Drought influences mixed-severity fire regimes across temporal and spatial scales in the Montane Cordillera of Canada

Understanding historical fire-drought associations, particularly in forests with mixed-severity fire regimes, is a research and fire management priority in western North America. My thesis investigates how drought variation across temporal and spatial scales drove such fire regimes in the Montane Cordillera of Canada. I developed three-interrelated studies written as independent chapters, all of which used crossdated fire-scars to represent historical fire years. The first two studies test fire-drought associations using monthly adaptations of the Drought Code (DC) from Canada’s Fire Weather Index System. First, I compared three monthly drought codes during the 20th and 21st centuries for montane forests of southeast British Columbia. Accuracy of monthly DC increased after accounting for overwinter drying, early fire season starts, and effective precipitation. June-August drought codes were significantly associated with historical fires. Variation in fire-season drought influenced fire severity, connecting modern fire-weather indices with historical mixed-severity fire regimes. Second, I investigated how historical drought variation drove mixed-severity fire regimes in the same location by developing a tree-ring proxy reconstruction of summer DC. Comparing summer DC against a local summer Palmer Drought Severity Index provided a nuanced understanding of inter-annual fire-drought associations and moisture content among forest fuels, namely in deep compact organics in the soil and large woody fuels, versus the duff layer. Fire years were associated with coinciding and previous year summer drought; but limited by coinciding and previous year summer wet conditions. Summer moisture conditions during fire years likely influenced ignitions and led to variable combustion of forest fuels. The final study encompassed broader spatial coverage by including 17 fire-history sites across the Montane Cordillera, and by testing historical associations between climate and fire based on years with evidence of fire at multiple sites, i.e., fire synchrony. Fire synchrony was historically common, and associated with droughts at regional and subregional scales based on tree-ring proxy reconstructions of climate. My thesis provides information on drought as a driver of mixed-severity fire regimes across temporal and spatial scales. Ultimately, understanding how drought drove mixed-severity fire regimes across scales, helps fire managers anticipate how these fire regimes are shifting due to climate change.Forestry, Faculty ofGraduat