Les pelouses xérophiles d'altitude du flanc sud de l'Alborz central (Iran)
Klein, J .C.
We recall the features which best render the specificity of the classes Prangetea ulopterae and Onobrychidetea cornutae, which enclose nearly all grassland communities in the subalpine and lower alpine belts of the Irano-Anatolian massifs. The Prangetea ulopterae are physiognomically marked by the dominance of big Umbelliferae and tall Polygona, belonging to the genera Prangos L., Ferula L.and Rheum L. In the Alborz, these associations grow in the upper area of Juniperus excelsa M. B. between 2400 and 3000 m and are therefore subalpine. The Helichrysetum oligocephali juniperetosum probably had formed the undergrowth of the juniper groves with the shrubs Rosa L., Cotoneaster Med., Lonicera L., Berberis L. etc; the H. o. prangetosum probably had been the Saum (forest hem) before the destruction of the junipers by man. Therefore these communities are equivalent to a paraclimax. The main species of the Onobrychidetea cornutae belonging to the genera Astragalus L. (subsect. Tragacantba Bge.), Onobrycbis Gaertn. and Acantbolimon Boiss., are thorny small cushions and endow them a very characteristic physiognomy. The Onobrychidetea cornutae are found in the altitudinal belt between 3200 and 3500 m, just above that of the Prangetea ulopterae and below the communities of the Oxytropidetea persicae, also forming small cushions, but without thorns. They correspond to the climatic climax of the lower alpine belt. Prangetea ulopterae and Onobryehidetea cornutae have an identical chorology, comprising - in a first estimate - the northern Iranian massifs such as the Alborz and the Kopet-Oagh, the northwestern massifs such as those on the borders of Iran, Turkey and Irak, and the western massifs such as the Zagros. Beyond the ecological differences due to the distinct altitudinal distribution of these classes, it is possible to recognize some common discriminant factors; these are: - a level of precipitation between 900 and 1300 mm, 50 % of which fall in spring (20 % in April alone) in essentially solid form, - a summer period of four months almost totally free from precipitation, - a rather cold thermal regime: the 0o-isotherm surmounts 3000 m only from April to November. In these special conditions the soil, formed by more or less coarse sand, little assorted, rich in blocks, with only traces of humus, is a very weakly developed lithosol with a pH-value of 6. 2-7.6.