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, but are now exposed and being attacked (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

Labels: ecology, environment, forests, insects, invasive species