Le bioclimat Mediterraneen: Analyse des formes climatiques par le systeme d'Emberger

Summary

In this second report on the Mediterranean bioclimate, the author treats the method developed by Emberger (1930 to 1971) to analyse the Mediterranean climate as defined in the previous report (Daget 1977).

First, the origins and justification of Emberger's pluviothermic quotient are treated. It is recalled how Emberger subdivided the climagram area into 6 zones corresponding to as many bioclimatic vegetation stages, based on studies of the Mediterranean vegetation. These are, in sequence from the most arid to the most humid and from bottom to top on the climagram, Saharian (or per-arid), arid, semi-arid (or sub-arid) sub-humid, humid and per-humid stages. Thermic variants are distinguished according to the values of m; there are variants with hot winters for 7<m, temperate for 3<m<7, cool for 0<m<3, cold for −10<m<0 and very cold for m<−10 in °C.

It is pointed out that these prineiples have been used widely in applied climatology: climatic maps, distribution of several species (Quercux ilex, Pistacia lentiscus, Pinus halepensis, etc.).

The modern concept of vegetation stage does not correspond to a simple vertical superposition of different bioclimates, but rather to a synthetic unit including altitudinal superposition and latitudinal succession like a geological stratum. In conclusion the Mediterranean character, linked to the precipitation-rate, is independent of the absolute value of the pluviometric modules, be it very small; the true desert is characterized by disorganized precipitations: no clear rate and interval between two showers of rain greater than one year.

Modifications applied by Emberger's successors are discussed. These relate chiefly to the presentation of the climagram in which a logarithmic scale has been adopted for Q ₂. The incluence of scale in cartographical applications in Tunisia on the results, and ecological applications, especially for analysing the distribution of certain species are discussed.

Emberger's pluviothermic quotient is compared with other climatic indices, and more particularly with evapotranspiration. It is demonstrated that Q ₂ varies as the ratio of the precipitation to the physical potential evapotranspiration above the stretch of water. There is a good correlation between Q ₂ and the empirical index of Turc although the linear link has only local value.

The influence of anthropic degradation of the environment on the aridity of the climate is discussed: a two-stage variation can be found. This climatic degradation results from the reduction in evapotranspiration and the increase in the positive balance of the radiative total amount linked to the disappearance of the vegetal covering and to the decrease of the water-reserve potential in the ground caused by erosion. It is concluded that Emberger's method is efficient for synchronic studies, but one should be most cautious in its application to diachronic situations (historical account of plant classifications or groundrestoration).