## 2 Several Concurrent Risks
### Present and Future Aridity: The Larger Threat
in southern Europe are mainly due to more frequent drought, often interacting with higher temperatures (higher water demand) and pathogenic attack [64; 65; 66; 67; 68; 69; 70], all of which are associated with further decreases in nutrient-cycling rates, changes to soil trophic-web structure, and lower soil fertility [71; 72]. Figure 2: Schematic of the main processes of forest degradation to shrublands and stypes linked to soil degradation, aridity rise, and inadequate human management in the Mediterranean Basin. Studies have also widely observed that tree dieback under drought conditions is associated with a depletion of reserves of nonstructural carbohydrates [73] and with a general increase in tree dormancy [74] and decrease in gross primary production (GPP) [75]. Mediterranean trees in their current distributions are thus frequently at the limit of their capacity of positive net primary production, so a further increase in aridity would be catastrophic [76; 77; 78]. The suitable period of growth of _Q. alex_ forests has been advanced by approximately 10 days by advancing the onset of spring by winter warming, but an early cessation of growth in spring and summer by approximately 26 days has reduced tree production and growth capacity [76]. This limiting situation has even been detected in the most sextic tree in the Mediterranean Basin, _Pinus halepensis_, whose populations in the semi-arid limit of its distribution in southern Europe have already been strongly affected by drought in the last two decades [77; 78]. The overall Mediterranean domain is projected to shift to North in the coming decades [5; 79]. Several Mediterranean forested areas are on steep slopes and thus frequently have shallow soils, so they are more sensitive to drought as observed in climate models in scenarios of atmospheric CO\({}_{2}\) concentrations affected, with reductions in growth during and after the severe droughts of the 1990s and rising [80]. Therefore, forests tend to be replaced with shrubland and even steppe vegetation as projected in climate models for scenarios of rising atmospheric CO\({}_{2}\) concentrations [81] (Figure 2). A recent study with satellite data found that an expansion of slow-growing evergreen hardwood Mediterranean species, forest dieback and defoliation of mainly non-Mediterranean species, and diseases and pests could be observed along the gradient between temperate wet and Mediterranean climates in western Europe [82]. Even replanting with the most drought-resistant Mediterranean tree species in desert areas of southeastern Spain has been negatively affected by the increasing frequency and intensity of drought, with more defoliation and decreased growth [83]. Some studies have observed how drought can limit the potential positive response to rising atmospheric CO\({}_{2}\) concentrations [84]. The expansion of shrublands at the expense of forests is thus expected from the interaction of more intense and extreme recurrence of droughts associated with warming and large fires [85; 86; 87; 88]. Interestingly though, the increasing patchiness of forests, however, can increase the capacity of forests to resist future drought. [PERSON] et al. (2015) [89] reported that decreasing the fragment size of holm oak forests in central and southern Spain improved resistance to further drought by improving local environmental conditions, with higher water-use efficiency and availabilities of soil water and nutrients. The situation of Mediterranean forest associated to global warming is especially critical in the North Africa Mediterranean forest. The impact of drought-aridity rise in the last few years of forest dieback in North Africa has been especially intense from 1970s-1980s and up to the current days [90; 91; 92; 93; 94; 95; 96; 97; 98]. This process is especially intense in several areas where the overgrazing pressure, the expansion of crops, and the urbanization, together with wood exploitation have continuously risen [98; 99; 100; 101; 102].