Bark Beetles: Cause for Concern in Snowy Western Watersheds?

A five millimeter long insect triggers complex changes in the hydrology of entire watersheds.

Unlike impacts from forest fire or clear-cutting, which are immediate and dramatic, tree death from bark beetles often results in standing dead timber that proceeds through a number of mortality stages following death. Though it depends on the tree species, dead trees have been known to remain standing for 50 years.

Figure 1: Unlike impacts from forest fire or clear-cutting, which are immediate and dramatic, tree death from bark beetles often results in standing dead timber that proceeds through a number of mortality stages following death. Though it depends on the tree species, dead trees have been known to remain standing for 50 years.

Unprecedented in human history, this insect outbreak has changed whole mountainsides from green to red to brown and then to grey (Figure 1), with the biggest devastation occurring in lodgepole pine forests that cover subalpine regions of the Rockies. Estimates indicate that, since the mid-1990s, nearly 400,000 square miles of total forest in western North America have been affected by bark beetles. It is important to remember, however, that although the current infestation may be larger than ever observed before, beetle epidemics are normal in coniferous forests. The hydrologic impacts of such a large landscape changes can last for decades, giving greater impetus to understanding how these infestations affect surface water supplies.

Although detailed investigations into beetle impacts on hydrology have been done relatively recently, research into the connections between other types of changes in forest cover and streamflow dates back to the early 20th century and has shaped much of the conventional wisdom about tree death and water supplies. At sites such as Wagon Wheel Gap in Colorado and Hubbard Brook in New Hampshire, scientists first assessed the effects of changes to forests, such as fire and clear-cutting, on streamflow. The results were clear, especially in snow-dominated areas–the removal of forest cover increased snow accumulation in the winter and reduced transpiration in the summer, leaving more water available to enter streams. Subsequent investigations found some mitigating effects of increased sunlight on the snow surface and increased wind speeds in open areas, but nearly a century’s worth of observations confirmed this general principle–removal of forest cover results in increased water yield, an effect that is more pronounced in areas with higher snowfall.

However, much of that research was based on rather dramatic changes to forest cover, usually through clear-cutting or patch cutting, which removes virtually all of the vegetative cover in certain areas. With bark beetle infestations like the current one that concerned Bob Steger, things are notably different. Despite the fact that the vast majority of a given patch of forest may be killed during a bark beetle infestation, the trees are not removed from the landscape. Furthermore, in contrast to fire and clear-cutting, the rate of forest disturbance is much slower with beetles–in the former, whole swaths of forest could be removed in hours, while beetle infestations kill trees slowly over years. Finally, those remaining living trees, which are usually the youngest ones, continue to grow, often at a faster rate. Thus we cannot assume that what past experiments found will wholly apply to beetle infestations; rather a mixed story emerges.