The current level of bark beetle mortality has resulted from a combination of natural factors. One group of factors influence the health of the trees; these include, but are not limited to drought, dense forest stands, and shallow/rocky soils. The second set of factors are human activities, such as fire suppression, past forest management practices, past grazing practices, and ongoing urbanization. These first two sets of factors influence the amount of water, light, and nutrients available to individual trees in the forest. Trees not receiving enough of these resources become stressed. Bark beetles can detect a stressed, susceptible tree, and they respond by colonizing it and effectively removing it from the population. This inadvertently makes a larger quantity of resources available for the surviving trees, shrubs, and herbaceous plants. Finally, the third factor is warmer than average winters and summers which have contributed to relatively large bark beetle populations. Warm winter temperatures allow more beetles to survive the winter, and warmer summers increase the number of generations per year which can greatly increase the size of a region's population.
Jeff Schalau, Associate Agent, University of Arizona, and Tom DeGomez, Area Agent and Regional Specialist, University of Arizona
