Fuel constraints, not fire weather conditions, limit fire behavior in reburned boreal forests

Fire frequency in boreal forests has increased via longer burning seasons, drier conditions, and higher temperatures. However, fires have historically self-regulated via fuel limitations, mediating the effects of changes in climate and fire weather. Early post-fire boreal forests (10-15 years postfire) are often dominated by mixed conifer-broadleaf or broadleaf regeneration, considered less flammable due to the higher foliar moisture of broadleaf trees and shrubs compared to their more intact conifer counterparts. However, the strength of selfregulation in the context of changing fire weather and climate combined with the emergence of novel broadleaf forest communities and structures remains unclear. We quantified fuel composition, abundance, and structure in burned and reburned forests in Interior Alaska and used a physics-based fire behavior model (the Wildland-Urban Interface Fire Dynamics Simulator) to simulate how these unique patterns of fuel influence potential rates and sustainability of fire spread. In once-burned forests dominated by mixed conifer-broadleaf regeneration, extreme fire weather conditions allowed for sustained fire spread, suggesting that intense fire conditions can enable reburning, even 10 to 15 years following a previous high-severity fire. However, fire spread was not sustained in thrice-burned regenerating broadleaf forests, where regeneration was often dense but more clumped, and thus less connected, separated by patches of bare soil. Crown fire traveled an average of 50 meters into thrice-burned forests before dying out, even under extreme fire weather conditions. This work suggests that fire spread may be possible in once-burned regenerating forests under extreme fire weather conditions but may be more limited in less connected and less fuel abundant thrice-burned regenerating forests, at least within the 10-15-year window post-fire.