Abstract.  Since 1990, native bark beetles have killed millions of trees across millions of acres of forest from Alaska to northern Mexico. While bark beetle infestations are a regular force of natural change in forested ecosystems, several of the current outbreaks, which are occurring simultaneously across western North America, are the largest and most severe in recorded history.

There are many species of bark beetles, but only a few are responsible for the large swaths of dead trees we see today. These species must kill their host tree to reproduce, and are most successful when they attack larger trees that are greater than about 80 years old. Bark beetle ecology is complex and dynamic, and a variety of circumstances must coincide for a bark beetle outbreak to succeed on a large scale. Only when specific conditions are met—ranging from temperature and precipitation patterns across an entire region, to the particular growing environment in a forest, to the existence of certain bacteria and fungi within the beetle and host tree—will bark beetle populations grow large enough to infest and kill large landscapes of trees. While outbreak dynamics vary from species to species and from forest to forest, the combination of two major factors appear to be driving the current outbreaks:
1. Changing climatic conditions, especially elevated temperatures and shifting precipitation patterns: Elevated temperatures can speed up reproductive and growth cycles and reduce cold-induced mortality during cold snaps. In addition, prolonged drought can weaken trees, making them more susceptible to bark beetle attacks.
2. Forest history and host susceptibility: Over the past century, natural disturbances such as stand-replacing crown fires and blowdowns, and human activities such as wildfire suppression and timber harvesting, have contributed to the existence of large areas of host trees that are very similar in size and age—what one scientist has described as a beetle “smorgasbord.” Also, when trees must compete for resources in overcrowded conditions, bark beetles can more easily overcome stressed trees’ defenses and initiate a severe outbreak.

Due to the complexity of bark beetle community dynamics and the specific ecosystems they inhabit, however, the roles these factors play differ from forest to forest. While research has uncovered a great deal of information about the life cycles and host interactions of some species of bark beetles, many gaps in our knowledge remain. In addition, because changing climate and forest disturbances have altered outbreak dynamics in recent years, some of what we have learned from past outbreaks may no longer hold true. There may be no equivalent in recorded history for the current outbreaks.

These recent infestations may result in dramatic changes to the long-term ecological pathways of some ecosystems, radically shifting vegetation patterns in some hard-hit forests. While there are no known management options to prevent the spread of a large-scale bark beetle outbreak, land-use activities that enhance forest heterogeneity at the landscape scale—such as creating patches of forest that contain diverse species and ages of trees—can reduce susceptibility to bark beetle outbreaks. However, because resource objectives often differ, and because the factors influencing a bark beetle outbreak vary depending on the species, host tree, local ecosystem, and geographical region, there is no single management action that is appropriate across all affected forests.

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