The Effect of Prescribed Burning in Decreasing Wildfire Extent in Portugal ^†

Abstract

Wildfire management in southern Europe is increasingly challenged by the combination of fire weather, flammable landscapes, rugged terrain, and the wildland–urban interface. Prescribed burning (PB) is a promising fuel reduction treatment, but evidence of its effectiveness in reducing wildfire extent in Europe remains scarce. Using 36 years of fire mapping in Portugal, we aimed to assess PB’s effectiveness in reducing wildfire extent. We quantified PB–wildfire encounter probability through survival analysis and evaluated the unburned fraction of PB patches upon wildfire encounter as a function of fuel-, topography-, treatment-, and wildfire-related characteristics, using generalized linear modeling and regression tree analysis. We used linear and generalized linear mixed models to quantify the annual wildfire extent decrease due to PB in nine selected sub-regions, i.e., PB leverage. Larger PB units survived more wildfire encounters, which were frequent (42% of the total) and happened soon after treatment (75% of the cases in the first 4 years). However, the unburned fraction was very low and was determined mostly by wildfire behavior. Wildfire extent was variable among the study regions, but the annual PB effort was typically less than 0.5% of the landscape. Wildfire extent was dominated by fire weather and ignition density but decreased with past fire extent. This resulted in an estimated PB leverage value of 0.10, i.e., 10 ha of treatment are currently required to decrease wildfire area by one ha, which is consistent with the PB-wildfire encounter findings. Our results highlight the need to scale up the PB effort in Portugal. Although the treatment location favors encounters, PB units should be larger and distributed in the landscape such that the wildfire spread is disrupted inside and around treatments, thus reducing its final size and increasing leverage. Our work provides information to fire management agencies towards improved planning and development of synergies between fuel management and fire suppression operations.