Cryopetrophytic communities of the Altai-Sayan mountain system

Floristic classification of the cryopetrophytic alpine communities of the Altai-Sayan mountain system have been performed. The Rhodioletea quadrifidae Hilbig 2000 class of the Altai-Sayan mountain system includes two order, one alliance and six associations. We were described new order (Valerianetaliapetrophyllae order nova prov.) and three associations (Ranunculo akkemensis-Valerianetumpetrophilae ass. nova prov., Mesostemmo martjanovii-Leiosporetum exscapae ass. nova prov., Potentillo biflorae-Sibbaldietum tetrandrae ass. nova prov.) These associations include communities with a predominance of Asian alpine and arctalpine petrophytes.


Introduction
Cryopetrophytic communities of the Altai-Sayan mountain system as those of the Asian highlands exhibit a unique type of vegetation confined to cold rock habitats. In the South-East Altai and central part of the West Sayan, they represent a type of vegetation characterized the belt [1,2]. Despite their wide distribution, these communities are the infrequently studied.
In terms of the Russian geobotanical school, the communities formed on scree slopes are rarely considered an independent assemblage. Most often they are referred to "scree communities" or "rock and scree vegetation" [3][4][5]. We do not agree with other researchers [4] that "scree slopes are not distinct in the originality of their flora." Our data demonstrate that these communities are characterized by high heterogeneity in species composition, and this is due to numerous factors: the feature of the underlying rocks, the mobility of the substrate, the nature and structure of scree slopes, chemical composition, the rate of silt accumulation, steepness of slopes, slope exposure, etc. Various combinations of these factors lead to formation of high coenosis diversity. R. V. Kamelin named these cryopetrophytic communities "cryopetrophyton oreoasiaticum" and conside them as different vegetation types [1].
The system of higher units of floristical classification of the cryopetrophytic communities of the Altai-Sayan mountain system was proposed by V. Hilbig [6,7,8]. Based on the geobotanical data collected in the highlands of northern Mongolia, he identified the class Rhodioletea quadrifidae Hilbig 2000 with two associations: Saxifrago setigerae-Rhodioletum quadrifidae Hilbig (1987Hilbig ( ) 1990 and Oxygraphio glacialis-Potentilletum biflorae Hilbig (1987Hilbig ( ) 1990. We found these communities on Katun Range of the Central Altai [9] and Sailig-Hem-Taiga Ridge of the Western Sayan [2]. We described new association of Saxifrago oppositifoliae-Rhodioletum quadrifidae Zibzeev 2013 on the territory of the Western Sayan [2]. An integrated classification system of the Rhodioletea quadrifidae class does not developed yet and there is no consensus of the authors about higher units of cryopetrophytic alpine communities.
This paper presents ecological characterization of the communities of the class Rhodioletea quadrifidae the Altai-Sayan mountain system and their coenotic diversity .

Study area
The highest mountainous area of the Altai-Sayan mountain system at altitudes of 1700-2900 m is dominated by ancient table-lands. The highest ridges (altitudes 2500-4500 m) havesteep slopes on metamorphic rocks, mainly base-rich chloride slates, with igneous rocks and limestones occupying small areas. Due to its location in the centre of Eurasia, the macroclimate of the Altai-Sayan mountain system is strongly continental. Local climate, however, is considerably modified by its mountainous topography.

Material and methods
The materials for this paper were 45 relevés performed using a standard technique when studying the alpine vegetation of the Altai-Sayan mountain system in 2012-2017. The Braun-Blanquet approach was used to classify the plant communities. The relevés were processed with the automatic classification technique using JUICE 7.0 software package. The nomenclature of the syntaxonomical units was done in accordance with the "International code of phytosociological nomenclature" [10]. Latin names of species and nomenclature follow The International Plant Names Index (IPNI) (http://ipni.org/).

Results and discussion
The class Rhodioletea quadrifidae Hilbig 2000 includes alpine petrophytic communities of the mountain systems of Siberia, northern Mongolia and East Kazakhstan, growing on eluvial scree slopes. These communities are characterized by a high heterogeneity in species composition due to factors such as altitude, slope exposure, slope steepness, nature, size and composition of rock screes. Such factors as the rate of silt accumulation and soil formation significantly affect the qualitative composition of these communities. Currently, the class is represented by alliance Rhodilion quadrifidae Hilbig 2000 and tow order Rhodioletalia quadrifidae Hilbig 2000 and Valerianetalia petrophyllae order nov. prov.
Ass. Saxifrago-Rhodioletum quadrifidae Hilbig (1987)  On the territory of South-East Altai (Kuraisky, Chikhachev and South-Chuisky ranges), we described сryopetrophytic communities are formed on unstable alpine scree. Soil layer is completely lacking. We propose to include these communities in the new order Valerianetalia petrophyllae order nov. prov. The order includes two associations Ranunculo akkemensis-Valerianetum petrophilae and Mesostemmo martjanovii-Leiosporetum exscapae.
Ass. Ranunculo akkemensis-Valerianetum petrophilae ass. nova prov. Thus, alpine vegetation on the screen and stone debris were ascribed to the class Rhodioletea quadrifidae. The class includes two orders, one alliance and six associations. Cryopetrophytic communities of the Altai-Sayan mountain system possess a group of regional Asian cryopetrophytic alpine species. It is cryo-petrophytes species. The order of Rhodioletalia quadrifidae and Valerianetalia petrophyllae differ from each other not only by species composition, but also by habitat features (the presence of soil horizon and substrate mobility).