Bark Beetles
Posted: November 18, 2011By: Professor Peter Kolb, University of Montana College of Forestry and Conservation
The bark beetle outbreak we are experiencing across the entire western portion of North America is the result of multiple ecological factors converging at the same time. Its occurrence is not a surprise for foresters across western forests as the current expansiveness of bark beetle activity has been building for many years. Bark beetles such as mountain pine beetles, one of the main culprits in the current outbreaks, have been extensively studied since the mid 1970s. Its life cycle and ecology are very well understood. It has been a natural part of western forests for millennia and its population cycles are fairly predictable. Under what we would characterize the average forest and climatic conditions of the past century it exists as a chronic population within pine forests, colonizing and killing trees that are unable or incapable of defending themselves due to a variety of physiological, genetic or environmental factors. It may be considered analogous to wolves circling a herd of caribou, culling out the weak, unfit and injured. As with any species, bark beetles have numerous pests and predators themselves including a variety of predatory beetles, wasps, nematodes, mites, fungal diseases, and larger predators such as bark gleaning birds and woodpeckers. Depending on the populations of these predators and pests, chronic bark beetle populations might be kept incheck.
The greater the suitable host tree number, the greater the potential food source and thus the larger the population of bark beetles that can develop. Likewise, the greater the percentage of host trees that are similar in age and size, the greater the probability of bark beetles successfully attacking and colonizing them at the same time.
A landscape such as Yellowstone National Park, that had a large acreage burn catastrophically in 1988, will develop an even aged forest of fire adapted lodgepole pine that are all similar in size equivalent in expansiveness as the area of disturbance. When these trees reach 90-100 years of age, they will mostly become suitable host trees at the same time that under the right climatic conditions can allow an epidemic of bark beetles to develop once again. The epidemic will then persist as long as there are host trees within flying distance of beetles and the climate remains favorable. The same is true, for example, of Colorado and Wyoming’s lodgepole pine forests. By and large, these forests are mature, even age forests of lodgepole pine stressed by drought and high densities of trees combined with warmer temperatures that foster mountain pine beetle population explosion. …
Mature forests with dense canopies have the additive effects of transpiring more water than forests of younger trees with less needle area, and intercepting rainfall and snowfall in their dense canopies that evaporates back into the atmosphere before having a chance to enter the soil where trees can absorb it. The additive impacts of greater water and energy production requirements, less soil water recharge, and limited space for photosynthetic (needle) area leads to significantly weakened trees. At this point the trees in this condition represent a large food source without any defenses, the perfect target for bark beetles and a host of other tree pests and pathogens.