## 2 Several Concurrent Risks  
### Present and Future Aridity: The Larger Threat  
The current increase in aridity in the Mediterranean Basin in recent decades is associated with higher temperatures in southern Europe and northern Africa [36; 37; 35]. Most models project larger increases in aridity in the Mediterranean Basin [37; 38; 39; 40] and a shift of current arid conditions towards northern latitudes [5]. Based on various scenarios of future climate projection, [PERSON] et al. (2016) [5] reported that the current climatic area of the basin would shift to other areas by the end of the 21 st century and that the current area  
Figure 1: Sites with observational/experimental data used in this study. would decrease by 15-23%. Central and southern areas of the Iberian Peninsula, southern Italian Peninsula, some Mediterranean islands, central Turkey, North Africa, and southeastern Greece are the most affected areas, with larger shifts in the area of Mediterranean climate towards hotter and drier climates. Extreme precipitation and a general decrease in total precipitation and increase in temperature, however, are also expected [41; 42]. A general increase in aridity, coupled with stronger and potentially destructive storms causing flood damage and extreme heatwaves, are thus possible [41; 42; 43]. These changes have already had several impacts on plant-soil systems in Mediterranean forested areas [18; 44; 45] and are expected to continue in the near future [18]. For example, a long-term (20 years) climatic manipulation in a holm oak (_Quercus izk_) forest has produced a continuously lower availability of soil water similar to that projected for the near future and has demonstrated that the capacity of the trees to adapt to this level of drought is higher than previously expected [46]. Most species of tall evergreen shrubs, however, have increased in coverage and biomass more than _Q. ilex_[47; 48], and the forest structure can be transformed into a shrubland structure under more frequent intense heatwaves and droughs [10; 18; 46; 49]. To complete this scenario, we must consider that the Mediterranean climate has frequent torrential rains [50] that are projected to increase [51] and thus produce larger nutrient losses in the near future. In this context, the rise in drought periods together with more frequent extreme rain events can exert a strong impact on Mediterranean soil C:N:P ratios due to the different cycling and solubility/volatilization of these three elements [18]. This can in turn also have asymmetrical effects favoring more some species than others [18]. A cascade of feedback effects among increased aridity, more frequent torrential rains, more intense and frequent fires, and changes in land use have thus exceeded the capacity of Mediterranean plants to cope with drought and other stresses [52; 53] (Figure 2). For example, several areas of central and southern Spain suffer from advanced stages of degradation and losses of biomass and soil, ultimately leading to desertification [18; 54; 55]. This scenario is not favorable for maintaining the current area of forests in the basin and thus for conserving their current structures, species compositions, ranges and diversities [18; 56; 57; 58]. In fact, forests have clearly moved towards higher altitudes of the basin in the last few decades [59; 60]. Several studies have consistently reported that annual growth and seed production in Mediterranean forests in recent decades are strongly correlated with annual drought intensity [61; 62]. Warmer and drier conditions linked to increases in Atlantic multidecadal oscillations (AMO) are associated with the increase in post-1990 defoliation in the forests of Spain [63]. Experimental, observational, and metadata studies have clearly indicated that the decreases (dieback, defoliation, and lower growth) in Mediterranean oaks (_Quercus_ spp.) and pines (_Pinus_ spp.)