Wildfires and western spruce budworm outbreaks: A multi-century dendrochronological record of forest disturbance interactions in the interior Pacific Northwest

Background/Question/Methods

Insect outbreaks are often assumed to increase fire severity or probability of occurrence, while fires may in turn alter susceptibility of forests to subsequent insect outbreaks through changes in the spatial distribution of suitable host trees. However, little is actually known about the potential synergisms between these natural disturbances. Assessing inter-disturbance synergism is challenging due to the short length of historical records and the confounding influences of land use and climate changes on natural disturbance dynamics. We assessed the relationship between natural disturbances (western spruce budworm outbreaks and fire), climate, and long-term changes in land-use practices in Douglas-fir forests in western North America. We used dendrochronological methods to reconstruct western spruce budworm outbreaks and fire occurrence at ten sites along a latitudinal transect running from central Oregon to western Montana. We quantified synchrony within disturbance types, explored the potential for synergism between disturbance types, and analyzed changes in disturbance dynamics that have occurred following Euro-American settlement of western North America. We compared these disturbance histories with dendroclimatological drought records to quantify the influence of moisture availability on disturbances.

Results/Conclusions

            Our sites sustained repeated, often decadal-length western spruce budworm outbreaks over the last three centuries. Fires occurred fairly frequently at these sites prior to 1890. Fires were more likely to occur during drought years, while outbreaks were most likely to begin near the end of droughts. After approximately 1890, fires were largely absent from these sites and western spruce budworm outbreaks became longer-lasting, more frequent, and more synchronous. Our results show no discernible impact of defoliation events on subsequent fire risk. We found no relationship between fire occurrence and the timing of outbreak initiations or the duration of outbreaks. Any effect from the addition of fine and coarse fuels during defoliation events appears to be too small to detect given the overriding influence of climatic variability. We therefore propose that if there is any relationship between the two disturbances, it is a subtle synergistic relationship wherein each disturbance type damps the severity, but does not alter the probability of occurrence, of the other disturbance type over long time scales. Although both disturbance types may increase in frequency or extent in response to future warming, our records show no precedent that western spruce budworm outbreaks will increase future fire risk.