## 2 Several Concurrent Risks
### Forest Wildfires  
Fire is caused by the integration of climate, topography, changes in land cover (fuel characteristics) and use, and socioeconomic factors. Fire is a frequent and historical long-term endemic disturbance in Mediterranean climates [57]. Large seed banks and the presence of savannoid woodlands are associated with the high resilience of some forest communities that conserve and retain nutrients and where soils quickly recover after a fire [194; 195]. The changes in climate and soil in several Mediterranean areas have already increased the risk of fire in the Mediterranean Basin [41; 196]. The increasing movement of human populations from rural to urban areas frequently tends to favor the build-up of fuel and thus larger and more intense fires. Decreases in the presence and density of human populations, however, reduce the probability of fires caused by humans [197; 198]. The decrease in forest biomass due to harvesting by humans for energy likely decreases the risk of fire in rural areas undergoing depopulation, since this practice reduces the fire hazard [199]. Depopulation provides a scenario of fewer but larger and more intense fires. We are thus assisting to the intensification of the classical role of fires in Mediterranean landscapes, with extensive and sudden destruction of forests and other ecosystems such as shrubland, grassland, and cropland, greatly impeding natural regeneration [200; 201].  
Drought has a negative synergy with the increase in more extreme wildfires and mainly extreme summer droughts [5; 202; 203; 204; 205; 206; 207; 208; 209]. This synergy, when fires follow droughts, promotes shifts in dominance from seeder to resprouter species [205]. Droughts after fires, however, strongly affect post-fire effects on the status of soil nutrients [208]. One hundred years of observational data from 77 sites with and without fires in Spain and France indicated that water stress converted forests in open shrubland by hampering recovery after a fire [209]. Thus, the high fire frequency in some Mediterranean areas can transform forest to shrubland [210; 211], and the combined effects of aridity and fire constitute another important positive synergy favoring the conversion of forest to shrubland [209]. The area damaged by fires has increased in the last few decades in several parts of the Mediterranean Basin [212]. Trade-offs between resistance to drought versus resistance to fire have recently been observed, e.g., resistance to embolism (a good adaptation to drought damage) has been negatively correlated with fire tolerance in conifers, which increases flammability when the water content of soil is low [213]. Forest fires in areas with Mediterranean climates greatly degrade the soil, mainly due to an increased erosion on steep slopes from torrential rain [214; 215]. Fire is a driver of desertification that is linked to a continual positive feedback of higher frequency and intensity of torrential rainfall (associated to climatic change) with an increase of soil erosion, which in turn leads to a loss of soil fertility and thus plant cover. These can also be aggravated by human activities such as excessive livestock pressure, and by increasing the frequency and extend of fires [18]. Fires briefly increase the concentrations of carbon (C) and N in the upper soil layers [216], but soils frequently lose C and N long after a fire [217; 218]. Modeling has strongly linked past fires with immediately previous droughts [219] and has projected high and important risks of fire in the near future in areas of the Mediterranean Basin where aridity is projected to increase [220; 221]. The soil temperatures reached during a fire in Mediterranean forested areas is another factor determining the effects of fire on the regeneration of soil quality. Temperatures <220 \({}^{\circ}\)C have little impact on soil conditions, enabling rapid recovery of vegetation, whereas the effects of temperatures >220 \({}^{\circ}\)C on soil degradation are more intense, causing a slower recovery of vegetation [222].