| Summary: | Understanding spatial and temporal patterns of fire regimes is critically important for
sustainable forest management and fire hazard mitigation. Mixed-severity fire regimes, in
particular, are poorly understood, yet increasingly recognized as important drivers of stand and landscape-heterogeneity. I address knowledge gaps pertaining to the management and
understanding of mixed-severity regimes including: (1) classification and mapping, (2)prevalence in mountain forests, (3) underlying topographical drivers, and (4) stand dynamics. Research questions were addressed using dendrochronological field data (fire scars, tree establishment dates, stand composition and structure) from 20 randomly selected research sites in southeastern British Columbia,I examined whether mixed-severity regimes, as currently represented in fire-regime
classification schemes, led to erroneous landscape-level fire regime mapping. I used my field data to evaluate the accuracy of two classification systems (Natural Disturbance Type (NDT)and Historical Natural Fire Regime (HNFR)) used by managers to map fire regimes in British
Columbia (Chapter 2). Each classification system made considerable and contrasting errors in identifying mixed-severity regimes relative to the field data and these misrepresentations were tied to elevation. I attributed these errors to assumptions about disturbances underlying each classification system, as well as limitations of the research methods used to estimate fire
frequency (i.e., using either stand-age or fire-scar data in isolation). I explored the prevalence of mixed-severity fire regimes, importance of underlying topographic drivers, as well as the influence of mixed- versus high-severity fires on forest composition and structure (Chapter 3). I found evidence of mixed-severity fires at 55%. At these sites, most reconstructed fires (73%) were documented solely by fire scars, indicating many were of low-to-moderate severity. The remaining 27% of fires were severe enough to create conditions suitable for even-aged cohort to establish. Spatial patterns of fire severity were primarily controlled by elevation (i.e., severity increased with elevation). Composition varied with disturbance history; however, structural differences (e.g., tree size classes) were subtle, with the exception of snag densities, which were much greater in old, high-severity forests (where time-since-last-fire >250 years). Understanding the ecological heterogeneity created by mixed-severity regimes potentially influences decisions related to conservation, silviculture, wildfire and fuel mitigation.
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