Disturbance–climate relationships between wildfire and western spruce budworm in interior British Columbia

Abstract

Defoliation by the native western spruce budworm (Choristoneura freemani Razowski; WSB) and wildfires are natural disturbances affecting the structure and function of montane forests in British Columbia, Canada. Limited research describes the climate conditions associated with WSB outbreaks and historical fire activity in these forests, and the spatial stability of these relationships across landscapes. Site-specific and regional characterizations of these relationships are required to establish the historical baselines necessary to document disturbance–climate relationships within dry conifer forests. We compiled site-level and regionalized tree-ring chronologies of fire (AD 1600–2013) and WSB (AD 1600–2009) outbreaks for both lower montane forests adjacent to expansive grasslands and middle montane forests (regionalized grassland and non-grassland chronologies). At site and regional scales, the relation between fire years, outbreaks of WSB, reconstructed values of the Palmer Drought Severity Index, and annual precipitation were examined using superposed epoch and bivariate event analyses. Regional disturbance–climate relationships were used to evaluate the influence of grassland proximity on these relationships and the disturbance history characteristics. The findings show that fires affecting grassland proximal sites in lower montane forests were historically more frequent than fires occurring in middle montane forests and were related to warm, dry conditions in the fire year and cool, wet conditions in the year(s) preceding the fire. These results indicate the climate conditions associated with both fine fuel growth and drying are key determinants for fire activity in lower montane forests. The initiation of WSB outbreaks was significantly related to drought, and this relationship was enhanced at lower montane sites. We also examined the interrelationships between fire years and WSB outbreaks, but no associations were discerned. Considering the recent escalation of widespread forest fire activity in the Pacific Northwest of North America and the potential for timber losses associated with WSB defoliation, the findings provide information important to forest scientists charged with promoting forest health and resilience in the dry montane forests of interior British Columbia.