Fig. 4: Conceptual figure illustrates the impacts of fire exclusion and suppression on area burned and fire severity in historically frequent-fire North American forests and woodlands represented by the majority of the fire scar sites used in our analysis.

This conceptual model generally applies to dry conifer forests in western North America, formerly red pine dominated forests in the southeastern boreal forest, and temperate broadleaf and conifer woodlands and forests of the eastern United States. Prior to ~1880, traditional burning and lightning-ignited fires contributed to an active historical fire regime with frequent, lower-severity fires that produced tree-ring fire scars, cumulatively burned a relatively large area, and maintained low fuel loads, thereby serving as a stabilizing (negative) feedback⁸⁸. The gradual increase in annual area burned from 1600 to 1880 may have resulted from a warming climate at the end of the Little Ice Age¹³⁶ and is generally based on Marlon et al.⁷, although this pattern may manifest due to reduced records in the earlier periods of analysis¹³⁷. The onset of sustained fire exclusion and suppression over many decades reduced annual area burned, reflected by a decline in tree-ring fire scars, during which time fuel loads accumulated. At present, fires generally burn during pronounced droughts, increasing the area burned in recent years, although total area burned remains below historical levels. Contemporary fires in historically frequent fire forests now often act as a destabilizing (positive) feedback⁸⁸, with tremendous social-ecological effects through smoke impacts, reduced community safety, and diminished forest recovery. The future is uncertain and depends on management actions, social decisions and priorities, and climate change. Figure credit: Jessie Thoreson.