Synthèse phytoécologique des forêts de l'arc alpin

4900 phytosociological forest releves of the alpine chain are analysed with CA and CBO. This synthesis allows to confirm first the scheme of the alpine chain unicity (OZENDA, 1985), based on both the climatic opposition between the external and inner Alps and the vegetation belt concept. An important floristic divergence appears secondly between the central-eastern alpine forests and the south-western one. The nitrogen availability and the soil porosity, seem to be the main ecological factors to explain this divergence and they are paterned by temperature and seasonal rainfall distribution.