Fire Written in Wood
Wildfire leaves indelible marks in the tree-ring record. When surface fire burns through a forest, it wounds susceptible trees, killing a patch of cambium and creating a 'catface' scar. As the tree heals, new annual rings grow over the wound, encapsulating the scar with a precise date — the year and even the season of the fire. By crossdating fire scars from multiple trees across a landscape, scientists reconstruct fire histories spanning centuries.
Composite Fire Chronologies
A single tree records only fires intense enough to scar it, and not every fire scars every tree. By combining scar records from many trees into a composite chronology, researchers detect the full range of fire events. A fire year is defined when a threshold percentage (commonly 10-25%) of recording trees show scars. This filter separates widespread fires from localized events.
Fire Interval Statistics
The intervals between successive fires follow statistical distributions — typically Weibull rather than normal. The mean fire interval (MFI), median fire interval, and Weibull parameters characterize the fire regime. Short MFI with low variability indicates a regular, climate-driven fire cycle. Long MFI with high variability suggests stochastic fire occurrence dependent on rare ignition-fuel-weather conjunctions.
Fire Suppression & Changing Regimes
Fire-scar records across western North America show a dramatic pattern: frequent fires every 5-25 years for centuries, then an abrupt cessation around 1880-1910 coinciding with livestock grazing (which removed fine fuels) and organized fire suppression. The resulting fuel accumulation has transformed formerly resilient surface-fire forests into landscapes prone to catastrophic crown fires — a legacy that drives modern wildfire management policy.