Fuel age, weather and burn probability in Portugal

The relative influence of the factors acting on burn probability, namely fuel and weather, is not well understood, especially in Europe. We use a digital fire atlas (1975-2008) and apply survival analysis to individual fires (1998-2008) to describe how burn probability changes with fuel age in Portugal. The typical fire return interval and median fire-free interval vary regionally from 23 to 52 and 18 to 47 years. Increase of the hazard of burning with time is generally near-linear, denoting moderate fuel-age dependency, as in some other shrub-dominated Mediterranean environments. Analysis of complete fire intervals resulted in shorter fire return interval and higher fuel-age dependency of burn probability than findings that included censored observations. Increasingly severe weather conditions either expressed through fire size or by extreme fire danger concurrently decreased fuel-age dependency and selected older fuels. The results are discussed from the viewpoints of fire suppression and fuel treatments.