Insect disturbance is often thought to increase fire risk due to enhanced fuel loadings, particularly in coniferous forest ecosystems.  Yet insect disturbances also affect successional pathways and landscape structure that interact with fire disturbances (and vice-versa) over longer time scales.  We applied a landscape succession and disturbance simulator (LANDIS-II) to evaluate the relative strength of the interactions between eastern spruce budworm (Choristoneura fumiferana, Clemens) and fire disturbances in the Boundary Waters Canoe Area (BWCA), a 430,000-ha wilderness in northern Minnesota, where historic fire disturbance is well documented, but insect disturbance history is less understood.  Disturbance interactions were evaluated for two different scenarios – presettlement forests and fire regimes vs. contemporary forests under fire suppression.  Short-term insect-fire interactions were simulated via fuel and forest type-specific fire spread rates based on the Canadian Forest Fire Behavior Prediction system, while long-term interactions were simulated through successional responses to the different insect and fire disturbance scenarios.

Results/Conclusions

Our results indicate that spruce budworm had little influence on presettlement fire regimes because short fire return intervals prevented the widespread establishment of the budworm’s primary host, balsam fir (Abies balsamea).  By contrast, fire suppression favors establishment of fir and spruce, and therefore the modern disturbance regime was dominated by insect disturbance.  In the contemporary simulations, fire risk was slightly increased during decades of budworm outbreaks.  However, longer term fire risk was reduced by the establishment of deciduous tree species within openings created by dying fir, which slowed fire spread rates and consequently reduced the total area burned.  Our results demonstrate the counter-intuitive interactions of two different disturbance regimes, where relationships based on one time scale do not necessarily relate to outcomes at other scales.  The results of the presettlement simulations further illustrate how the strength of disturbance interactions depends on the state of the system.  Hence while insect disturbance can influence fire risk in boreal coniferous forests, the nature of the interaction depends on both the time scale of interest and the interplay between fire, insects, and succession that determine the susceptibility of the system to each disturbance type.