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Biomes and Regions of Northern Eurasia

The Mountains of Central Asia and Kazakhstan

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Mountainous Biota

The mountains of Central Asia and Kazakhstan are one of the four centres of biological diversity within the territory of the FSU. The peculiarity of biota is due to its mixed character: Indo-Hymalayan, Mongolian, Eurasian, and Mediterranean species are represented as well as local endemics. Under equal conditions, species-richness and endemism in the mountains are always higher than on the surrounding plains, first, because mountains serve as refugia during unfavourable climatic epochs, and second, because of higher habitat diversity. Thus, the flora of the Central Asian mountains comprises about 5500 species, while the surrounding deserts accommodate about 2000 species (Table 16.6).

Number of plant species in various regions of the Central Asian mountains

Table 16.6 Number of plant species in various regions of the Central Asian mountains

Altitudinal vegetation zones range from the deserts of the foothills to the alpine environments of the high mountains (Figure 16.8).

Altitudinal zonation of vegetation

Fig. 16.8 Altitudinal zonation of vegetation.

There are two general types of altitudinal sequences: a more humid sequence with forest belts, which occurs in the Dzungarsky Alatau and in the northern part of the Tien-Shan mainly on the slopes of northern and western exposure, and an arid sequence typical of more southern locations (e.g., the south-western Tien-Shan, Pamir-Alay) and slopes located in the rainshadow in which forests are not represented as a continuous belt. Both types of altitudinal zonation are most clearly expressed in the marginal regions. In the arid interior regions (i.e., the central Tien-Shan and eastern Pamir), the occurrence of elevated plateaux and continentality of climate mask altitudinal differentiation. Human activities, which began in the Central Asian mountains thousands of years ago, have had strong impacts on vegetation, ranging from creating areas with a rich collection of cultivated plants to the destruction of montane forests. In this chapter, most attention is given to the sequences with forest belts and for the Pamir and Tien-Shan these are based on the models by Agakhanyants (1981, 1986). Stanyukovich (1973), Walter and Box (1983a, b), and Ogureeva (1999) provide more comprehensive surveys.

Kopetdagh

Biogeographers are in agreement that the Kopetdagh (together with the rest of a larger Turkmen-Khorossan mountainous system, the major part of which is located in Iran) is a separate biogeographic entity. However, because of the transitional nature of the biota, its status and affiliation with units of the higher level are debated (V. Fet, 1994). The most distinct feature of the Kopetdagh biota is the combination of Iranian and Turanian species, which constitute the core of its flora, with European and Mediterranean species within a very distinctly defined geographical area. For many species, the Kopetdagh marks a limit of their distribution. The specific biogeographic position, combined with the prolonged isolatioji in the Paleogene-early Neogene, resulted in the development of numerous endemics. Many species have a very narrow range of distribution which distinguishes this area from the conventional transitional zones. Plant endemism is unusually high: about 18 per cent of plants found in the Kopetdagh are endemic (Kamelin, 1970). Most of them exhibit a taxonomic relationship to the Iranian and eastern Mediterranean species, indicating that the vegetation of the Kopetdagh evolved from the ancient Mediterranean flora under the influence of two major factors: continuous aridization since the end of the Tertiary and orographic uplift (Kamelin, 1973; Kurbanov, 1994). In the Tertiary, the south of Central Asia was dominated by subtropical evergreen forests of the laurel type and by tall-grass savannahs. However, even arid woodlands, which had been formed in the Kopetdagh by the middle Pliocene, became impoverished later and a facilitated evolution of the xerophilous flora occurred. There are a number of texts dealing with the history of the Kopetdagh biota (Kamelin, 1973; Korovin, 1961; Ovchinnikov, 1971) and a more recent review has been given by Kurbanov (1994).

Although the mountains do not reach the snow line and only in the central Kopetdagh subalpine plant communities develop between 2500 m and 3000 m, vertical zonality has a complex structure (G. H. Fet, 1994). One of the first models of altitudinal zonation was proposed by Chernyakhovskaya (1927) who distinguished six vegetation zones: submontane plain, sagebrush steppe, grass and herbaceous-grass steppe, juniper forests, highland vegetation, canyon vegetation, and intrazonal vegetation of rocky outcrops. All subsequent schemes were largely variations of this original model. However, because boundaries between the altitudinal belts are not distinct and plant communities have a mosaic distribution and vegetation is often disturbed, there is still a disagreement on altitudinal division (as well as regionalization) of the Kopetdagh vegetation. Thus, Kamelin (1970), who also distinguishes six altitudinal zones, comments that only two are distinctly different: a shiblyak (a diverse community of low tree and shrub species) and semi-savannah belt between 400 m and 1500 m and a juniper and steppe belt between 1600 m and 2800 m.

In the foothills, including adyrs which occupy the area between 400 m and 800 m, subtropical desert vegetation develops. Sagebrush communities predominate together with numerous ephemerous and ephemeroid species whose participation predetermines a clearly expressed seasonal change in vegetation. A large extent of soil erosion, associated with poor vegetation cover and strongly enhanced by overgrazing, leads to the widespread occurrence of species adapted to stony soils. Under the conditions of heavy grazing, sagebrush coverage declines from 40 per cent to 5-10 per cent and in the strongly eroded locations petrophytes may completely replace sagebrush communities (G. H. Fet, 1994). The low mountain belt, between 800-900 m and 1200-1400 m, is occupied by the tall-grass semi-savannah composed mainly of the communities dominated by the perennial grass Elytrigia trichophora. Ephemeroid species and meadow steppe herbaceous vegetation co-dominate in the lower part of this belt while in the upper part the participation of the steppe species, such as Festuca valesiaca, increases. At higher altitudes, between 1200-1400 m and 1900-2300 m, steppe vegetation develops, dominated by Stipa spp. and Festuca valesiaca. Above this boundary, upland xerophilous plants predominate.

Although woody vegetation occupies small areas in the Kopetdagh, developing mainly on the northern slopes, in river valleys and along the beds of seasonal streams, tree and shrub communities are diverse. Most widespread are shiblyak communities, dominated by Paliurus spina-christi, Acer turcomanicum, Primus, Amygdalus, and Rosa spp. (G. H. Fet, 1994; Popov, 1994). Shiblyak comprises a variety of fruit trees and vines suitable for cultivation, including pomegranate, fig, wild grapes, apple, cherry, and many others. It develops mainly in the low mountains: Paliurus spina-christi occurs between 400 m and 800 m, Acer turcomanicum is limited to the 800-1500 m zone and only Berberis integerrima and Crataegus pontica can penetrate the middle mountains to an altitude of 1900 m. Typical of the higher altitudes are juniper woodlands. These are composed ofjuniperus turcornanica, the only juniper species found in the Kopetdagh and one of the most drought-resistant species which can grow under the conditions of annual precipitation below 150 mm and summer temperature above 46°C (Popov, 1994). While at present juniper woodlands are confined mainly to the middle mountains, as recently as in the 19th century they were widespread at 400-600 m. Juniper, which was widely used as firewood, is also the only tree in the Kopetdagh which can provide timber for construction. Between the 1930s and the 1960s, the area occupied by juniper woodlands declined from an estimated 79 000 ha to 20 000 ha (Koksharova, 1970). The easily accessible lower mountains in the western ridges were affected particularly badly and only on the steep slopes of the central ridges does tree cover reach 40-50 per cent and old trees reaching 18-20 m in height can still be found (Popov, 1994).

Riparian forests develop in the south-western Kopetdagh in the mountain river valleys which have a more humid climate as a result of their proximity to the Caspian. Such forests, composed mainly of Juglans regia with the participation of Fraxinus syriaca, Ulmus carpinifolia, Thelycrania meyeri, and Platanus orientalis are confined to a very narrow (between 50 m and 100 m) zone along rivers at altitudes from 1000 m to 1500 m. Vines, which are not found in the Central Asian mountains further east, are common and a representative of the relatively moisture-demanding Miocene flora, Periploca gracea, occurs (Kamelin and Zabelina, 1987). Because of the abundance of fruit trees and vines, riparian forests of the south-western Kopetdagh have been a centre of genetic and selection studies conducted by the Vavilov Institute of Plant Breeding since the 1930s. Of particular practical interest was such a valuable species as Juglans regia and already in the 1930s, a lack of young trees was observed resulting from overharvesting of walnut seeds (Popov, 1994). By the early 1990s, the population of Juglans regia had been reduced to less than 2000 trees partly due to the continuing overharvesting and partly due to the occurrence of unusually strong mudflows in the 1980s. With the continuing depletion of Juglans regia, the participation of light-loving shrubs and trees increases. In particular, a hydrophilous relict species of Mediterranean origin, Platanus orientalis, which in Central Asia occurs only in riparian habitats, has become more widespread. The presence of relatively moisture-demanding riparian forests in the southwestern Kopetdagh and their composition as well as the proximity to the Transcaucasia, led to suggestions that the flora and fauna of the Kopetdagh belong to the Hyrcanian type. Hyrcania, which comprises the southern and south-western shores of the Caspian Sea, is a unique relict biogeographical region. As V. Fet (1994) points out, there are indeed many relict elements in the riparian biota of the south-western Kopetdagh. Some specific Hyrcanian elements (such as the ground beetle Broscus karelini and the darkling beetle Metadisa virdis) can be found in the humid valleys of the Kopetdagh and many genera have variant species (e.g., the spiders Dysdera concinna in Hyrcania and D. pococki in the Kopetdagh). However, the Tertiary relicts, typical of the Kopetdagh riparian forests, are characteristic of the whole of the ancient Mediterranean region. Species with the Hyrcanian-Kopetdagh distribution are few, and such characteristic dominants of the Hyrcanian forests as Quercus castaneifolia and Parrotia persica do not occur in the Kopetdagh. Therefore, assigning the Kopetdagh to the Hyrcanian biogeographical region is not justified but questions remain about the ways of penetrations of the European, Mediterranean, and Caucasian elements eastwards which, being limited by the Caspian in the north and deserts of Iran in the south, led through Hyrcania.

Tien-Shan and Pamir

The enormous heights of the Tien-Shan and Pamir predetermine the development of full spectrum of altitudinal zones. However, because of the complex orography and non-uniformity of climate, floristic variations occur between ridges and slopes of different exposure. Acknowledging the strong heterogeneity of the vegetation, Agakhanyants (1981) introduced a term 'mountain plakor' and suggested that models of vertical zona-tions can only be developed for these 'typical' slopes. 'Mountain plakor' is certainly a contradiction in terms, since in the Russian-language literature the term 'plakor' is used to describe flat surfaces or watersheds on which zonal vegetation develops, but its use certainly highlights the complexity of vegetation classification. Agakhanyants (1981, 1986) distinguishes six biogeographical provinces according to the history of floristic development, modern composition of vegetation and vertical structure of biota: northern Tien-Shan, south-western Tien-Shan, western Pamir-Alay, western Pamir, and central Tien-Shan and eastern Pamir (Figure 16.1), although Zlotin (1975) argues that because of different altitidues, degree of continentality, and specific features of community structures, central Tien-Shan and eastern Pamir should not be viewed as a single province.

Northern Tien-Shan

The northern Tien-Shan (including Dzhungarsky Alatau, Saur, Tarbagatay, and Zailiisky, Kyrgyz, and Kungey-Alatau Ridges) has a full spectrum of altitudinal zones (Figure 16.8a). Temperate semi-deserts dominated by sagebrush communities occur in the foothills extending from the westernmost regions of the Tien-Shan to the Dzhungarsky Alatau. Above these, a zone of feathergrass steppe develops typical of which is a widespread occurrence of shrubs, in particular Rosa spp. and Lonicera microphylla, on wetter and shaded slopes and in the valleys. At the upper part of the belt, feather-grass steppes pass into steppes and woodlands composed of Primus racemosa and Mains sieversii become a notable part of the landscape. In contrast to the meadow steppes of the southern regions, the meadow steppes of the northern Tien-Shan do not experience a severe deficiency in moisture and remain green throughout the summer. The origin of the flora is also different: while many plants in the southern regions are of Mediterranean origin, the flora of the meadow steppes of the northern Tien-Shan is closely connected to the steppes of the Kazakh plains. In the Dzhungarsky Alatau, species of the Altay-Siberian origin predominate while for many of the Central Asian species the ridge marks the northern and eastern boundaries of their distribution. Forests succeed steppes first on northern slopes at an altitude of 1500-1700 m and then on southern slopes. In contrast to other regions where walnut forests prevail, deciduous woodlands in the northern Tien-Shan, developing in valleys in the lower part of the forest belt, are composed of Populus tremula. Characteristic of the northern Tien-Shan are coniferous forests, comprised of Picea schrenkiana and Abies semenovii at higher altitudes with the participation of Betula, Sorbus, and numerous shrubs in the undergrowth. In the Dzhungarsky Alatau, Picea obovata is a typical forest-forming species. Although this belt receives about 800 mm of precipitation per annum and is generally reminiscent of the taiga landscapes, the dark forest soils which develop under the montane forests have much higher humic content and lower acidity than podzols. This is because the chemical composition of needles of Picea schrenkiana is different from that of other species of spruce: the content of calcium oxide in needles of Picea schrenkiana reaches 44 per cent while in needles of the other spruce species it does not exceed 12 per cent (Glazovskaya, 1955). Forests are succeeded by the floristically rich subalpine zone which is a complex of tall-grass meadows and woodlands formed by Juniperus sibirica. Junipers have a bush-like form at lower levels and take a creeping form at higher altitudes. Tall-grass meadow communities are dominated by sedges (mainly Helictotrichon asiaticum, Alopecurus songoricus, and Festuca rubra) with the participation of cold-tolerant flowering species (e.g., Geranium spp., Anemone narcissiflora, Aster alpinus) and form vast and productive summer pastures. At an altitude of 2800-3000 m, the alpine low-grass meadows dominated by Kobresia spp. develop. The diversity of species as well as productivity of the alpine meadows is considerably lower than those of the tall-grass meadows and locally vegetation does not form a continuous cover.

Western Tien-Shan and Pamir-Alay

With respect to biogeography, there is much similarity between the south-western Tien-Shan and the western Pamir-Alay. The western Tien-Shan has a temperate climate but south of the Chatkalsky Ridge, climate and landscapes assume subtropical features. The amount of precipitation received by individual ridges and slopes differ widely across the region. The mountains of southern Tajikistan, for example, have an arid climate while the Gissar and Darvaz receive an ample moisture supply. The uneven distribution of precipitation predetermines local differences in altitudinal zonation particularly with respect to the occurrence of forests. Forests develop mainly on wetter northern and western slopes. Thus, juniper woodlands occur on the northern macroslope of the Turkestan Ridge while its southern slope is devoid of woody vegetation and its landscape is that of an extremely arid environment. In all, there are three types of zonal woodlands (sparse arid Pistacia woodlands, broad-leaved deciduous woodlands, and open juniper woodlands) and seldom are all three types represented in the altitudinal sequences.

A model of altitudinal zonation suggested by Agakhanyants (1981, 1986) for moister slopes is shown in Figure 16.8b. The sequence begins with the belt of deserts or semi-deserts which assume a subtropical character, expressed in the domination of ephemeral species, in the Fergana valley. These are succeeded by dry steppe (semi-savannah) belt, whose vegetation is composed mainly of tall forbs. Meadow steppe dominated by large umbellates, among which Prangos pabularia and species of Ferula are most widespread, develops above. In contrast to the northern Tien-Shan, most plants end their growth by late July and meadow vegetation dies with increasing moisture deficit. Under the meadows on fine alluvium, mountainous chernozem, and chernozem-like soils form with a high humic content and a well-expressed carbonate layer. In the lower part of this zone, shrub vegetation develops in shaded sites. On western slopes and in the peripheral parts of the region, particularly in the Gissar, Darvaz, and Peter the First Ridges, deciduous woodlands occur as isolated groves on wetter slopes. The main forest-forming species in broad-leaved woodlands are Juglans regia and J. fallax with co-domination of Acer spp. or Malus sieversii. The Fergana Ridge marks the eastern limit of distribution of walnut groves and while they occur as far north as the Talas-Alatau, in more northern regions walnut groves usually prefer sites protected from the north and exposed to the south-west. In all, there are over a hundred tree and shrub species in deciduous woodlands; fruit trees are particularly abundant but notable is the absence of oak. Because deciduous woodlands have a mosaic distribution, their classification into a separate altitudinal belt is debated. Thus Agakhanyants (1981, 1986) distinguishes a belt of 'woodlands and shrubs' rather than woodlands and in the model put forward by Pavlov (1980) woodlands are not recognized as a separate entity. In the upper part of the meadow steppe zone, deciduous woodlands are succeeded by open juniper woodlands comprised of Juniperus species of the Central Asian origin (J. seravschanica, J. semiglobosa, and J. turkestanica). In more arid locations, juniper woodlands (locally with Pistacia in the undergrowth) develop in place of broad-leaved forests and mountainous feather-grass steppe communities occur instead of the meadow steppe vegetation. From an altitude of 2500-2700 m, woody vegetation disappears, giving way to the tall-grass meadows. Similar to the meadow steppe zone, there is a notable domination of umbellates while in drier localities elements of steppe vegetation occur. Chernozem-like soils develop under the subalpine meadows but in contrast to the soils of the meadow steppe, these are devoid of carbonates and have a brown sod layer. The subalpine meadows form the most important summer pastures with productivity of fodder species of 0.8-1.8 tonne ha-1a-1 (Ovchinnikov, 1977). Low-grass alpine meadows succeed the tall-grass meadows and vegetation adapted to low temperatures develops above.

Western Pamir

The western Pamir is distinguished from the western Tien-Shan and Pamir-Alay by the absence of zonal woody vegetation. With the increasing continentality of climate, the sequence of altitudinal zones becomes more similar to those of the eastern Pamir and central Tien-Shan. Attempting a general model, intended for both the western and eastern Pamir and Central Tien-Shan, Agakhanyants (1981, 1986) divided the mountains into four belts (Figure 16.8c). What distinguishes montane deserts of the western Pamir from those located landwards is the presence of ephemerous plants at lower altitudes. Vegetation communities are dominated by Artemisia vachanica, A. lemanii, Hammada vachanica, Peganum harmala, and Poa bulbosa. Xerophilous vegetation (a very broad term favoured by Agakhanyants but not by other authors), represented mainly by the communities of Acanthophyllum pungens, Acantholimon, Astragalus, and Onobrychis spp., develops above on the rocky slopes. It is succeeded by feather-grass steppes. However, steppe communities occur widely in the neighbouring zones and the altitudinal boundaries of their distribution vary widely across the region.

Eastern Pamir and Central Tien-Shan

Characteristic of the eastern Pamir and central Tien-Shan are extremely severe climates, with respect to both temperature and aridity, and the widespread occurrence of permafrost above 3000 m. The duration of growing season is short and ranges from six months at 3000 m to six weeks at 4000 m (Zlotin, 1997). The mean annual temperature is below zero and there are no periods without frosts. Snow cover stays from two to six months and, under the conditions of very low precipitation and strong winds, particularly in the eastern Pamir, it is thin and uneven which causes the soil to freeze deeply. The environments of the eastern Pamir and central Tien-Shan are similar to those of the Arctic tundra and, as a result of these severe conditions as well as past glacia-tions, vegetation is poor in comparison with that of the other regions of the Central Asian mountains. Thus, in the high mountain deserts of the eastern Pamir, vegetation cover is 8-15 per cent and as few as 3-7 species are found in 100 m2 (Walter and Box, 19836). Most of the eastern Pamir and central Tien-Shan belongs to the alpine zone. However, because of the dryness of climate, alpine vegetation develops only in better moistened locations while most of the area is dominated by cold mountainous deserts and steppes similar to those of Mongolia and Tibet. Due to the characteristic local levelness, altitudinal zonality is not clearly expressed. The belt of xerophilous vegetation of rocky deserts is fragmented in the Tien-Shan and the belt of mountain steppes has a patchy character in the eastern Pamir.

Focusing on the central Tien-Shan, Zlotin (1997) suggested a more detailed model which classifies vegetation into seven altitudinal 'steps' within the alpine belt (Figure 16.8d). This scheme recognizes the alpine meadows as a separate altitudinal step because in the syrt regions of the Tien-Shan meadow communities may occupy over 80 per cent of the area, reflecting the relatively high moisture availability. Meadow communities are dominated by species of Kobresia and in the Tien-Shan they include about 70-80 species of flowering plants (Zlotin, 1997). The humic content of soils developing under the Kobresia meadows reaches 8-11 per cent. On a broader scale, meadows are less notable as a separate step. Outside the syrt region, meadows occupy not more than 6-8 per cent of the territory, forming intrazonal communities in stream valleys, while steppe and desert environments account for about 50 per cent and 20 per cent, respectively.

According to the scheme put forward by Agakhanyants (1981, 1986), cold deserts occupy the lowest belt (in the central Tien-Shan, semi-deserts, dominated mainly by Artemisia, develop in the intermontane depressions between 1500 m and 2500 m with elements of dry steppe feather-grass vegetation in the localities with better moisture supply). On gently sloping surfaces, Artemisia rhodantha, Ajania tibetica, Kalidium screnkianum, and Limonium hoeltzeri form mixed communities. Typical of drier locations, especially in the eastern Pamir, is a dwarf shrub Eurotia ceratoides — the only local plant which can be used as fuel wood. On the steeper, rockier slopes, xerophilous vegetation develops, formed by Onobrychis echidna, Hedysarum minjanense, Acantholimon, and Astragalus spp. One of the plant adaptations to the severe conditions is their cushion-like shape. Low temperatures and strong winds set a limit to the vertical growth of plants, which expand in a horizontal direction, and whereas fine soil is blown on to the edges of the plant, it develops a characteristic circular form creating within itself a specific moisture and temperature regime. Large cushions often retain seeds of other plants transported by wind and the intergrowth of one plant with another is common. Deserts and xerophilous vegetation are succeeded by montane steppes, formed mainly by species of Stipa, Festuca, and Helictrotrichon, which are particularly typical of the syrt regions of the Tien-Shan. Above, vegetation adapted to low temperatures occurs in place of alpine meadows. In the lower part of the belt, cushion-plant communities similar to those of the frost-boil Arctic tundra develop. In the central Tien-Shan, they are dominated by large (about 50 cm in diameter) cushions ofDryadanthe tetrandra which, covering between 10 per cent and 30 per cent of the surface, constitutes about 9 0 per cent of the total plant reserve (Zlotin, 1997). Typical of the eastern Pamir are cushions of Acantholimon. Herb communities, similar to those of the polar deserts, occur at the uppermost limit of vegetation where higher plants do not form a continuous cover.

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