Characterizing combined fire and insect outbreak disturbance regimes in British Columbia, Canada
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Fires and insect outbreaks are important agents of forest landscape change, but the classification and distribution of these combined processes remain unstudied aspects of forest disturbance regimes.
We sought to map areas of land characterized by homogenous fire regime (HFR) attributes and by distinctive combinations of fire, bark beetles and defoliating insect outbreaks, and how their distribution might change should current climatic trends continue.
We used a 41-year history of mapped fires and forest insect outbreaks to classify HFRs and combined fire and insect disturbance regimes (HDRs). Spatially constrained cluster analysis of 2524 20-km grid cells used mean annual area burned, ignition Julian date, fire size and fire frequency to delineate HFR zones. Mean annual areas burned, affected by bark beetles, and affected by defoliators were used to delineate HDR zones. Random forests classification used climate associations of HDRs to project likely changes in their distribution.
Eighteen HFR zones accounted for 30% of variance, compared to 27 HDR zones accounting for 59% of variance. Fire regime designation had low predictive power in explaining 23 homogenous insect outbreak regimes or the 27 HDRs. Climate change projections indicate a northward migration of current HDR zones. Conditions suitable for defoliator outbreaks are projected to increase, resulting in a projected increase in the total rate of forest disturbance.
When describing forest disturbance regimes, it is important to consider the combined and possibly interacting agents of tree mortality, which can result in emergent properties not predictable from any single agent.
KeywordsDisturbance interactions Disturbance regime Fire regime Insect outbreaks Spatially constrained cluster analysis Climate change
We thank Steve Taylor and Gurp Thandi for compiling the B.C. Natural Disturbance Database, and Bryan Pettit for preparing the gridded polygon intersections and summaries. This work was supported by the Future Forest Ecosystems Science Council of British Columbia (Project FFESC-B1-2009-12), the Forest Change Program of the Canadian Forest Service (Natural Resources Canada), and the Research Office of the University of Northern British Columbia (Fund 26911). The constructive suggestions of two anonymous reviewers are gratefully acknowledged.
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