Flore et végétation actuelles de l’Afrique du nord, leur signification en fonction de l’origine, de Involution et des migrations des flores et structures de végétation passées

PRESENT VEGETATION AND FLORA OF NORTHERN AFRICA, THEIR MEANING IN RELATION TO THEIR ORIGIN, EVOLUTION AND MIGRATIONS OF FLORAS AND THE STRUCTURES OF PAST VEGETATION In the light of recent works and biogeographic synthesis, the Mediterranean flora appears more and more as a heterogeneous entity, reflecting, to a great extent, the palaeogeographic and palaeoclimatic history of the region. In particular, the co-existence of elements of southern stock and of northerly elements, presently points to the possibilities of exchange which occurred very early in the Tertiary between the Gondwanian type of floras or the less tropical types and the Laurasian floras.The tropical elements are numerous and can be linked to various entities according to their age; a pantropical entity comprising in particular, Tetraclinis and Warionia, but also various families, is common to all the tropical regions and, without any doubt, contemporary with the dismemberment of Gondwana; a north-tropical entity peculiarly common to California and the Mediterranean region; a palaeotropical entity strongly heterogeneous and complex. One finds there: — thermophilous sclerophyll types often linked to the African rainforest species, — old xerophilous types, distributed in South Africa and north of the Equator (randflora), — endemic taxa of high African mountains, showing affinities with Ethiopian species or of the high African mountains, — taxa more recently arrived or even common sahelian species settled during the last pluvial. The elements of extratropical stock are composed of autochthonal or Mediterraneo-Tertiary elements, and of northern elements. The Mediterraneo-Tertiary elements are the remnants of differentiated floras generally in situ on the banks of the Tethys and on the micro-plates which occur there. The role of the Iberian micro-plate is particularly important in the western Magreb. It is advisable to associate them with various species belonging to the Irano-touranian and Saharo-Arab stocks, whose settlement is often recent. An oro-Mesogean entity is particularly important and brings together the endemo-vicariant taxa generally occurring from the Atlas to the western Himalayas. The northern elements bring together a mesothermal entity, a remnant o f the pre-glacial Lauresian floras, poorly represented in north Africa, a microthermal northern entity generally comprising species recently established and a north-alpine entity contemporary with the last glaciations, extremely localized on the high Atlas mountains. Finally, the origin of the main characteristic formations of the Mediterranean stages is examined and discussed.