Converging and diverging burn rates in North American boreal forests from the Little Ice Age to the present

Warning: This article contains terms, descriptions, and opinions used for historical context that may be culturally sensitive for some readers. Background: Understanding drivers of boreal forest dynamics supports adaptation strategies in the context of climate change. Aims: We aimed to understand how burn rates varied since the early 1700s in North American boreal forests. Methods: We used 16 fire-history study sites distributed across such forests and investigated variation in burn rates for the historical period spanning 1700–1990. These were benchmarked against recent burn rates estimated for the modern period spanning 1980–2020 using various data sources. Key results: Burn rates during the historical period for most sites showed a declining trend, particularly during the early to mid 1900s. Compared to the historical period, the modern period showed less variable and lower burn rates across sites. Mean burn rates during the modern period presented divergent trends among eastern versus northwestern sites, with increasing trends in mean burn rates in most northwestern North American sites. Conclusions: The synchronicity of trends suggests that large spatial patterns of atmospheric conditions drove burn rates in addition to regional changes in land use like fire exclusion and suppression. Implications: Low burn rates in eastern Canadian boreal forests may continue unless climate change overrides the capacity to suppress fire.