Электронный атлас "Биоразнообразие животного и растительного мира Сибири"

Zonal and subzonal types

Forests, Scrub and Fringe Communities

Types of Vegetation: Forests, Scrub and Fringe Communities

Zonal and subzonal types: Continental Hemiboreal Forests

Zonal Features Of The Vegetation Type: Hemiboreal forests in North Eurasia are the intermediate zonal type between true boreal forests (taiga) and temperate broad-leaved forests as well as between the boreal and the forest-steppe zones in the relatively narrow geographical subzone between the latitudes 52o and 58o N. The term "hemiboreal forest" has been taken from AHTI et al. (1968) who classified the European forests of the southern part of the boreal zone as a peculiar geographic type. Later, HAMET-AHTI (1981), singled out a subzone of circum-hemiboreal forests in the territory of both continents of the North Hemisphere. Similarly, in the works of Russian geobotanists, forests of the southern part of the forest zone are seen as belonging to a special subtaiga subzone (BUKS et al. 1977; SOCHAVA, 1979; GRIBOVA and ISATCHENKO, 1979).

Phytocoenotic Featuries: The reason for the classification of these forests as a boreal type of vegetation is the dominance in the woody layer of few but widespread boreal coniferous and small-leaved deciduous species. They form physiognomically similar communities in the vast area of North Eurasia, causing an impression of syntaxonomic uniformity of the forests of this region. Nevertheless, there are significant phytosociological and floristic distinctions between hemiboreal forests and typical boreal (taiga) communities. The former are characterised by a leading phytosociological role of species typical of the vegetation of temperate zone in the composition of herbaceous layer. This peculiarity was shown in the scheme of plant-geographical zones of Eurasia (MEUSEL et al. 1965; 1978). According to this scheme, the subzone of hemiboreal forests was included in the temperate transcontinantal zone.

Geographical Peculiarities: Hemiboreal forests in the continental part of Eurasia (from the Urals in the west to the Greater Khingan Mountains (Da Hinggan Ling) in the east) form a southern peripheral forest zone in contact with steppe as well as being a component of the temperate forest-steppe zone. In the latter case, they represent zonal analogues of broad-leaved forests, replacing them in conditions of increased climatic continentality of Central Eurasia at the same latitudes. Similarly, in the mountain systems of South Siberia and North Mongolia, orohemiboreal forests are a component of the lower part of the forest belt and that of mountain forest-steppe. Continental hemiboreal forests in the main part of their vast range extend across the largest mountain systems of North Eurasia: the South Urals, Altai, Sayani, Transbaical mountains, Khangai, Khentei, Grater Khingan and low mountains of Middle Siberian Plateau. These mountain systems determine variation in the macro- and meso-climates in their different sectors as well as large geographical and plant-geographical boundaries of Eurasia - between Europe and Asia, North and Central Asia, North and East Asia.

Ecology: The majority of the orohemiboreal forests found as components of the mountain forest-steppe occur at a varying range of altitudes: 250-1200 m in semi-humid temperate-continental climate and 1400-2600 m in semi-arid ultra-continental one. There they inhabit the mesic sites of shaded slopes and are replaced by steppes on the more xeric southern slopes. The only exceptions are the orohemiboreal small-leaved-coniferous mixed forests of the hyperhumid sectors of Altai and Sayani, which dominate in the lower part of the forest belt on the slopes of different aspects. Continental hemiboreal forests show one common ecological peculiarity over their vast area: everywhere they occupy the warmest sites in landscapes which are available for supporting the forest vegetation types. Distribution of hemiboreal forests is not limited by paricular soil characteristics. They grow both on rich loamy soils and poor sandy, stony soils developed from chloride slates, carbonates, sandstones, granites as well as alluvial and fluvio-glacial parent materials. Continental hemiboreal forests form a relatively continuous geographical subzone only in the territory of the West Siberian Plain between latitudes of 490 50' N and 520 17' N. There they occur in flat drained watersheds on widespread zonal grey loamy soils or on knolls of the boggy areas. Southwards, in the forest-steppe zone of the West-Siberian Plain, the hemiboreal forests are replaced by meadow-steppes in the flat and convex parts of watersheds. There they occupy the numerous moist and moderately moist small shallow depressions and are a component of the special type of so-named "West-Siberian kolok forest-steppe". The southernmost hemiboreal forests occur in the zone of dry steppes of North Kazakhstan. These are azonal psammophilous and petrophilous pine forests forming isolated forest areas on the sandy river terraces, fluvio-glacial deposits and lower granite mountains.

Phytocoenotic Characteristics: The woody layers of the continental hemiboreal forests are often monodominant or formed of few species with a large ecological amplitude which can survive in extreme climates: Pinus sylvestris, Betula pendula, B. platyphylla, Larix sibirica, L. gmelinii. North Asian Betula pendula and Pinus sylvestris forests, unlike their European analogues are primary zonal communities and grow on zonal grey and dark-grey loamy soils. High fertility of the soils and good light penetration due to poor closure of the canopy promote a luxuriant development of the herbaceous layer. It characteristically has 60-90 % cover, height up to 1.5-1.8 m, rich assemblages of 40-90 species per 200 m2 and a structural subdivision into several sublayers. This distinct phytocoenotic feature provides the basis for the Russian synonym of the term "hemiboreal forest" - namely, "grass forest". It points to the key physiognomic difference between the Siberian grass forests and widespread typical coniferous taiga. The principal peculiarity of the herbaceous layer of the widespread types of continental hemiboreal forests lies in the fact that it is formed mainly of species which are not true forest plants. They are, in fact, light-demanding, mesophilous and meso-xerophilous species of meadows, steppes, forest edges, subalpine meadows, high mountains, as well as species without definite phytosociological adaptation to particular habitats (forest-grassland, forest-steppe, meadow-steppe species). Typical shade taiga species do not play any significant part in the composition of the vegetation. Thermophilous nemoral species are not characteristic either for most of the communities of continental hemiboreal forests. In the territory of Southern Siberia, these species are relic plants with disjunctive ranges. Concentration of nemoral species (including single broad-leaved tree species like Tilia cordata subsp. sibirica) is shown only in Siberian relic small-leaved dark-coniferous subnemoral forests which are locally widespread in the warmest part in Siberia - the hyper-humid foothills of the Altai and Sayani. A further important phytocoenotic peculiarity of the majority of continental hemiboreal forests is the absence of moss and lichen layers. The exception is provided by communities of ultra-continental larch forests of Mongolia ("pseudotaiga forests"). What they have in common is a layer of xerophilous moss species which do not play a significant role in the communities of the true taiga, but nevertheless characterize some physiognomic similarity of this zonal forest type.

Николай Ермаков

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