Assessing wildfire potential in a coastal forest watershed, British Columbia, Canada

Wildfire disturbance in coastal western Canada has varied with changes in climate and human activity. Although infrequent during the past century, large fires in coastal forests occurred in the past, and current and future fire hazard are uncertain. We used multiple lines of evidence to characterize the contemporary regime of a forested water supply area around greater Victoria (GVWSA), British Columbia (BC), Canada, to provide a framework for assessing fire hazard more broadly in Pacific coast watersheds. Threshold conditions for locally significant fires (LSF; >10 ha) were established using recent fire and weather records. These data were used to 1) define baseline fire danger climatology for the GVWSA; 2) model fire behaviour and annual or cumulative burn probability (BP) across the region; 3) compare BP with late holocene fire return intervals derived from paleo-charcoal samples; and 4) evaluate modelling using recent fire records and case studies. The fire season was 14 days longer in the drier eastern portion of the watershed compared with the western portion. The fire season in recent years (2010–2022) was more pronounced, with higher Fire Weather Index (FWI) quantiles compared with older (1996–2009) records. LSF occurred under Fire Weather Index (FWI) ≥ 32 conditions (∼93rd percentile), although rare landscape-scale fires (> 1000 ha) were associated with more extreme events (approx. FWI ≥ ∼50, ca. 99.6th percentile), particularly summer outflow conditions. Models suggest that stands of mature Douglas-fir, the dominant vegetation type, require high sustained winds during high to extreme danger conditions for crown fire occurrence. The overall BP was lower (by 50–70 %) than values from interior BC, while the distribution of BP across the GVWSA reflected the influences of lightning maxima on exposed ridges and potential human ignitions outside the perimeter. Simulated fires were mostly small to moderate in size (median= 52 ha; 90 % < 308 ha), with the largest reaching ∼2800 ha within a 7 km fireshed. Charcoal-based paleofire reconstruction suggest the persistence of fire occurrence patterns. Data from four lakes around the watershed generally supported the landscape BP variability. Late-Holocene fire frequency was consistently 50–100 % higher than surrounding modelled present-day BP, reflecting the persistence of local climatic and topographic influences despite changes to anthropogenic influences. Wildfire potential in the forests of SVI is lower than in continental regions but poses a growing risk to water resources and other values. Environmental variability and sparse fire records suggest a multi-proxy approach can be effective for informing managers of fire hazard in similar areas of the Pacific coast.

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