Contemporary conditions of plain and mountain phytocenoses: composition and structure (the Yamal penincula, the Polar Urals)
The paper gives the results of a comparative analysis of the composition and the structure of plain and mountain phytocenoses (the Yamal Peninsula, the Polar Urals). A particular attention was paid to vascular plants. Geobotanical descriptions of undisturbed phytocenoses (the southern part of the Polar Urals) allowed us to reveal enough clearly the processes of herbalization and delichenization of tundra phytocenoses caused by reindeer overgrazing (the Yamal Peninsula and the northern part of the Polar Urals). The disturbed plain and mountain phytocenoses are at the stage of pasture digression (overgrazing), this caused significant changes in the ratio between the components. If the diversity of vascular plants did decrease on the disturbed territory, this decrease was insignificant due to the fact that the ground cover (moss, lichens) and organogenic soil horizons are not destroyed presently. The percentage of herbaceous plants in the total number of vascular plants were also similar in all three investigated regions (78-85%). In the floras of all three investigated regions the species of the Arctic fraction prevailed - an evidence of the similarity of macroclimatic conditions. At the same time, the comparison of the ecological and geographic structure of a series of concrete cenoflo-ras evidenced of regional differences in the environment. Only in disturbed - periodically dry and moderately humid - ecotopes (the northern part of the Polar Urals), the process of herbalization caused mesophytisation of the plant cover and was accompanied by a greater role of the species of the Arctic fraction. In the cenofloras of the disturbed - both plain and mountain - ecotopes the indices of floristic similarity varied within nearly the same limits (К <sub>С</sub> = 41-69% and К <sub>С</sub> = 4163% respectively) and the values of Sorensen's coefficients frequently exceeded 50%, predominantly at the expense of common species of herbaceous plants. There were more significant differences between cenofloras of the disturbed plain and mountain ecotopes (К <sub>С</sub> = 26-45%); relatively high similarity (К <sub>С</sub> = 54%) was observed only between the composition ofgrass-dwarf shrub-moss-lichen tundras which occupied relief elements with a thin snow cover, the most stable environmental conditions and a rigid ecotope selection.