## 2 Several Concurrent Risks
### Present and Future Aridity: The Larger Threat
#### 2.1.1 Drought Is Currently Shaping the Structure of Mediterranean Forests  
An increase in drought, however, may also have different effects in different forest Mediterranean species and communities. Mediterranean plants have developed various morphological and physiological strategies to adapt to drought [36], but the largest trees in a community are frequently more sensitive and less resistant and resilient to increases in aridity [36; 103]. [PERSON] et al. (2015) [104] observed that semi-deciduous and deciduous species responded differently from evergreen species to severe summer drought. The impact of drought on defoliation and dieback can asymmetrically affect individual trees based on size, even in the same species, shorter trees are more resistant and resilient to increases in drought than taller trees, both in intra- and interspecific comparisons [105]. Tallshrub species defoliate less than dominant canopy trees under more intense drought, so tall shrubs tend to produce more leaves in the upper canopy levels in a clear step towards replacing forests with shrubland [106; 107]. The effects of drought across current environmental gradients of aridity may differ more between current xeric sites than mesic sites. [PERSON] et al. (2012) [108] reported that a specific decrease in annual precipitation was responsible for canopy defoliation in most xeric forests, dominated mainly by species such as _P. halepensis_ [PERSON] and _Q. liex_, whereas the worsened annual water balance was responsible for defoliation in more mesic forests, dominated by species such as _P. sylvestris L., P. nigra_ [PERSON] or _Abies alba_ Mill. [PERSON] et al. (2011) [109] found that the growth of species at xeric sites responded to the amount of spring and summer precipitation, whereas the growth of species at mesic sites responded more directly to the amount of summer precipitation only.  
Most studies have nevertheless found that the effects of drought on the fitness of various species of Mediterranean forest trees were much more detrimental to mesic than to more xeric species [110; 111; 112]. Genera that have evolved under Mediterranean climatic conditions, mostly shrubs, however, were more damaged by the severe drought in 1994 in Spanish woodland areas than pre-Mediterranean species, mostly trees, that evolved earlier such as _Pinus_ sp, _Quercus_ sp. or _Olea europea_ L. [49]. Contrasting results, though, have also been reported [113]. These asymmetrical effects are even more evident when comparing the Mediterranean with temperate and/or boreal tree species coexisting in mountainous areas within Mediterranean latitudes where the Mediterranean species are expanding to sites previously occupied by wet-temperate forest species [114].  
Tall shrubs such as _Phillyrea_ sp. have a higher capacity than trees to adapt and resist intensive droughts [115; 116] and have a higher capacity than sympatric trees to maintain their foliage and concentrations of nonstructural carbohydrates after droughts [116]. Experimental and observational studies have clearly demonstrated a scenario of gradual replacement in communities from dominance by trees to dominance by tall shrubs [115; 116].