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Biomes and Regions of Northern Eurasia

Steppe and Forest-steppe

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Biological Diversity

Specific Features of Steppe Flora

The biological diversity of natural steppes is extremely rich both with respect to number of species and lifeforms. Between 37 and 56 species per square metre occur in the protected steppes of the Kursk and Voronezh regions in Central Russia and up to 80 species are found in the meadow steppe of the region (Alekhin, 1986). There are a number of specific features distinguishing steppe vegetation. First, steppe ecosystems are composed of plants among which xerophilous sedges predominate. Second, most plants of the steppe have a well-developed fibrous root system with a large surface area, a characteristic which is important under the condition of low soil moisture. Biomass of the below ground parts considerably exceeds biomass of the above ground parts and is concentrated mostly in the uppermost 50 cm in the humus horizon of soil. Third, dead parts of the steppe plants quickly decompose down to mineral components.

In response to the dryness of steppes, plants have developed many adaptive features. One of the adaptations, an extensive and deep root system, has already been discussed. Others include the narrow shape of leaves and the bunch (tussock) form of plants. Narrow folded leaves (1.5-2 mm wide) reduce moisture loss through transpiration and allow plants to tolerate hot, dry weather. The bunch form allows the accumulation of snow and dust and plants therefore receive more moisture and nutrients. The base of tillers in bunch-plants is positioned rather deep in soil, thus protecting them from trampling by ungulates. Light colours, typical of the steppe plants, help to reflect sunlight, thus protecting them from overheating.

The open character of the steppe landscape and frequent strong winds have led to the development of extraordinary adaptations with respect to the dissemination of seeds and fruits. One of the most interesting adaptations is the so-called 'tumble weed'. By the time when seeds are fully ripe, spherical inflorescences break off at the root collar and, steered by the wind, roll across the steppe, scattering seeds. Crambe tatarica, Gypsophila paniculata, Ceratocapnos arenarius, Phlomis pungens, and Salsola belong to this group of plants.

Before the development of agriculture, zonal steppe vegetation was dominated by the turf-forming grasses, such as Stipa, Festuca, Koeleria, Ðîa, and also Carex. Ephemeroids, that is, plants that develop in spring when the soil is wet and flower and produce seeds by the beginning of the dry period (this is yet another adaptation to the dry conditions) are widespread. Typical ephemeroids of steppe are Tulipa, Iris, Gagea, Adonis spp., and some species of Astragalus. However, under conditions of insufficient moisture supply, the distribution and character of vegetation are strongly affected by microrelief and the lithology of soils and rocks. Meadow vegetation often develops in topographic depressions. Variations from the zonal vegetation occur in sandy, stony, and gravelly soils. The sandy steppes, whose origin is linked to aeolian and river sands, occur in almost all provinces. Vast areas are occupied by sandy steppes in the lower course of the Dnieper (the so-called Aleshkovskie Sands), in the basin of the river Don, and on the watershed of the Volga and Ural. Psammophytic vegetation dominates these landscapes. The most typical species are Elymus giganteus, Gnaphalium arenarium, Stipa rubens, and Koeleria.

The stony and gravelly steppes are typical of the southern Urals, central Kazakhstan, the Altay and Sayan mountains, and the Transbaikal region. Fragments of the stony steppes also occur in the Ukraine (in the Donetsk upland and Crimea). Soil cover is poorly developed in the stony steppes and erosion rates are particularly high. Compared to other ecosystems, stony steppes are less productive. However, the share of endemic plants is much higher than in zonal steppe ecosystems because stony steppes served as a refugium for many species during unfavourable climatic epochs. According to Gorchakovsky (1997), vegetation of the stony steppes are not just a petrophilous type of zonal steppe but an ancient floristic complex. Endemics and relics include petrophilous species especially of the taxa Astragalus, Oxytropis, Hedysarum, Miniuartia, and Dianthus.

The character and distribution of vegetation in steppe and semi-desert is affected by the salinity of the soils. Three groups of steppe ecosystems are distinguished with respect to salinization: calcareous and those developing on solonetz and solonchaks. On calcareous soils, the floristic composition is shifted towards xerophilous species such as Stipa capillata on calcareous chernozems and S. lessingiana on calcareous dark chestnut soils. In contrast, Stipa is practically absent from the solonchak steppes, where species of Artemisia, Grinitaria, and Kochia prostrate widely occur. Silaum besseri, Galatella, and Artemisia dracunculus develop on the well-moistened saline chernozems. The flora of the solonchak steppes, which are widespread in the south of the biome, consists almost entirely of halophytes such as Salicornia, Halocnemum, Suaeda, and Salsola.

Plate 11.2 Stony steppes typical of the southern Urals and Asiatic territory (photo: A. Chibilyov)

Tree and Shrub Vegetation in Steppes

The steppes and even the semi-deserts of Northern Eurasia are not completely treeless. A typical feature of steppes is the occurrence of shrublands which are composed of Primus fruticosa, P. spinosa, Amygdalus, Caragana, Spiraea, and Rosa. Apart from the open steppes, shrublands often form a dense belt surrounding woodlands. Isolated woodlands occur mainly in the specific habitats characterized by higher moisture supply and often represent peculiar oases of unaltered nature in the steppe biome. Typical woodland ecosystems include Quercus woodlands on the watersheds, Betula and Populus tremula woodlands developing on sandy soils, and in the topographic depressions, open woodlands composed of Pinus sylvestris, which occur on granite rocks, and riparian forests comprised of AInus alnutinosa and Populus spp. Ravines, which are a typical feature of steppes, are mostly covered by woody vegetation, among which Quercus, Tilia, Acer, Fraxinus, Ulmus, and Betula predominate. The composition of woodlands changes from west to east. In the European steppes, Quercus is the dominant tree species while east of the Urals Betula, Pupulus tremula, and Pinus sylvestris occur. The occurrence of patches of woodland in steppes often makes it difficult to distinguish between steppe and forest-steppe, particularly in the context of the complex topography of Southern Siberia and the Baikal region (Bannikova, 1998).

Fauna

A characteristic feature of the animals of the steppe and forest-steppe zones is that most of them (e.g., wolf, fox, elk, badger, wild boar, roe deer, grouse) have polyzonal areas of distribution. The number of endemic species is much lower than in the neighbouring forest and desert biomes. Thus, mammal species endemic to the steppe biome account only for 30 per cent of all mammal species that occur in steppe, while in boreal forests and in deserts about 70 per cent of all mammals are endemic to these biomes. This is because in the steppe and forest-steppe there is a large variety of habitats ranging from wetlands to semi-desert and from woodlands to sandy or stony steppes. Ungulates and rodents dominate among vertebrates. According to Formozov (1981), ninety-two species of mammals occur in steppes at present. The animal population of the steppe is highly variable both in terms of number of individuals and diversity of species. Some animals (e.g., saiga — a type of antelope, and the Asiatic wild ass) migrate to the steppe seasonally, some rodents (ground squirrel, marmot) are 'active' only for a few months of the year and others (e.g., griffon-vulture) reproduce only in favourable years. The animal population is also sensitive to ecological factors and the interannual variability in environments which is so typical of the steppe biome.

As the main feature of steppes is the open character of the landscape, many species developed various adaptations to existence in open spaces. Thus many rodents inhabit the below ground part of the ecosystem and form colonies. Out of 92 species of steppe mammals, 72 live underground (Formozov, 1981). Life in colonies allows rodents to move safely across large areas. For example, the population density of marmots often reaches 20-30 individuals per hectare. Burrows are located close to each other and connected by passages along which marmots can move very fast. Grazing marmots always closely observe each other and quickly react to each other's warning signals. This is a very effective system of protection which makes it difficult for carnivores to attack marmots. Many rodents feed on the below ground parts of plants (i.e., rhizomes, bulbs and tubers), which are particularly well developed in the steppe biome. Fifty-three species that inhabit burrows are active all year round and 50 of these species accumulate stores for winter. For example, steppe pika accumulates hay in the early summer when plants have the highest nutritional value. It stores hay in burrows, hollows or on the surface, forming small haystacks.

The burrowing activity of rodents is an important factor in the formation of soils (Dokuchaev, 1883; Grinnell, 1923), the nature of microrelief and the vegetation in all types of steppe ecosystem from the meadow steppes in the north to the desert steppes in the south (Formozov, 1981). The interrelationships between vegetation and the animal population were studied in detail by Lavrenko (1952) who suggested that rodents (especially marmots and ground squirrel) have a considerable influence on the structure of steppe vegetation and attributed the complexity and irregularity of vegetation patterns, observed across vast areas in pristine steppes, to the burrowing activity of rodents.

Ungulates of the steppe are gregarious and are able to move fast in the search of food such as, for example, saiga, a typical species of the Kalmyk, southern Urals, and Kazakh steppes. The wild horse (now extinct) and the Asiatic wild ass (Kulan) (at present not found in the steppes) used to comprise herds of 50-100 and sometimes up to 1000 individuals. The effects of ungulates on the development of steppe ecosystems are well known. Research conducted by Lavrenko (1980) in protected steppes has shown that the true steppe ecosystems dominated by bunch-plants can exist under conditions of moderate grazing. The absence of grazing results in the accumulation of dead phytomass, a changing microclimate of the plant canopy, and the replacement of grasses by rhizomatous species and shrubs, while extensive grazing leads to a decline in the abundance of forbs and later species of Stipa, followed by an increase in species of Festuca (Chibilyov, 1990; Lavrenko and Karamysheva, 1993).

The transformation of the steppes has caused an irreversible decline in the diversity of animals, fn prehistoric times, the population of saiga and the Persian gazelle exceeded 10 million and 5 million individuals respectively, while the population of the wild horse and auroch accounted for dozens of millions (Mordkovich, 1982). The development of pastoralism and agriculture over 2-3 millennia destroyed the habitats of wild ungulates and most species had become extinct by the beginning of the 19th century. For example, the Asiatic wild ass disappeared from the Ukrainian steppes in the 16th century and from the Volga-Urals watershed and Kalmykya at the end of the 18th century. At the beginning of the 19th century it was still widespread in Western Siberia and central Kazakhstan. However, already at that time the boundary of its distribution was rapidly receding southwards and by the 1930s it had become extinct in the steppe biome. Well documented is the extinction of the wild horse, hunted widely both for meat and skin, and because it often distracted females from the domestic herds. Its population had been critically depleted by the beginning of the 19th century and the last individuals are known to have been killed in the 1870s (ironically in the region of the current steppe nature reserve, Askania Nova). Among ungulates, only saiga and the Persian gazelle still occur in the wild. However, by the beginning of the 1990s saiga had diverted its migration routes away from the northern steppes, and the Persian gazelle has not been seen in agricultural regions for many years. Other species, including such typical bird species of the steppe as the great and little bustard, have been affected by both development and hunting. Nature reserves could play an important role in the protection of the steppe species. However, protected areas account for a rather small territory. After the Askanya Nova nature reserve was established in 1898 in the southern Ukraine, the only large reserve (the Orenburgsky reserve in the Orenburg region) capable of protecting typical wildlife has been established in the steppe and forest-steppe zones.

The expansion of agriculture has affected both the environmental characteristics of habitats and species selection and has led to the formation of a new fauna that did not exist in prehistory. The modern fauna of the steppe biome comprise domestic animals and species that have adapted to agricultural landscapes. This fauna is species-poor and its composition is uniform across the biome. Typical species include ground squirrel, common hamster, European hare, and small birds feeding on grain, and insects such as locust.

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