# Mapping the Most Susceptible Regions to Fire in Portugal
## 1 Introduction  
Over the last decades, an increasing trend in larger and more severe wildfires has been generally observed over many fire-prone regions, namely in the Mediterranean basin [1, 2, 3], California [4, 5, 6], and Australia [7, 8]. More frequent hot and dry summer conditions, combined with climate change and high fuel accumulation over time and space, have already been shown to promote large and intense fire seasons across these regions [9, 10, 11]. Severe wildfires result in biodiversity destruction, forest damage, timber losses, carbon and nutrient cycling disruption [12, 13, 14], economic losses and human casualties, and potential post-fire effects, such as soil erosion and debris flows [15]. Portugal has a Mediterranean climate, which is predominantly characterized by hot and dry summers [16]. In this season, due to the geographical position of the country, the advection of dry and warm air from Continental Europe and North Africa can amplify the exposure of Portugal to high temperatures [17] and, consequently, to fire incidence, as 97% of extreme wildfires occurred under heatwave conditions in the period 1981-2010 [18]. Among the Mediterranean countries, Portugal has been recording a high extent of burned areas during the last decades [10]. Hundreds of thousands of hectares have burned, and a wide range of studies have shown that the recent occurrences of extreme fire seasons, suchas in 2003, 2005, and 2017, were closely linked to anomalous hot and dry summers, and also to high fuel availability, promoted by favourable conditions in the pre-fire season [10; 18; 19; 20]. During the last four decades, [PERSON] et al. [21] observed a positive trend in the number of fires, as well as in the burned area in Portugal. This positive trend can be partly related to the high number of fragmented landscapes with multiple owners (public and private) that have resulted in the strong dynamics of the country's landscape [22]. Many properties have been abandoned over the last years due to population migration from rural to urban areas [23]. As a consequence, an increase in areas covered by disorganised tall shrublands and forests, and a decrease in managed agricultural and short shrublands areas, resulted in fire risk enhancement [23; 24]. In the context of the occurrence of recent large fires, and especially after the catastrophic fire season of 2017, the Portuguese government and scientific institutes have been implementing strategies and policies regarding fire prevention and firefighting [25]. Many studies have focused their analysis on improving fire prediction and assessment, using machine learning techniques, e.g., [26], and on the identification of the areas that are more vulnerable to fire occurrences. The latter usually relies on meteorological variables and fire danger ratings [27], or on their combination with morphological factors, such as elevation, slope angle, population density, fire's exposition, and land cover [28; 29; 30; 31]. These studies strongly improved the knowledge of fire regimes in Portugal and showed where the responsible authorities should act in order to mitigate the fire risk in certain regions which are highly exposed to fire. Nevertheless, the understanding regarding the role of vegetation activity in fire susceptibility is not fully clear. The fuel accumulation and availability in ecosystems constitutes a triggering factor in enhancing fire intensity and, as pointed out by [PERSON] et al. [32], non-managed landscapes, which have the potential to gather similar conditions to those experienced during the 2017 fire season, will promote the continuity of potentially destructive wildfires. Within the scope of fire prevention, the identification of the most susceptible areas to burning is essential, for example, for policy and prevention decision making.