Shrub and dwarf shrub communities of Sangilen Plateau

The paper presents a detailed ecological and caeno tic characteristic of dwarf shrub and shrub communities growing in the highmountain belt of the Sangilen Plateau. The communit ies nvestigated were referred to the class of Loiseleurio-Vaccinietea Eggler 1952. The dwarf birch thickets with prevalence of Betula rotundifolia and the high incidence of Caragana jubata, Juniperus pseudosabina, J. sibirica, Pentaphylloides fruticosa, Salix glauca, Spiraea alpina were referred to the union of Carici tristis–Betulion rotundifoliae Zibzeev et al. 2018 of the order of Betuletalia rotundifoliae Mirkin at al. ex Chytrý Pešout et Anenkhonov 1993. Communities with dominant calciphil ous shrubs and dwarf shrubs, such as Rhododendron adamsii, Caragana jubata, Salix berberifolia, S. reticulata, were referred to the union of Saxifrago oppositifoliae–Rhododendrion adamsii Zibzeev et al. 2018 of the order of Rhododendro-Vaccinietalia Br.-Bl. ex Daniels 1994.


Introduction
Shrub communities are the most common type of vegetation growing on the Sangilen Plateau. Prevalent are the communities formed by Betula rotundifolia, also common are communities with dominant Caragana jubata and Rhododendron adamsii.
Caragana jubata (Pall.) Poir. Is the Asian montane calciphilous shrub species, the habitat of which is related to the mountain systems of Central Asia, western Mongolia, China, Tibet, and the Himalayas. In the territory of Russia, the species covers the mountainous regions of Eastern Siberia and the Far East; the north-western boundary of its habitat goes along the Sangilen Plateau and certain ridges of south-eastern Altai (the Kurai and Chikhachev Ridges).
In the territory in question, Caragana jubata is found in the range of altitudes from 1900 m to 2550 meters. In deciduous forests consisting of Larix sibirica, it participates in the formation of the shrub tier. In river valleys and surrounded by cryophyte steppe, the species forms communities with co-dominance with Pentaphylloides fruticosa. In the mountain tundra belt, Caragana jubata forms vegetation communities with co-dominant Rhododendron adamsii and Betula rotundifolia.
Rhododendron adamsii Rehder is the north-Asian high-mountain calciphilous shrub species. In the territory of Russia, the western boundary of the habitat of Rhododendron adamsii goes along the Yenissei River, in the south, in western and eastern Tannu-Ola Range, the Sangilen Plateau, to go further to the mountain ridges of Eastern Sayan mountains, Khamar-Daban, Barguzin ridges, and in the north, it reaches the Aldan Highlands. In the Russian Far East, the habitat of Rhododendron adamsii stretches as far as the shore of the Sea of Okhotsk. Outside Russia, the species is commonly found in the mountain systems of Mongolia, and its southern boundary is in Tibet. In the territory of the Sangilen Plateau, Rhododendron adamsii grows in the high-mountain belt, where it forms communities on the carbonate rocky slopes of different exposure and steepness.

Materials and Methods
The study is based on 94 releves of high-mountain shrub communities performed by the authors in the upper reaches of the Naryn and Balyktyg-Hem Rivers in July 2014. The descriptions were made on the sample plots 100 m 2 in area. The classification of the vegetation communities was made using the Braun-Blanquet method [1]. In this work, we used the concept of a differentiating species proposed by German geobotanists [2,3]. The names of the syntaxa are provided in accordance with the International Code of Phytosociological Nomenclature [4]. Resulting from the conducted classification of the shrub communities with dominant Betula rotundifolia, Ass. Saussureo schanginianae-Caraganetum jubatae Zibzeev et al. 2018 were referred to the union of Carici tristis-Betulion rotundifoliae, whereas the communities with Rhododendron adamsii and Caragana jubata were referred to the union of Saxifrago oppositifoliae-Rhododendrion adamsii of the order of Betuletalia rotundifoliae of the class of Loiseleurio-Vaccinietea.

Results and discussion
The class of Loiseleurio-Vaccinietea unites alpine and arctoalpine dwarf shrub and shrub communities of Eurasia and North America. The communities of the Altai-Sayan mountain region and of Northern Mongolia were referred to the order of Betuletalia rotundifoliae.
In the southern part of the Altai-Sayan mountain region, situated in the rain shadow zone, grass-shrub communities are most common. Their geographic isolation, the physiognomic image and the high species saturation and specifics allowed us to refer them to a separate union of Carici tristis-Betulion rotundifoliae [5].

Conclusion
The characteristic features of caenotic variability and spatial organization of the vegetation cover of the Sangilen Plateau are determined by the conditions of the semiarid climate and the characteristics of the geological and geomorphological structure of the area, edaphic factors, in particular, a high degree of soil mosaicism, related to outcrops of marmorized limestone and granite.
Resulting from the performed classification of the shrub and dwarf-shrub communities of the high-mountain belt of the Sangilen Plateau, three associations have been described. The shrub tundras of the class of Loiseleurio-Vaccinietea are the landscape-forming type of vegetation; in the lower part of the high-mountain belt, they form their own sub-belt, which occupies up to 2/3 of the area of the highlands.