The Mediterranean ecoregions are usually defined by their particular climates, which are transitional between temperate and dry tropical climates. The main characteristic is the existence of a combined dry and hot summer period of variable length, which imprints a strong water stress on species and ecosystems during summer. A high unpredictability characterizes these Mediterranean climates, with high yearly variation in timing as well as amount of rainfall or occurrence of extreme temperatures.

Rainfall is extremely variable, with mean annual values ranging from 100 to 2000 mm. The lowest values are found at desert margins, especially in North Africa and the Near East. The isohyet of 100mmyr— represents the borderline between the Mediterranean and the Saharan climates. Rainfalls higher than 1500 mm are mostly found at medium altitudes of some coastal mountain ranges. But Mediterranean-type climates differ markedly between and within the Mediterranean regions, in terms of total rainfall and seasonality. For example, annual rainfall of parts of southern California and central Chile are comprised between 250 and 350 mm, whereas some Mediterranean montane sites of SW Africa receive as much as 3000 mm by year, and the summer rainfall is similar here to annual totals for California or Chile.

Mean minimum temperatures of the coldest month (m) are often used to define climatic subdivisions in the Mediterranean Basin (Table 2). These values are correlated to elevation and to a lesser extent to increased latitude and continentality. In most places, m is between 0 and +3 °C although extremes can reach +8 to +9 °C in desert margins and —8 to —10 °Con the highest mountains.

Aridity and temperature play an essential role in the structure and composition of Mediterranean ecosystems.

The Emberger pluviothermic quotient (Q£) constitutes the most utilized index for classifying Mediterranean climates:

_ 2000P

where P is the annual rainfall (in mm), M is the mean maximum temperatures of the warmest month of the year, m is the mean minimum temperatures of the coldest month of the year.

According to the levels of humidity and the winter severity, several bioclimatic zones and thermal variants are respectively defined, and they can be included in the climagram of Emberger; their combination permits to define six main bioclimatic types (Table 3).

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