations
Bentz, B.J., J. Regniere, C.J. Fettig, E.M. Hansen, J.L. Hayes, J.A. Hicke, R.G. Kelsey, J.F.
Negron, S.J. Seybold. 2010. Climate change and bark beetles of the Western United
States and Canada: direct and indirect effects. Bioscience 60:602-613.
Bright, BC, JA Hicke, and AT Hudak. 2012. Landscape-scale analysis of aboveground
tree carbon stocks affected by mountain pine beetles in Idaho. Environmental
Research Letters 7: 1-6.
Carlowicz, Michael. "Seeing Forests for the Trees and the Carbon: Mapping the World's
Forests in Three Dimensions." Earth Observatory. NASA, 9 Jan. 2012. Web. 13 Dec.
2012.
Choi, W. 2011. Influence of global warming on forest coleopteran communities with
special reference to ambrosia and bark beetles. Journal of Asia-Pacific Entomology
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Evangelista, P. H., S. Kumar, T. J. Stohlgren, and N. E. Young. 2011. Assessing forest
vulnerability and the potential distribution of pine beetles under current and future
climate scenarios in the Interior West of the US. Forest Ecology and Management
262:307-316.
"Forest Health: Mountain Pine Beetle." National Parks Service. Web. 12 Dec. 2012.
Gan, J. 2004. Risk and damage of southern pine beetle outbreaks under global
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climate change. Forest Ecology and Management 191:61-71.
Gorte, W.R. 2009. Carbon sequestration in forests. Congressional Research Service Report
for Congress.
Kurz, W.A., C.C. Dymond, G. Stinson, G.J. Rampley, E.T. Neilson, A.L. Carroll, T. Ebata,
and L. Safranyik. 2008. Mountain pine beetle and forest carbon feedback to climate
change. Nature 452:987–990.
Nikiforuk, Andrew. "Bark beetles, aided by climate change, are devastating U.S.
pine forests." Washington Post. N.p., 5 Dec. 2011. Web. 1 Nov. 2012.
Pfeifer, E.M., J.A. Hicke, and A.J.H. Meddens. 2011. Observations and modeling of
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western United States. Global Change Biology 17: 339-350.
Powell J.A., Logan J.A. 2005. Insect seasonality: Circle map analysis of temperaturedriven life cycles. Theoretical Population Biology 67: 161–179.
Seidl, R., W. Rammer, D. Jager, and M. J. Lexer. 2008. Impact of bark beetle (Ips
typographus L.) disturbance on timber production and carbon sequestration in
different management strategies under climate change. Forest Ecology and
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California Agriculture and Natural Resources, Nov. 2008. Web. 01 Nov. 2012.
<http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7421.html>.