Pine Beetle-Caused Forest Death, And Climate Change

There is some interesting new work carried out by researchers at Dartmouth College and the USDA Forest Service on the relationship between the Mountain Pine Beetle, major die-offs of forests in North America, and climate change.

The Mountain Pine Beetle (Dendroctonus ponderosae) is a kind of “bark beetle” (they don’t bark, they live in bark) native to western North America. They inhabit a very wide range of habitats and are found from British Columbia all the way south to Mexico. In British Columbia alone, the pine beetle, though a fairly complex process, has managed to destroy 16 of 55 million acres of forest. This epidemic of tree death is seen in mountain forest regions all across the western United States. The beetles affect a number of species of pine trees.

The beetle lays its eggs under the pine tree bark, and in so doing, introduces a fungus that penetrates adjoining wood. This fungus has the effect of suppressing the the tree’s response to the Pine Beetle’s larvae, which proceed to eat part of the tree. This suppressive effect blocks water and nutrient transport, together with the larvae eating part of the tree, quickly kills the host tree. The process can take just a few weeks. It takes longer for the tree to actually look dead (note the evergreen tree you cut and put in your living room for Christmas is dead the whole time it is looking nice and green and cheery). By the time the tree looks dead, i.e., the needles turn brown and fall off, it has been a dead-tree-standing for months and the Pine Beetles have moved on to find other victims.

It has long been thought that climate change has contributed to the western epidemic of Pine Beetles, as well as a similar epidemic in the Southeastern US (different species of beetles). The primary mechanism would be increasing winter extreme low temperatures. The very low temperatures would kill off the larvae, removing the threat of the beetle’s spread locally after that winter. Extreme winter temperatures have warmed by around 4 degrees C since 1960 across much of the beetle’s range. The lack of killing colds itself does not cause a beetle epidemic, but simply allows it, or produces a “demographic release.” If the beetles are already there, they have the opportunity to spread.

A recent study, just out, (see reference below) confirms this basic model but also adds a considerable degree of complexity. The study shows that there is not as strong of a correlation between raising winter temperatures above typical killing levels and the spread of the beetle. The study indicates that demographic release form an increase in extreme winter lows is part of the equation, but the situation is more complex and likely warming in general enhances beetle spread and reproduction during the summer part of its lifecycle, and may weaken the trees to make them more vulnerable to attack. In addition, other non-climate related factors probably play a role.

The study looked at several regions and assembled data on beetle frequency and spread over time, and various climate related data. From the abstract:

We used climate data to analyze the history of minimum air temperatures and reconstruct physio- logical effects of cold on D. ponderosae. We evaluated relations between winter temperatures and beetle abundance using aerial detection survey data… At the broadest scale, D. ponderosae population dynamics between 1997 and 2010 were unrelated to variation in minimum temperatures, but relations between cold and D. ponderosae dynamics varied among regions. In the 11 coldest ecoregions, lethal winter temperatures have become less frequent since the 1980s and beetle-caused tree mortality increased—consistent with the climatic release hypothesis. However, in the 12 warmer regions, recent epidemics cannot be attributed to warming winters because earlier winters were not cold enough to kill D. ponderosae…There has been pronounced warming of winter temperatures throughout the western US, and this has reduced previous constraints on D. ponderosae abundance in some regions. However, other considerations are necessary to understand the broad extent of recent D. ponderosae epidemics in the western US.

“This amount of warming could be the difference between pests surviving in areas that were historically unfavorable and could permit more severe and prolonged pest outbreaks in regions where historical outbreaks were halted by more frequent cold bouts,” says first author Aaron Weed, an ecologist at the National Park Service.

In the 11 coldest regions, winter temperatures cold enough to e lethal to D. ponderosae have become less frequent since the 1980s, and this is associated with an increase in tree mortality, confirming the link between warming conditions and increased parasite caused tree death. However, in the 12 regions with the warmest climate, recent epidemics are not clearly linked to warming winters simply because the earlier, colder, winters were already not cold enough to repress the tree-killing mountain pine beetle. This suggests that other factors may play a role in the epidemics in the western United States.

Evens so, the pattern of warming (including increase of minimum winter temperature) correlates to the demographic release of the mountain pine beetle. The authors note that “warming year-round temperatures that influence generation time and adult emergence synchrony … and drought effects that can weaken tree defenses …” are plausible explanations, but further note that a simple single explanation is not likely to be sufficient to explain the overall phenomenon. The link between warmer years, added number of generations per year, and the epidemic is explored here.

This is, in a sense, a numbers game. A cold winter does not kill off all of the beetles. However, no matter how cold the winter is, no beetles will be wiped out if they are not there to begin with. So, demographic release, which makes possible but does not cause an outbreak, could cause an abundance of beetles across a much larger area where, no matter what natural suppression may occur, they will then become more abundant over time.

As noted, the trees themselves matter. We can safely assume that generally changes in overall climate will mean that plant communities adapted to a given region might lose that adaptive edge and be subject to a number of problems which can then be exploited by a potentially spreading parasite. These changes in viability of plant communities are not all climate change related. Forest management, disturbance, and regional demographics (as forests age, they tend to change what they do) are also factors in this complex set of ecological relationships.

The bottom line. This study confirms the effects of warming, especially the increase of winter low temperatures, on the potential for D. ponderosae to spread rapidly locally and regionally. The study also calls into question the simplistic model that this is all that happens to explain the widespread epidemic of this beetle. Other factors, including other aspects of global warming, also contribute to the epidemics. In addition, and importantly, the study demonstrates a high degree of variability in the outcome of ecological and climate change.

This epidemic is probably the largest observed kill-off of forests caused by a parasite. So far it is much more severe in its effects than forest fires, but over the long to medium term, we will probably see increased frequency and severity of forest fires because of the abundance of fuel provided by the die-off.


Weed, A. S., Bentz, B. J., Ayres, M. P., & Holmes, T. P. (2015). Geographically variable response of Dendroctonus ponderosae to winter warming in the western United States. Landscape Ecology. doi:10.1007/s10980–015–0170-z

Text for the image at the top of the post, from the USDA:

The Mountain Pine Beetle is at epidemic levels throughout the western United States, including here in the Rocky Mountain Region … Forests affected here include several in Colorado, Wyoming, South Dakota and Nebraska. In northern Colorado and southeastern Wyoming, Mountain Pine Beetles have impacted more than 4 million acres since the first signs of outbreak in 1996. The majority of outbreaks have occurred in three forests: Arapaho-Roosevelt, White River and Medicine Bow/Routt.

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Thanks for linking, Greg. It endlessly amuses me that I am accused of being an "ozonista" as though I don't recognize other threats to trees, especially climate, when you would be hard pressed to find anyone more pessimistic about climate change than me...and it is actually the mainstream foresters who are single-mindedly fixated on one influence - drought from climate - who with rare exceptions refuse to factor in ozone, at all!

Additionally I read something a couple of weeks back (probably in 3quarksdaily) where a forester claims there are two populations of pine trees, ones that thrive in cold and ones that thrive in (slightly) warmer temp. Her claim was that during the little ice age, the cold hardy trees came to dominate, but now these are vulnerable to beetle predation, and the few warm hardy trees are supposedly immune, so eventually will come to be the dominate subspecies. Of course the transition will be a bitch.

By Omega Centauri (not verified) on 30 Mar 2015 #permalink

the pollution absorbed by trees is what is weakening their natural resistance, leading to epidemics of insects, disease and fungus all over the world, irregardless of any correlation with warming or drought.

I concur, Gail Zawacki. Mostly. Yet, I'm conflicted on what is actually causing it -- More and more, I keep coming up with aluminum toxicity and certain herbicides as it is not just trees but also broadleaf plants such as Passiflora (maypops).

I have witnessed the damage in the mountains of eastern America -- Back then, it was attributed to various sulfur and nitrous oxides from metal smelting and paper manufature just to the west.

I've also witnessed the spotty destruction of pine (we call them 'borers', here) on a piece of land I have helped maintain for the past thirty years. This land is in 'the valley' and the destroyed trees are the ones sparce and separated from the bordering dense growth. Amongst various cases of lightning, tornadoes, and ice storms the pine borer beetle got the most. It does seem confined to indiviual 'sick'(??) trees. I've also frequented nearby areas with dense pine growth and no sign of any disease.

Concerning ozone: I wish to point out that some of the healthiest 'old growth', rich soil areas are the ones that seem to produce the greatest concentrations of that gas on a sunny summer day.
Cade's Cove monitoring:

Regarding spontaneously falling trees: In my experience, it is invariably an infestation of those big, black ants that smell like acetic acid which can cause an otherwise healthy looking tree to just 'pop and drop'.

By Élan vital (not verified) on 30 Mar 2015 #permalink

LOL - Why didn't I think of that? I get it - how clever! Like using "climate disruption" instead of "global warming". Semantics make all the difference, it's amazing. So maybe if I said "chocolate pudding" instead, people wouldn't be so resistant to the idea that a known toxin is toxic?

Wikipedia may help here:

"Ozone /ˈoʊzoʊn/ (systematically named 1λ1,3λ1-trioxidane and catena-trioxygen), or trioxygen, "

"Her claim was that during the little ice age, the cold hardy trees came to dominate, but now these are vulnerable to beetle predation, and the few warm hardy trees are supposedly immune, so eventually will come to be the dominate subspecies."

Yeah, that hypothesis worked really well for the American Chestnut... Let's all depend on that, right? NOT !

By Brainstorms (not verified) on 30 Mar 2015 #permalink

“The beloved aspen forests that shimmer across mountainsides of the American West could be doomed if emissions of greenhouse gases continue at a high level, scientists warned on Monday. That finding adds to a growing body of work suggesting forests worldwide may be imperiled by climate change.

The new paper analyzed the drought and heat that killed millions of aspens in Colorado and nearby states a decade ago. Such conditions could become routine across much of the West by the 2050s unless global emissions are brought under control, the study found.”

“Dr. Anderegg’s paper fits with other recent findings suggesting that forests may not be as resilient to global warming as once hoped. For instance, a paper published two weeks ago found that the ability of the vast Amazon forest to pull carbon dioxide out of the air was weakening through time, with trees growing faster and dying earlier.”

“Forest experts, including Dr. Allen, are particularly worried about future “hot droughts,” similar to the one that struck Colorado and nearby states in the early 2000s. Huge stands of aspens died, and heat-loving beetles killed millions of acres of pine trees.

These droughts are characterized not just by a lack of rainfall but by high temperatures that suck residual moisture out of the soil. They are predicted to increase in a warming climate.”……- carbon-decade-ago-emissions

By cosmicomics (not verified) on 31 Mar 2015 #permalink

I think its really sad that the growing population- which it indirectly leading to all of this- is still denying their share in warming of our planet. just wish the people out the whould start to realize what they are doing.

By lorinda- 15018939 (not verified) on 31 Mar 2015 #permalink

Thanks, Greg, for a balanced discussion of this issue.

As someone who is greatly concerned about AGW, I am always aware of how balanced, fair, and yes, even skeptical science is regarding climate change. I have used the issue of the pine beetle numerous times as an example of ill effects from climate change, and yet here is a study that says yes, climate change is part of the problem, but no so fast there, big fella....

Sadly, the other side uses a study such as this to say, "See, all that fear about climate change and the pine beetle isn't confirmed at all." And my side, trying to depend on science, always has science being conservative and balanced in the discussion.

But that is science, and you follow it wherever it leads.

The contortions the scientists go through to push forest decline into the warming bag never cease. The Gillis article is another case in point. The paper he writes about is here - take a look at figure 3.

Notice the blue areas - that the model predicted would survive the drought, but the trees died there too, anyway.

Maybe the reason the model is only 75% accurate is that it isn't taking into account the air pollution?

By Gail Zawacki (not verified) on 31 Mar 2015 #permalink

Millions of trees are more valuable than one species of beetle.I honestly think that pine beetles should be exterminated.What are the chemicals that can be used to stop this outbreak?

By Maria Moreroa (not verified) on 31 Mar 2015 #permalink


I'm not sure that would be a great move..

Think about it. You'd have to spray some VERY powerful insecticide over vast areas, repeatedly. After all, these beetles or their larvae are inside the trees.

There will be a huge side-kill, which could itself lead to the trees dying by removing some symbiosis. At the least, you'll wipe out the predators of the beetles - they will get an extra dose of the insecticide and/or starve.

So what will almost certainly happen is that a year or two later, the problem re-explodes - with insecticide resistant beetles - and it's worse than ever.

To be fair, there might be a parasitic worm or wasp that preys exclusively on these beetles.. in which case we still have the huge logistic challenge of captive breeding and mass release over vast areas.

By Andrew Dodds (not verified) on 31 Mar 2015 #permalink

Do the pine beetles have a benefit to their environment?

Since this epidemic is more severe than forest fires and trees are very important. Could we not find a way to reduce and control the population size Pine beetles, as to lessen the effects of this epidemic?

By Megan (u15031153) (not verified) on 01 Apr 2015 #permalink

Megan - I would hazard a guess that in one sense they do have a benefit. As Greg said all those dead trees will end up as fuel for forest fires. Assuming those were mostly mature stands (old growth - as the western pine beetle goes after trees older than about 80 decades) then new growth in the burned out areas will absorb a lot more CO2 than the pine forest would ongoing - that does not however take into account the amount of carbon locked in that old growth until it burns. Not sure if that would all balance out or if there would be a benefit there.

By Doug Alder (not verified) on 01 Apr 2015 #permalink

To add to that one of the things happening here in BC is attempts to massively harvest affected pine - if you see purple stained pine lumber for sale you'll know what it is from :) If they manage to do something with it other than burn it then that will have a positive effect on the carbon cycle

By Doug Alder (not verified) on 01 Apr 2015 #permalink

I agree with Andrew. It would not be a good idea to exterminate the beetles, because whatever insecticide they decide to use will, in one way or another, have a negative impact on the trees. Furthermore, as time goes by, insecticide-resistant beetles develop, and the trees may be weaker (due to the continual spraying of the pesticides), which would make the problem much worse.

By TN Mphateng (1… (not verified) on 01 Apr 2015 #permalink

I definitely agree that "extermination" is the wrong way to go. Since the beetles are only native to western North America, we would hate to start a downward spiral that would ultimately lead to the extinction of the species. This is especially true since the dead trees are not completely useless to us. Incentives for companies to use beetle-killed trees for bio-fuel or biopower applications have been created by western U.S. states and Canadian provinces. However, it is important to remember that the trees are also important and should also be conserved.

To answer Megan’s question, various methods have been developed in order to control and reduce the beetle population. These include “Pheromone Baiting” which involves attracting the beetles to selected areas using synthetic hormones; from there a certain percent can be eliminated. There is also the option of relocating either single or groups of infested trees to other larger areas, in an effort to conserve and distribute the beetles. Pesticides such as chitosan or permethrin could be used. However this would not be ideal as, just as TN Mphateng said, it could possibly lead to the development of a resistant population. Scientists are also currently looking at an interesting solution at the moment which involves working with the trees to activate and enhance their resistance mechanisms.

I personally feel that burning of areas where infested trees are concentrated is the best option. It will control the beetle numbers and not cause the same environmental and health damage as pesticides would. I may also just feel that way since I grew up on a farm in South Africa where seasonal burning is a norm.

Now that all these various options have been developed, the difficult stage starts. Decisions need to be made about which method to apply, and this is only after considering a number of aspects: Which will be most effective? Which is sustainable? Which is the most cost effective?

At the end of the day this all just goes to show that our reckless actions have led to yet another major environmental issue that we must scramble to find a solution for before it is too late. When do you think we will learn our lesson and stop making extra work for ourselves?

By Sam (15036198) (not verified) on 01 Apr 2015 #permalink

Burning is a terrible idea. Agricultural burning in the Southern Hemisphere is a major contributer to ground level ozone. Prof. Jack Fishman, author of "Global Alert" explains this in his lecture titled "Tropospheric Ozone - Are We Creating a Toxic Atmosphere?" which is featured here: direct link to the video:

By Gail Zawacki (not verified) on 02 Apr 2015 #permalink

In reply to by Sam (15036198) (not verified)

I think that burning the trees is a good idea. Its the most natural way of getting rid of the beetles. They will die and the trees will grow back.

I don't think burning the trees is a good idea. By burning the trees, vast areas of the natural habitat of other animals will be destroyed, causing the destruction of whole ecosystems. Several other species of insects or animals will be put into danger. If these areas infested with the pine beetles were to be burnt, there will be no insects to pollinate plants after the plants grow back. Another reason why burning isn't a good idea is because of all the air pollution that would be taking place if millions of acres of forests would be burned to get rid of the beetles. Are there no natural predators of these pine beetles that could help lessen the numbers the beetles infesting forsest, if only in a small area?

By AG Woodley - u… (not verified) on 04 Apr 2015 #permalink

The beetles are behaving exactly as any population does when they come across a surfeit of food - their numbers explode. This is what happened to the human population due to the "green revolution" and it is the same pattern for bacteria in a petri dish given a glut of sugar. The "food" the beetles have are the dying trees. The trees would be dying anyway, from pollution. The beetles and fungus they spread are hastening the decay. If you want to address the issue of forest decline and premature tree mortality, you would have to figure out how to curtail the human habit of burning fuel and spreading artificial fertilizers on agricultural crops and drilling for more fuel, releasing toxic gases. So since nobody wants to do that, nothing will be done until the ecosystem collapses and humans are left with the desert they created.

By Gail Zawacki (not verified) on 04 Apr 2015 #permalink

In reply to by AG Woodley - u… (not verified)

While climate change is definitely due to human activity, it has become part of process of natural selection. As temperatures begin to increase and these species of trees die out is it not part of the introductory phase of a better adapted species to move into the area?
Perhaps Man is able to restore some of the damage caused to these forests...
It may seem as though this loss of trees is only a negative issue, it does provide a new opportunity for new life. While not much is able to grow in these cold regions, with rising temperatures the conditions are becoming favourable for a wider variety of plant species which could be introduced into the area.
Together with this, the death of these trees provides material which can decay naturally in the area providing nutrients for new growth.
Burning would not be a good idea since it would only lead to increased air pollution, cancelling out any efforts that Man may have for reparation in the area.
As degradation may take very long in these environments, it would be particularly challenging yet a new approach if entomologists and conservationists alike could join ideas to introduce a species of decomposers as well as a predator species that may control the population of beetles.
Personally these new innovative attempts to rebuild nature with nature may be more effective than trying to step in and do it ourselves, as Man alone.

By Shannon-Lea O'… (not verified) on 04 Apr 2015 #permalink

The effect of clobal warming is just too much, especially when its effect is indirect. But still I believe that nature has a way of regulating itself. In so saying, I mean that the beetles will exit the carrying capacity of its habitats then eventually 'die', its pupation controlled then the pine trees will have a chance to recover.

Unless if there is another thought about this?

As seen the pine beetles are a more severe threat to the pine trees than forest fires. Is their no way to reduce te population of pine beetles because of the great effect it hase of the pine tree population?
Are the pine beetles in anyway beneficial for the enviroment?

By Heinrich(14124506) (not verified) on 08 Apr 2015 #permalink

This is a very good example of one ways in which nature plays a roll in climate change and if we were to interfere with these pine beetles it would probably lead to even further damage.

By M.R Revolta (u… (not verified) on 17 Apr 2015 #permalink

So, Greg, what do you make of this - according to this article, native trees should be MORE resistent to invasive attacks of insects, not less:…

and yet, next to climate change/drought, almost all foresters blame invasive species for tree decline such as in this horrifying instance (among many) where 286 different species of trees have been attacked…

I have never bought the "blame the invasives" argument for epidemics of insects, disease and fungus since, first of all, native pathogens are also on the rampage and furthermore, people have been exporting exotic plants, fruits, living trees and lumber products for centuries in very large quantities. If merely importing aliens were enough to destroy the world's forests, it wouldn't have taken until the last 10 years or so to reach emergency proportions (yes, the UK Environment Dept. has declared a state of emergency -


By Gail Zawacki (not verified) on 17 Apr 2015 #permalink