When I hear the word ‘beetle’ I automatically think of mountain pine beetle (Dendroctonus ponderosae):
Much of my research in the past 6 years has focused on the effects of mountain pine beetle (MPB) on forest structure, and the subsequent impacts on snow accumulation and melt under the forest canopy. And since large areas of MPB-killed forest can increase wildfire risk (a recent synthesis paper in Forest Ecology and Management discusses this in detail), my research also examines the effects of wildfire on snow processes and hydrology.
But MPB isn’t the only insect affecting our forests on such a large scale. Many different insects feast on our forests each year, and the scale of attack and subsequent impact on forest health and structure is closely related to climate.
Spruce (Dendroctonus rufipennis) and Douglas-fir (Dendroctonus pseudotsugae) beetles
These two beetles are from the same Dendroctonus family as the mountain pine beetle, but affect different tree species. Spruce beetle outbreaks appear episodically in the century-long stand history of spruce-dominated regions, with the most recent one from 1990-2000 occurring on the Kenai Peninsula of Alaska, and the Bowron Lakes area of British Columbia. Given the relationship between beetle attack and air temperature, studies suggest these outbreaks may increase with warming climate. Outbreaks of Douglas-fir beetle also occur episodically, with the most recent happening in Washington right now. These beetles thrive in areas that have recently been affected by wildfire or blowdown – so with increasing wildfire incidence and extreme weather events leading to treefall, Douglas-fir beetle may continue to thrive.
Ips – also called the pine engraver or engraver beetle – is prevalent in pine and spruce forests of the western Cordillera, and has many different subspecies that specialize in which trees they attack (spruce, pinon pine, lodgepole, limber pine, etc.). It looks a lot like the pine beetle, but with a dent in its bum. Ips will attack drought-stressed trees, but is also drawn to newly cut ones. Ips infestations are not as aggressive as Dendroctonus infestations, and are of more limited spatial extent and severity.
Budworm (Choristoneura spp.)
This one is a killer of spruce and Douglas-fir, from the West to the East coasts of Canada and up into the Territories and Alaska. Episodic outbreaks of budworm have been linked to weather conditions – namely winters with below average precipitation followed by early springs with average air temperature.
So why do we care about all these bugs? Our forests are affected by pests – and have been cyclically for at least the past century. But recent – and predicted – changes in climate can significantly alter the recurrence interval of infestation, and the resulting severity and extent. For example, pine beetle populations exploded partially as a result of warmer winters that failed to kill larvae overwintering beneath tree bark. Now, warmer summer temperatures are allowing them to breed twice as fast. Increased air temperatures are also implicated in spruce and Douglas-fir beetle outbreaks, and indirectly with Ips, which is attracted to drought stressed trees. Given the variety of tree species affected, the spatial range of each of these species, and the likelihood of enhanced climatic changes – we can expect more insect attacks in the coming years.
This will have implications for everything from hydrology to land surface albedo. Snowmelt-dominated headwater basins that feed downstream systems are likely to see major hydrologic effects. If large areas of forest are affected by these types of pests – similar to the large area of BC now in the grey stage of mountain pine beetle attack – the surface albedo can increase with subsequent cooling effects on the overlying atmosphere.
It’s an excellent reminder of the interconnections between environmental systems, in this case forests, water, and atmosphere, and the cascading effects of change in just one small part of this system.