Mediterranean climates induce severe and contrasted stresses to habitats and species. These stresses are compounded by the unpredictable nature of weather patterns, and organisms have to cope with this temporal variation in climate and resource availability. Ecological and eco-physiological studies indicate that Mediterranean species demonstrate similar strategies to resist climatic and edaphic stress. Drought stress proves to be the essential climatic factor responsible for the restriction of productivity, growth, and survival of several groups of
Mediterranean plant species, notably the evergreen woody plants. Sclerophyllous leaves also exhibit other water conservation features such as sunken stomata and low cuticular conductance. Other strategies to cope with water stress are related to complex root systems, cellular tolerance to low water potentials or high secondary compound production (e.g., terpenes, tannins).
The drought summer season also induces original physiological strategies for the two main groups of the Mediterranean soil microfauna; oribatid mites are more resistant to dryness and only migrate into the deep soil layers when the soil water content becomes too low; collembolas cross the summer in an egg stage, and several species can surmount the dryness by a deshydratation process similar to anhydrobiosis. There exists a balance of the collembola composition between, on one hand, the winter populations which are composed of common species but highly diversified qualitatively and quantitatively, and the other hand, the 'reserve populations' present in a latent state in the soil which are expressed only when exceptional summer rainfalls occur or during the onset of the wet season.
On the community level, a striking example is represented by the biotic assemblages of vernal pools, precipitation-filled seasonal wetlands found mainly in the Mediterranean climate regions. Inundation during the growing season largely eliminates colonization by upland species in the pools whereas the terrestrial phase is sufficiently desiccating to prevent establishment of typical wetland species. Several cosmopolitan aquatic plant genera are shared between the five Mediterranean ecoregions, such as ferns (Pilularia, Marsilea, Isoetes) or dicots (Callitriche, Elatine, Ranunculus), whereas vernal pool specialists are often derived from genera of terrestrial origin. The essential ecological characteristic ofthese temporary ponds is the stochastic alternation of flooded and dry ecophases within and between years. This induces strong year-to-year differences not only in pool hydrology, but also in the composition and dynamics of vernal pool communities, with a strong temporal and spatial segregation which limits competition as it is the case of the larvae of anuran amphibians. The succession of contrasted phases favors the emergence of varied and highly specialized plant and animal communities, particularly adapted to this high habitat instability. These environmental factors also played a significant selective force in shaping life strategies of temporary pool species. Dormancy provides a determinant means of enduring prolonged unfavorable dry periods, and several typical vernal pool species possess mechanisms that keep them from emerging under unsuitable conditions: drought-resistant reproductive organs such as seeds or oospores for the well-represented annual plants (c. 80% of the whole vernal pool specialists in California and the Mediterranean Basin), and eggs or cysts for crustaceans
(e.g., cladoceran and anostracan branchiopods). A great adaptability of the life cycle often exists, and when water levels are shortened and water temperature increases, invertebrates and amphibians can present an advanced metamorphosis and several annual plants known as ephe-merophytes can complete their entire life cycle within only a few weeks.
If we consider the main ecological processes linked to climatic stress, several studies have demonstrated that competition increases with aridity. Summer drought and poor soil nutrients, coupled with frequent disturbances, explain the reduced rates of competitive displacement observed in most of the Mediterranean communities. Changes in species interactions along water gradients were also demonstrated in arid Mediterranean environments. The importance of positive interactions through facilitation in arid plant communities represents a complex and unrecognized process. Nevertheless, some recent studies suggest that facilitation decreases with elevation in dry Mediterranean mountains (Sierra Nevada in Spain, central Andes of Chile) because of the prevalence of water stress over temperature stress compared to mesic alpine communities.
In addition to summer drought, low winter temperatures and episodic frost exert a strong influence in limiting distribution range and causing alteration in species composition and productivity, notably for ecosystems situated in the northern limits of the Californian and Mediterranean Basin ecoregions. From freezing tests, three groups of plants can be distinguished with regard to frost susceptibility in the Mediterranean Basin: (1) the most sensitive species with 50% frost injury to the leaves at —6 to —8 °C and to the shoots at —9 to —15 °C (Ceratonia siliqua, Myrtus communis); (2) the medium sensitive species with serious damage to the foliage at —12 to — 14 °C, and to the stems at —15 to —20 °C (Olea europaea, Quercus coccifera, Pinus halepensis); (3) the resistant species, not seriously damaged until —15 to —25 °C (Quercus ilex, Cupressus sempervirens). The distribution of Mediterranean species of the Northern Hemisphere is indeed shaped by frost events such as late spring below freezing temperatures and absolute minimum temperatures. For example, the extremely cold winters of 1956 and 1985 in the northern Mediterranean contributed to the determination of the distribution of some keystone trees such as olive tree, Aleppo pine, and holm oak.
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