An Agro-Bioclimatic Classification of Arid and Semiarid Lands in the Isoclimatic Mediterranean Zones

The author favors a quantitative bioclimatic approach to describe and discuss the concept of “Mediterraneity,” its various criteria and acceptations. According to the criteria elected, the Isoclimatic Mediterranean Zone represents some 15 million km2 (including Mediterranean deserts), almost 12% of the land mass of the planet. The Mediterranean biomes habor some 75,000 species of vascular plants, including about 54% endemics. That means 25% of the world's vascular flora are present on 12% of the land mass. This bioclimatic zone is concerned with parts or the whole of more than 50 countries. This huge area is not homogenous, as one could anticipate, although summers everywhere are dry and warm (or hot) and winters rainy and mild (or cold). There are many and large variations within the overall framework. A critical analysis of some 1,500 weather stations from more than 50 countries revealed the existence of a number of submodels within the Mediterranean bioclimatic entity, including in the Mediterranean Basin itself. Definitions of such submodels are based on various significant and discriminating quantified criteria. Rainfall seasonality, characterized by a Mediterraneitry Index, may be monomodal, bimodal or trimodal, with peaks in any season but summer. Drought may be of a short or long duration, early or late, mild or severe. Summer rain may be totally missing for up to 4–5 months in the rainier zones (as in the east Mediterranean Basin) or significantly high (up to 22–23% of the annual total) in truly Mediterranean countries, such as in the west Mediterranean highland zones, not necessarily continental, for instance.