Biology in the News Explained

Climate change + fire suppression = ecological disaster?

The mountain pine beetle (Dendroctonus ponderosae, Coleoptera: Scolytidae) is native to western North America. A finer resolution of its range, however, reveals that it is historically native to some parts of the West, but not others. Specifically, it has generally had a limited presence in Canada, primarily due to very low winter temperatures. Although the pine beetle’s cold tolerance is incredibly high because they have the anti-freeze compound glycerol in their bodies, generally sustained (5 or more days) temperatures below -30F kill most of them off. This has reduced the likelihood of mountain pine beetle outbreaks in Alberta, and thus susceptible trees there have historically been protected (Rice et al., 2007).

In the last 5-10 years, however, conditions in the West, including Alberta, have changed. Rising temperatures have meant that for several winters in a row, the northern Rockies have not reached low enough temperatures to kill off the mountain pine beetles infesting the trees there. Even in the U.S., the historical trend was that every few years most of the beetles are killed due to cold, and thus the outbreaks were knocked back. So the pine beetles, which are a native species, have begun behaving like an invasive one: they are multiplying rapidly without a natural check, and expanding their range, attacking populations of trees that are not adapted to them.

Compounding this problem is the recent history of fire suppression in the West. One of mountain pine beetle’s favorite hosts, lodgepole pine (Pinus contorta) is a fire-adapted species; it is common for lodgepole stands left undisturbed to burn once or twice a century, and be replaced by seeds from serotinous cones (cones in which the seeds are sealed unless they reach the high temperatures of a fire). Lodgepole stands are striking in that usually all the trees are the same age and size due to the burn regimen. Mountain pine beetles prefer older, larger trees. The larger the tree, the more food available for the developing beetle larvae, and the larger the increase in population the next year, if there is not a sustained hard freeze. By suppressing natural fires in lodgepole habitat, we may have enhanced the long term outbreak we are seeing now.

But here’s the flip side: mountain pine beetle outbreaks make lodgepole pine stands more susceptible to fire down the road (Page and Jenkins, 2007). For instance, the 1988 Yellowstone National Park fires were highly correlated spatially with trees affected by a mountain pine beetle outbreak about fifteen years before (Lynch et al., 2006). What we may be experiencing now is a mega-outbreak, due to warming and fire suppression, which will eventually contribute to massive forest fires throughout the West in the future (also increasing of course from drier weather), which may have the benefit of being a different kind of check on mountain pine beetle populations. But instead of the historical ecology, in which mountain pine beetle outbreaks occurred for maybe 3-4 years, decades apart, a whole new, different ecology driven by constant high beetle populations decimating the forest, which as a result may burn more often, will remake the landscape in ways that we cannot yet imagine.

Of course there are those who believe that we can replicate the ecological benefits of fire, while keeping the timber available for human use. However, thinning trees mechanically is a blunt instrument that does not mimic the effects of fire at all in the case of lodgepole (Sibold et al., 2007). In fact, there is the danger of unintentionally increasing the density of trees (and necessitating, further, constant thinning effort) if enough of the canopy is opened to encourage new seeds to germinate and grow.

There are those who believe humans are all powerful and can easily control insect outbreaks and fires through management if only the wicked, meddling environmentalists would let them (never mind that somehow the forests managed themselves just fine for millennia). In fact, many species are adapted to respond to biotic (e.g. herbivory pressure) and abiotic (e.g. weather) influences in ways we don’t even understand. Global climate change is now accepted by anyone rational to be at least partly enhanced by the massive release of carbon dioxide into the atmosphere by industrial humans that would not have occurred otherwise. Fire suppression is an active (and expensive) choice that trades short-term convenience for long-term ecological disruption, whose consequences we are barely beginning to understand. Those who blame “environmentalists” for the hundreds of acres of brown pines they see spreading like a cancer in the West, would find that ecologists (pretty much environmentalists by default) only wish they had such god-like power to affect the ecology of our forests, so they could save them from 150 years of disastrous “management.”

References

Lynch, H.J., Renkin, R.A., Crabtree, R.L. & Moorcroft, P.R. (2006) The influence of previous mountain pine beetle (Dendroctonus ponderosae) activity on the 1988 Yellowstone fires. Ecosystems, 9:1318-1327.

Ono, H. (2003) Mountain Pine Beetle Symposium: Challenges and Solutions. Kelowna, British Columbia. T.L. Shore, J.E. Brooks, and J.E. Stone (editors). Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Information Report BC-X-399, Victoria, BC. 298 p.

Page, W.G. & Jenkins, M.J. (2007) Mountain pine beetle-induced changes to selected lodgepole pine fuel complexes within the intermountain region. Forest Science, 53:507-518.

Rice, A.V., Thormann, M.N. & Langor, D.W. (2007) Mountain pine beetle associated blue-stain fungi cause lesions on jack pine, lodgepole pine, and lodgepole x jack pine hybrids in Alberta. Canadian Journal of Botany-Revue Canadienne de Botanique, 85:307-315.

Sibold, J.S., Veblen, T.T., Chipko, K., Lawson, L., Mathis, E. & Scott, J. (2007) Influences of secondary disturbances on lodgepole pine stand development in rocky mountain national park. Ecological Applications, 17:1638-1655.

Thanks to T. Etienne for initial information on mountain pine beetle

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2 Responses to “Climate change + fire suppression = ecological disaster?”

  1. Travis says:

    Dr. Henneman – I believe you need to research the literature you cite a little better. The Lynch et al. 2006 paper doesn’t report a high degree of correlation as you state, if my memory serves me correct it was somewhere around 11%, which although was significant, is very low. I also think you overstep the findings of Page and Jenkins 2007 a bit. They did not test susceptibility to fire, they just inferred surface fire potential and spread based on custom (untested) fuel models and modeling efforts using these. For a great discussion on most if not all of the research relevant to this topic, see Simard et al. 2010.

    In addition many lodgepole pine fire return intervals are on the order of 150-200 years, much longer then the fire suppression era. Also, the 1910 fires created a landscape of similar aged lodgepole pine that has contributed to the food source availability for MPB, and is another non-climate, non-human related reason for some of the current large-scale epidemics. I do believe that climate change has a role in all of this, but I don’t think it’s the major cause, and many of my entomology colleagues agree.

  2. biotunes says:

    Travis,
    Thanks for your thoughtful reply. You have some good points.

    In reference to the Lynch study, “correlation” is really the wrong word (even though they used it in their abstract). What they did is match a model of the burn to the observed burn pattern, and found that their model predicted correctly 87% of the pixels on a map that actually burned. Based on this model, areas with beetles were 11% more likely to burn. Drought was actually the most important factor predicting burn, as one would expect. But from what I could tell they included drought and beetles as separate factors, when we know that they are not independent – beetles are more successful during droughts. But what that means is that a beetle x drought interaction term is key. (Drought may or not be affected by climate change; it is most conservative to say not necessarily, because recent tree-ring data (as you probably know) shows that the west of the past few hundred years has been very dry, and the period of intense settlement actually coincided with an unusually wet period: see http://bioblog.biotunes.org/bioblog/2007/03/16/the-definition-of-drought/)

    Still, it is certainly more complicated than I presented in this brief column – as Lynch et al state in their discussion:

    “…we concur with Bigler and others (2005), who also concluded that the more important effect of beetle activity on fire risk is through a change in stand structure and composition, as opposed to an increase in fuels.”

    Even if some lodgepole stands burn over a larger interval of 150-200 years, then suppressing fire in stands getting close to those ages, which haven’t yet burned, has increased the interval unnaturally until the next burn, so it doesn’t really matter. That said, I had a really interesting discussion recently with a fire specialist from our local FS office, which enlightened me more about the history of fire suppression, which shows it to have been a lot more complicated than just policy positions. I am planning to do a follow-up on this post soon.

    Finally though, you state that entomologists you know don’t think that climate change is a major cause of the spread of MPB. Could you please elaborate? Given that they are spreading into areas that until recently were regularly cold enough to kill them off, it seems as though climate change is a pretty strong contender. Do your colleagues believe that this is just a natural cycle unrelated to a longer term trend? Or do they think that the spread into historically too-cold areas is unimportant? The former is certainly a possibility; one of the challenges in climate change work is separating natural, shorter cycles from the overall global trends. We may not definitively be able to determine the truth until a hundred years from now.

    But, I’m one entomologist who believes that the evidence, although circumstantial, suggests that climate change is a big player in MPB demographics. Whether the changes we will see will really amount to an “ecological disaster” is a more open question to me, despite my hyperbole, because long-term ecological change (on a scale beyond human lifetimes) itself is indeed a natural process. We humans just tend to think of changes that happen during our lifetimes as subjectively bad.

    Could you please give me a more complete reference for the paper you cite? I certainly am interested in what the latest thinking on this is – I’m sure it has evolved since I wrote this three years ago.

    Cheers.

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