The effectiveness of past wildfire at 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 scarcely been attempted in Europe. We examined to what extent previously burned areas restricted the development of individual large wildfires (> 500 ha) in mainland Portugal. Results For the 1984–2021 period, we (1) modeled the proportion of large wildfire perimeters coinciding with transitions to shorter time since fire (TSF), i.e., locations where fire spread ceased upon encountering assumedly less flammable fuels, and (2) characterized the prevalence of different TSF in the composition of the area burned by large wildfires in relation to available TSF. Only 4% of the large wildfires did not comprise edges intersecting past wildfires. Low TSF (especially up to 8 years) resulted in large-wildfire perimeter limitation at TSF transitions. This effect was further enhanced by high historical burn probability and proximity to roadways and watercourses. Perimeter limitation did also increase under high (but not very high or extreme) fire danger, benefiting from maximum seasonal firefighting preparedness. TSF prevalence in the composition of large-wildfire area was extremely variable and thus an overall weak pattern emerged, with minimum and maximum prevalence respectively at TSF < 2 years and TSF ≥ 6 years. Conclusions Large wildfire limitation in Portugal is hampered by fast fuel build-up after fire, indicating a short-lived fire-hazard reduction effect under the prevailing Mediterranean humid climate of the study region. Nonetheless, such effect should be considered when planning fuel-reduction treatments and can be used opportunistically during large-wildfire suppression operations.

Time for Managed Wildfire in Europe: The Pioneering Integrated Fire Management Plan of the Aran, Pyrenees

“…Results show that 0.75 hectares burnt in high intensity are avoided for every hectare of controlled burns invested. Recent work in Portugal (Davim et al 2022;Davim et al 2023) found a lower ratio, i.e., 1:0.2, whereas another work in the same area found an effect of 1:0.9 (Price et al 2015). Our results are within the range found in the literature and denote a modest effect of a prescribed burn in reducing highintensity res predicted under climate change.…”