Synopsis:
Because recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years, researchers have been prompted to determine what factors trigger broad scale outbreaks, and how do these factors interact? How do human activities, such as forest management, alter these interactions, and thus the frequency, extent, severity, and synchrony of outbreaks? Extensive host tree abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales.
The structure of the greater landscape critically influences pine beetle eruptions. Because bark beetles are relatively poor dispersers, connectivity of suitable habitat is highly correlated with dispersal. Temperature, drought, and processes that homogenize forest age, genetic, or species structure, such as stand-replacing disturbances or widespread management activities, may synchronize spatially disjunct populations. Localized habitat fragmentation can predispose stands to attack, alter water flow, and uncouple predator-prey tracking. Landscape-scale management and land-use activities can reduce forest heterogeneity, a major constraint against populations surpassing the eruptive threshold. Additionally, transport by humans poses an ever-present risk of introducing invasive bark beetles.
Management practices in some regions have also increased the abundance of susceptible hosts. Because of aggressive fire suppression, the annual burned area declined from about 100,000 ha to less than 1000 ha over the last five decades (Taylor and Car- roll 2004). This reduced rate of disturbance yielded forests in which nearly 70% of lodgepole pine was more than 80 years old, therefore resulting in an overall threefold increase in the amount of susceptible pine from 1910 to 1990.
Conclusions:
Pine beetle eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. The structure of the greater landscape critically influences pine beetle eruptions. Because bark beetles are relatively poor dispersers, connectivity of suitable habitat is highly correlated with dispersal. Localized habitat fragmentation can predispose stands to attack, alter water flow, and uncouple predator-prey tracking. Landscape-scale management and land-use activities can reduce forest heterogeneity, a major constraint against populations surpassing the eruptive threshold. Additionally, transport by humans may introduce invasive bark beetles.
Thresholds/Learnings:
Caveats:
Future research should determine precise thresholds at which pine beetle eruptions take place, in order to help pin-point specific management tactics.
