Actualisation des limites de l’aire du bioclimat méditerranéen selon les critères de Daget (1977)
Authors: Roumieux Camille , Raccasi Guillaume , Franquet Evelyne , Sandoz Alain , Torre Franck , Metge Gérard 
Published: April 2020

The Mediterranean basin is a set of geographical areas at the edge of the Mediterranean sea, but its boundaries are not unanimously established. They result in a multi-millennial coevolution between natural ecosystem and human societies with a complex geological and biogeographical history. The Mediterranean basin shows a remarkable diversity of species, habitats and landscapes. The Mediterranean climate is a transitional stage between temperate and tropical climate (Emberger 1971 ; Quézel 1985) with a dry and warm summer period (Quézel et Médail 2003). The question of a delimitation of a Mediterranean climate envelop (MCE) is discussed from mid 19th century (Grisebach 1866). Numerous definitions according to floristic, climatic or bioclimatic criteria have been proposed with corresponding geographical maps. Daget (1977) proposed the two different following criteria : (1) summer is the driest season and (2) a drought physiological indicator < 5. Criterion 1 is analogous to the one of Emberger (1941) and criterion 2 is in agreement with Bagnoul et Gaussens (1953, 1957). Daget (1977) used a standard seasonal approach (4 calendar seasons). Daget approach is the result of a long sequence of publications. Another justification of the choice of Daget criteria is the popularity of the resulting map. Historical map of Daget (1977) is based on a network of a maximum of 520 meteorological stations from the Meteorological Office of 1954. Our updated map is based on theWorldclim database, version 1.4 which gathers monthly precipitation and temperature from a network of about fifty thousand worldwide stations (averaged on a 1950-2000 period). Worldclim database proposes interpolated data on a 1 km2 definition at global scale. The method used for interpolation is proved to be especially accurate in regions with important topographical variability (Hutchinson 2004). At the scale of the Mediterranean Basin, the interpolation is based on 10177 stations. Our work proposes an updated map of the MCE of Daget taking advantage of the densification of the meteorological stations network (10177 vs. 520 stations). Historical and updated maps represent respectively areas of 11155000 and 11988000 km2. Differences are mainly explained by the need of subjective interpretation of low density station areas. Some zones were excluded by historical MCE and now included : South part of Arabian Peninsula, Kazakhstan, Kyrgyzstan, Tajikistan, Pakistan and Russia. Other regions are now excluded in the updated MCE : Libyan and South Egyptian regions, the Northeast of Spain, the North of Italy and the Balkans. The updated MCE from Daget (1977) is constrained by criterion 1 in the South. European north-western limit is constrained by criterion 2. In Asia, criterion 1 constrains the Northern limit of the updated MCE. Mapping information was obtained by a specific geographical system from the synthesis of different climate information layers (precipitation, temperature). Original map from Daget (1977) has been digitised to allow direct comparison. These tasks were processed in Arcgis 9.2 software. Finally, the question of the multiplicity of ECM’s proposed in the literature is addressed and criterion convergence perspectives are questioned. Updated maps from concurrent historical maps have been obtained. In a near future, we will propose an evolution of the MCE using a more relaxed definition of seasons. Also in the midterm, distinction of bioclimatic stages in the Mediterranean region will be proposed. This subdivision of MCE could be obtained using altitudinal stages, pluviothermic ratio of Emberger (1955) or the detailed seasonality of maximum precipitation (spring, autumn or winter). Downloading of the corresponding map is possible at http://ecologia-mediterranea.univavignon.fr/fr/mini-site/miniecologia/archiveselectroniques.html