© The Authors, published by EDP Sciences, 2019

1 Introduction

The South-East Altai and the South-West Tuva (Fig. 1) form the northern outpost of Central Asian arid area, while the main part of Altai-Sayan mountains belongs to the boreal zone. The South-East Altai center is the largest Altaian depression − the Chuya depression (1800-2100 m); the steep flanks of mountain ranges encircle the depression. Kuraisky ridge (2800-3800 m) forms its northern border, the spurs of North Chuysky ridge (2500-3000 m) − the western, and Chikhachev ridge (3000-4000 m) − the eastern, Ukok plateau and the Sailugem ridge (2900-3200 m), − the southern. Rivers of the South-East Altai originate in nearby ridges and flow into Chuya river [1]. The South-West Tuva includes Mongun-Taiga mountains (3400-3900 m), the southwestern flank of Tsagan-Shibetu ridge (3300-3500 m) and Kargy depression (1800-2100 m). Shallow mountain rivulets flow into Kargy river.

Three main types of climate appear to correspond to three main geomorphological units: (1) the climate of Chuya and Kargy depressions, (2) the climate of the flanks of the mountain ridges encircling depressions and (3) the climate of high-altitude ridges and plateaus. Low winter temperatures are caused by Siberian anticyclone; the cold air flows down from the slopes, fills the intermountain depressions and creates temperature inversions (January temperature in Chuya depression is −32 °С). Mountain slopes are noticeably warmer than depressions: the average January temperature is −16°C. The climatic conditions of mountain flat tops are similar to those in a free atmosphere [2] : the average January temperature at altitude of 3000 m is −18°C. In the summer, the Chuya depression is the warmest place in the South-East Altai: the average temperature in July is + 14°C, while on the surrounding mountains slopes it is +11°C. At an altitude of 2500 m, the average July temperature goes down to +8°C, and at an altitude of 3000 m − to + 4°C. Annual rainfall in depressions varies from 100 to 150 mm. The surrounding mountain slopes receive up to 400 mm of precipitation; annual rainfall at the height of the snow line reaches 600 mm on the Chikhacheva ridge and 800-1000 mm on the South Chuysky ridge [2].

The altitudinal zone series is known to comprise two belts (altitudinal zones): steppe (1800-2400 m) and alpine (2400-2800 m) one. The limits of altitudinal zones correspond to the main relief forms [3].

Fig. 1 Geographycal position of arid regions of Altai-Sayan mountains.

2 Results

2.1 Biodiversity

In arid regions of Altai-Sayan mountains we revealed 8 basic types of plant communities: i − shrub tundra, ii − cryophytic forest, iii − alpine meadow, iv − Kobresia tundra, v − tundrasteppe, vi − cryophytic steppe, vii − petrophytic steppe, viii − dry steppe (bunch-grass typical and desert steppe).

We carried out floristic classification of plant communities of all basic types: dataset of 858 geobotanical releves (published information and unpublished materials of the authors) was analyzed. Most of releves belongs to published syntaxa, some new ones were ascertained on the basis of our unpublished data. The biodiversity of arid region vegetation is 25 associations and communities from 5 classes, 6 orders and 8 alliances (Table 1).

To find out the significance of differences of floristic classification syntaxa and their accordance with basic types of plant communities, we carried out a cluster analysis of floristic classification syntaxa. The dendrogram was made in Past [4] using Ward method.

Two main groups were revealed (Figure 2): 1) high mountain communities and cryophytic forest (i − v), 2) steppes (vi − viii). Shrub tundra (i) belongs to class LOISELEURIO-VACCINIETEA, cryophytic forest (ii) − to class RHYTIDIO- LARICETEA. Kobresia tundra (iv) and tundra-steppe (v) represent the class CARICI- KOBRESIETEA: the first one − alliance Kobresion myosuroidis, the second one − alliance Potentillo niveae-Caricion pediformis. Alpine meadow (iii), because of the lack of regional higher syntaxonomic units, was pre-assigned to the class JUNCETEA TRIFIDI. Kobresia tundra, tundra-steppe and alpine meadow form one common cluster. All steppe communities were ascribed to class CLEISTOGENETEA SQUARROSAE. Dry steppe was included in the order STIPETALIA KRYLOVII, the others − in the order HELICTOTRICHETALIA SHELLIANI: petrophytic steppe belongs to alliance Helictotrichion schelliani, cryophytic one − to alliance Stellario petraeae-Festucion tschuensis.

Fig. 2 Dendrogram of syntaxa. Arabic numerals in the dendrogram correspond to those in the prodromus. i − shrub tundra, ii − cryophytic forests, iii − alpine meadows, 4 − Kobresia tundra, 5 − tundra-steppes, 6 − cryophytic steppes, 7 − petrophytic steppes, 8 − bunch-grass steppes.

2.2 Characteristics of basic plant communities

To give short characteristics of basic plant communities we chosen active species, defining community visual appearance. The activity of the species was calculated as the square root of the multiplication of constancy by mean cover [5]. Very active (activity ≥40), active (40> activity ≥ 20), and moderately active (20 > activity ≥ 10) species form the core of active species (Table 2). Very active species dominate in plant communities, active ones − codominate; moderately active species are constantly present.

Betula rotundifolia, mosses (Aulacomnium turgidum, Hylocomium splendens, Rhytidium rugosum), and lichens (Cladonia arbuscula, C. stellaris, C. amaurocrea) define the appearance of shrub tundra (i). Larix sibirica dominates in tree layer of cryophytic forest (ii); Betula rotundifolia and Lonicera altaica form shrub layer; tolerant to cold species make up the herb layer. Alpine meadow (iii) has no obvious dominants; Pentaphylloides frticosa, a wide range of alpine species, and tolerant to cold meadow- steppe species (Pulsatilla patens) codominate. Kobresia tundra (iv) has two main dominants: Kobresia myosuroides and Dryas oxyodontha. Tundra-steppe (v) and cryophytic (vi) steppe occur only in arid high mountains; they possess the numerous group of cryophytic species, the most part of which are inactive ones. Their distinctive feature is the simultaneous presence of species of different altitudinal zone groups. Alpine (Carex rupestris, Festuca kryloviana), forest-steppe (Carex pediformis), and steppe (Helictotrichon altaicum) species codominate in tundra-steppes (v); cryophytic species Eremogone meyeri and Saussurea schanginiana are moderately active. Steppe (Koeleria cristata, Helictotrichon altaicum) and forest-steppe (Carex pediformis) species make the core of cryophytic steppe (vi), alpine species have no significant role; in numerous group of cryophytic species only Festuca tschujensis is active. In petrophytic steppe (vii) steppe grasses (Agropyron cristatum, Poa botrioides) dominate; numerous petrophytic herbs are present, but not active. Dry steppe (viii) are composed of only steppe species (Agropyron cristatum, Artemisia frigida, Poa botryoides).

2.3 Vegetation structure

The steppe belt (1800-2400 m) is confined to the Chuya and Kargy depressions and the adjacent flanks of mountain ranges. The lower subbelt (1800-2100m) occupies the bottoms of depressions. The background plant community is dry steppe (viii). There are two main forms of relief here. The first form occurs on mountain flanks of the Chuya depression. Steep rocky slopes and screes dominate; the background vegetation is petrophytic steppe (vii). The second form of relief predominates in arid regions; the background plant community is cryophytic larch forest (ii) and cryophitic steppe (vi). In the arid conditions of regions under discussion, forests can exist only due to the moisture received in summer during the melting of the ice permafrost. Cryophytic larch forest (ii) covers concave slopes, while cryophytic steppe (vi) occupies convex ones. In the lower part of the alpine belt (2400-2600 m), the slopes of shadow exposures are covered with shrub tundra (i); the sunny slopes − with Kobresia tundra (iv) and tundra-steppe (v). Tundra-steppe is confined to the stony part of the slopes; Kobresia tundra occupies areas with more developed soils. In the vegetation cover of the upper part of alpine belt (2600-2800 m), Kobresia tundra absolutely dominates. Alpine meadows (iii) occur throughout all alpine belt.

3 Conclusion

Thus, 8 basic types of plant communities were revealed in Altai-Sayan arid region vegetation. Shrub tundra was included in class LOISELEURIO-VACCINIETEA, cryophytic forest − in class RHYTIDIO-LARICETEA; alpine meadow was pre-assigned to class JUNCETEA TRIFIDI. Kobresia tundra and tundra-steppe represent class CARICI-KOBRESIETEA; steppes were ascribed to class CLEISTOGENETEA SQUARROSAE.

The authors are grateful to the Russian Foundation for Basic Research for providing financial support to the research (grant № 18-04-00822 А).

References

All Tables

All Figures

	Fig. 1 Geographycal position of arid regions of Altai-Sayan mountains.
In the text

	Fig. 2 Dendrogram of syntaxa. Arabic numerals in the dendrogram correspond to those in the prodromus. i − shrub tundra, ii − cryophytic forests, iii − alpine meadows, 4 − Kobresia tundra, 5 − tundra-steppes, 6 − cryophytic steppes, 7 − petrophytic steppes, 8 − bunch-grass steppes.
In the text