The effectiveness of past wildfire in limiting reburning is short-lived in a Mediterranean humid climate

Abstract Background The study of wildfire interactions (i.e., spread limitation and reburns) is gaining traction as a means of describing the self-limiting process of fire spread in the landscape and has important management implications but has not been attempted in Europe. We examined to what extent previously burned areas restricted the development of large wildfires (>500 ha) in mainland Portugal. Results We analysed wildfire areas in Portugal (1984-2021) and (1) modelled the proportion of large wildfire perimeters that ceased spread at fuel-age transitions to younger fuels resulting from previous wildfires, and (2) characterized patterns of large wildfire preference (and avoidance) for fuel age. Only 4% of the large wildfires did not comprise edges intersecting past wildfires. Low fuel age (especially up to 8 years post-fire) resulted in a high probability of a large wildfire perimeter being limited at fuel-age transitions, further enhanced by high historical burn probability and proximity to roads and watercourses. Perimeter limitation was lowest for Summer wildfires (and high fire-suppression preparedness). The response of wildfire preference to increasing fuel age was extremely variable and thus an overall weak pattern emerged, with avoidance of fuels < 3 years and equal preference for ≥ 6-year-old fuels. Conclusions Large wildfire growth was hampered by building-up fuel resulting from antecedent wildfires. The fuel-reduction effect is short-lived in the prevailing Mediterranean humid climate of the study region. Nonetheless, it should be considered when planning fuel-reduction treatments and can be used as an opportunistic fuel treatment during large wildfire suppression operations.