Les groupements chionophiles de l'Alborz central (Iran) Comparaison avec leurs homologues d'Asie centrale
Klein, J. C.
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 Oxytropidetea persicae, the orders Trachydietalia depressae and Catabroselletalia parviflorae and the associations Jurinelletum frigidae and Erigeronetum elbursensis. The ecology of these communities is closely related to the snow cover. The water supply of Oxytropidetea persicae is only made by melting of snow and consequently depends, 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 Jurinelletum frigidae which is rapidly uncovered by snow is consequently the most xerophilous and the most cryophilous association. In contrast, the Erigeronetum elbursensis is longer under snow. Duration of snow cover differentiates the subassociations; these are considered mesophilous and hygrophilous respectively. The affinities linking Oxytropidetea persicae and Salicetea harbaceae Br.-Bl. and the communities of dry pozzines described by Quézel in the high mountains around the Mediterranean (Sierra Nevada, North Africa, Greece and Turkey) are ecologic and physiognomic, but never floristic. So it is with the corresponding syntaxa of Caucasus. However, the Oxytropeta immersae described by Ukatcheva, in oriental Pamir, display several physiognomic, ecologic and floristic similarities with Oxytropidetea persicae. They constitute a remarkable example of vicariance. In fact, the area of this class borders on the area of Oxytropidetea immersae, as is shown by scattered phytosociological data, some of them unpublished, about Indu-Kush and the Gissar mountains. The lowering of the mountains creates a major discontinuity which isolates the area of the Oxytropidetea persicae from that of the Oxytropidetea immersae. 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 glaciations, 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 chronological unity of the ancestral syntaxon which gave rise to the Oxytropidetea immersae and the Oxytropidetea persicae. Therefore, the chionophilous communities of Central Alborz have a remarkable floristic component of Central Asiatic origin, which they share with other syntaxons, which are now being studied.