Les groupements à grandes ombellifères et à xérophytes orophiles: Essai de synthèse à l'échelle de la région irano-touranienne
Klein, J. C.
In this phytogeographic synthesis, concerning the subalpine and lower alpine belts of the irano-touranian region, two types of communities are compared. Both are correlated with a poorly developed soil, a pronounced summer drought and microthermic conditions. Each of these units offers a characteristic physiognomy: The groupings with tall ephemeroids (Prangos L., Polygonum L., Rheum L.. ..) are found in the belt between 2000 and 3000 m and are linked to the remains of the thermophilous juniper groves, along with Juniperus excelsa M. B. and the shrubs Rosa L., Cotoneaster Med., Lonicera L., Berberis L.. ..; in the lower alpine belt above 3000 m we find the communities with xerophytic orophilous thorny cushions (Onobrychis Gaert., Astragalus L, subg. Tragacantha Bge., Acantholimon Boiss.). They are replaced at about 3500 m by the chionophilous communities, also forming small cushions but without thorns (Oxytropis DC.) (Klein 1982). The tall umbelliferae communities seem to proceed from thermophilous pleistocene juniper groves after their destruction by man. The hypothesis is: These juniper forests originated by transformation from the preshibljak, a xerophilous formation with trees, shrubs and herbaceous elements, which has developed either during the contact with arcto-tertiary (turgaj) and tropical (poltav) floras (Ovchinnikov 1971) or from the simple autochthonous mesogean flora (Kamelin 1967). In the same way, the orophilous xerophytes may have constituted synusiae in the thermophilous juniper groves, becoming distinct communities afterwards in relation to the climatic conditions and the orogenesis at the end of the tertiary and at the beginning of the quaternary. Appearing at the post pliocene, they may have belonged to the mesogean flora (Kamelin 1967). However, the majority of authors (Tolmachev 1949, Ovchinnikov 1971, Popov 1929, Lavrenko 1961. ..) think that the appearance and the extension of the xerophilous xerophytes coincide with a continuous long period of orogenesis, beginning at the upper miocene. This includes two main periods marked by aridity (end of miocene - beginning of pliocene) and coldness (end of pliocene - pleistocene). Ovchinnikov 1971 assigns them a mesophilous arctotertiary origin. According to the chorology of the characteristic species and some scarce phytosociological or physiognomic informations, these two types of communities defined as syntaxa in the Alborz mountains and, concerning only those with tall umbelliferae in Helgurd dagh (Iraq), occur probably in the whole mountains of the irano-anatolian province and belong to the same upper units (Prangetea ulopterae and Onobrychidetea cornutae. In the mountains of the middle Asia province (Darvaz, Gissar, Pamir, western Tien-Shan), some homologous groupings have been identified and described in so far as formations. Therefore, the existence of plant communities, which are at the same time homologous and distinct, confirms on the one hand the individuality of the irano-anatolian and the middle Asia provinces, and on the other hand their relationships, belonging to the irano-turkestanian region (Rechinger 1986), unities which were founded so far only on floristic criterions.