## 2 Several Concurrent Risks
### Present and Future Aridity: The Larger Threat
#### 2.1.2 Grazing Pressure under Drought Enhancement  
Disturbance by livestock and over-exploitation have frequently increased, thus aggravating the negative effect of the reduction in natural precipitation [117; 118]. These disturbances have been especially observed in savannoid woodlands and dehesas, the agroforestry human managed ecosystem where the Mediterranean evergreen genus _Quercus_ is dominant [54; 119]. This savannoid system combines livestock exploitation with pigs and bovine with grain crops mainly located in the central and southern Iberian and Italian Peninsula [54; 119]. These systems are currently frequently accompanied by encroachment that has even worsened the tree status by increasing the competition for soil water between deep-rooted shrubs and trees [120]. However, the livestock pressure is decreasing very fast in large parts of the northern Mediterranean basin, to such an extent, that herbivory has fallen to levels unnaturally low [119]. This is leading to dense pioneer forests going hand in hand with massive loss of botanical diversity of small light demanding species that need levels of grazing [119]. The increasing frequency of annual extreme summer droughts in recent decades has been directly monitored at the plot level [121; 122] or by multitemporal imagery from satellites such as Sentinel-2 [123]. These studies have found that extreme droughts can have more detrimental effects on Mediterranean forests than a long-term constant increase in drought intensity [122], and that the increasing frequency of extreme droughts decreases resistance and resilience of forest ecosystems, exacerbating the recovery of Mediterranean forests [121]. 1.3 Aridity and Warming: An Open Door for Pests. The Case of \"La Seca\" (_Plyotrophthora cinnamomi_ [PERSON]) and Insect Outbreaks  
The increases in heatwaves, warming, and droughts have been associated with pathogens in Mediterranean forests, such as the pine processionary moth (_Thaumetopoea pityocampa_ [PERSON]), which has been expanding even more to northern forests and that hasbeen correlated with a reduction in the growth of pine trees in several pine forests in the Mediterranean Basin [124; 125; 126]. The aggressive _Pluophthora_ fungi (mainly _P. cinnamomi_), widely known in the Iberian Peninsula as \"_La seca_\" [127], however, is causing the largest areas of mortality. Infestation by _P. cinnamomi_ is currently the main direct cause of declines in _Q. alex_ and _Q. suber_ forests in several areas of southern Europe [128], with a large capacity to kill these two species [129]. The increasing spread of this pathogen has been favored by warmer temperatures and increased aridity [130; 131; 132; 133; 134], making it very difficult to discern between drought, rise in temperature or _P. cinnamomi_ as the main cause of _Quercus_ forest decline [135]. This pathogen is expected to spread further with the projected warmer temperatures and increase in drought in Mediterranean and sub-Mediterranean areas of southern Europe, mainly in _Quercus_ forests [134; 135; 136; 137]. Recent satellite images and photointerpretation studies in locations of affected trees in southwestern Spain from 2001 to 2016 have identified spatial patterns consistent with the hypothesis that _Pluytophthora_ is the main cause of open oak forests (dehesas) defoliation and decline in the study areas that may also be influenced by increased drought [138]. Infestation by _P. cinnamomi_ can interfere with and suppress the expression of tree genes involved in chemical defenses such as tannins [139], and the metabolomic shifts of trees affected by this pathogen are similar to those for drought [140]. The early stages of infestation by _Pluytophthora_ spp., however, are difficult to detect [140]. Many recent studies and experiments have provided more information about _Pluytophthora_ spp. infestation. The decline of _Quercus_ forests caused by _Pluytophthora_ spp.