Shifting wildfire spread dynamics in Canada: declining spread rates but increasing spread days under more severe fire weather from 2002 to 2024

Anthropogenic climate change has driven a marked increase in wildfire activity across Canada in recent decades, yet it remains unclear whether the mechanisms governing fire spread have changed over time. Using the Canadian Fire Spread Dataset, we quantified trends in wildfire spread dynamics from 2002 to 2024 at national and regional scales, examining whether changes were associated with increasing fire-conducive weather or with shifts in landscape resistance to fire spread. We found that the mean number of days with observed fire spread per fire more than doubled (+138%) over the study period. This increase was driven by a growing number of days with fire-conducive weather, which promoted longer fire durations and greater cumulative potential spread. In contrast, mean daily spread distance (DSD) per fire declined substantially (−43.9%), while the frequency of extreme spread events (above the 90th percentile) remained relatively stable. Concurrently, the ratio of observed to potential DSD decreased markedly, indicating that fire spread is increasingly constrained by landscape features beyond the effects of climate alone. Two competing hypotheses may explain this pattern individually or collectively. First, landscape resistance to fire spread may be increasing due to interacting processes such as fire-driven negative feedbacks, fuel fragmentation, shifts toward less flammable broadleaf vegetation, and forest management. Alternatively, increasingly severe fire-weather conditions may be enabling fires to spread into stands that were previously less likely to burn, creating the appearance of greater landscape resistance. Although fire spread regimes differ regionally, temporal trends were broadly consistent across Canada, suggesting common but interacting underlying drivers. Despite potential increasing landscape constraints, we conclude that the growing number of fire-conducive days remains sufficient to promote larger fires and rising annual area burned. These findings highlight the need to account for evolving landscape resistance when projecting future wildfire activity under continued climate warming.