Les groupements chionophiles de l'Alborz central (Iran) Comparaison avec leurs homologues d'Asie centrale par J.C. KLEIN, Paris avec 1 carte, 2 figures et 3 tableaux Abstract. An analysis of species combinations and its correlation to ecological factors was applied to the study of chionophilous communities of central Alborz. The following five phytosociological units could be defined : the class 0 x y t r 0 p ide tea per sic a e , the orders T rae h y die t a I i a d e pre s s a e and Cat a b r 0 s e I let a I i a par v iflo rae and the associations J uri n e I let u m f rig ida e and E rig e ron e tum elbursensis. The ecology of these communities is closely related to the snow cover. The water supply of 0 x Y t r 0 p ide tea per sic a e is only made by melting of snow and consequently de pends, in duration and quantity, on the volume of snow accumulated on the stand or nearby. During the bad season, they are protected by the snow cover. The J ur i nell e tum f rig ida e which is rapidly uncovered by snow is consequently the most xerophilous and the most cryophilous association. In contrast , the E rig e ron e tum e I bur sen sis is longer under snow. Duration of snow cover differentiates the subassociations ; these are considered mesophilous and hygrophilous respectively. The affinities linking 0 x y t r 0 p ide tea per sic a e and S a lie e tea her b ace a e Br. BI. and the communities of dry pozzines described by QuE ZEL in the high mountains around the Mediterranean (Sierra Nevada, North Africa, Greece and Turkey) are ecologic and physiog nomic, but never floristic. So it is with the corresponding syntaxa of Caucasus. However, the 0 x y t r 0 pet ai m mersaedescribed by UKAlCHEVA , in oriental Pamir, display several physiognomic, ecologic and floristic similarities with 0 x y t r 0 p ide tea per sic a e. They constitute a remarkable example of vicariance. In fact, the area of this class borders on the area of 0 x Y t r 0 p ide tea i m mer s a e, as is shown by scattered phytosociological data, some of them unpublished, about Indu-Kush and the Gissar moun tains. The lowering of the mountains creates a major discontinuity which isolates the area of the 0 x y t r 0 p ide tea per sic a e from that of the 0 x y t r 0 p ide tea i mmer s a e. During the Pleistocene, the rising of Kopet-Dagh which attained a relatively low altitude, placed these two massifs contiguous to each other ; but above all, during the gla ciations, when the vegetation belts were lowered, the orophilous flora coming from the massifs of Central Asia, was able to reach Alborz via Kopet-Dagh. At that time, only one chionophilous syntaxon was able to spread from the massifs of Central Asia to Alborz. The changes of climate, which took place after the glaciations, could have disrupted the chrono logical unity of the ancestral syntaxon which gave rise to the 0 x y t r 0 p ide tea i mmer s a e and the 0 x y t r 0 p ide tea per sic a e. Therefore, the chionophilous communities of Central Alborz have a remarkable flo ristic component of Central Asiatic...
