The Tobol and the Irtysh watershed occupy the western part of West-Siberian plain. Area studied covers latitudinal band between 55° and 57° n. l. It includes the southern part of subtaiga subzone and northern part of forest-steppe zone. This territory has been used for agriculture since XVIIth century. In the middle of XXth century the local landscapes were represented by endless arable lands alternated with tiny patches of forests on the watersheds and swamps and solonchaks prevailing at lowlands. In the end of XXth century the large areas of arable lands, especially in northern part of mentioned area, were abandoned. Nowadays this area is covered by fallow meadows on different stages of demutation.
Vegetation studies of the region have started in the end of XIXth century. One of the best scientific works on local grasslands was written by B. N. Gorodkov (1915).
The spectrum of natural plant communities demonstrates some changes fr om south to north. These changes enclose both zonal and intrazonal vegetation. The southern part of the area studied belongs to forest-steppe zone. Its zonal vegetation is represented by small-leaved forests, grasslands (steppe meadows) and meadow steppes. Zonal vegetation of northern part — subtaiga subzone — includes only small-leaved forests. The watershed grasslands replace the cut-off forests. Intrazonal vegetation of southern part consists of various grasslands on salty soils and swamps predominated in the northern part. The border of forest-steppe and subtaiga represents a combination of zonal and intrazonal communities disturbed by man.
The goal of this article is to reveal the diversity of grasslands on drained soils of mentioned area. The analysis of 151 geobotanical relevйs obtained fr om subtaiga and northern forest-steppe of Omskaya and Tyumenskaya administrative districts has been carried out. The syntaxonomy of the Tobol-and-the Irtysh watershed grasslands is represented by 6 associations, 4 subassociations and 2 communities of 4 alliances, 4 order and 2 classes.
Subtaiga and northern forest steppe are characterized by different spectra of grasslands. This can be explained by a presence of wide range of saline soils in forest-steppe and their absence in subtaiga. The floristic composition of grasslands of forest-steppes zone includes the plants tolerant to soil salinity such as Artemisia pontica, Galatella biflora, Plantago maxima, Eryngium planum, Saussurea amara, Limonium gmelinii. The listed species are rare in subtaiga grasslands while Festuca pratensis, Phleum pratense, Agrostis gigantea of alliance Festucion pratensis are dominated in subtaiga grassland meadows.
Steppe meadows of this territory are represented by two classes. Northern forest-steppe subzone demonstrates the northern lim it of Festuco-Brometea (alliance Galatellion biflorae of order Festucetalia valesiacae). The associations Galatello biflorae–Calamagrostietum epigeii and Galio borealis–Artemisietum ponticae occupy the zonal positions. Their communities are characterize by high activity of meadow-steppe mesoxerophytes that are the diagnostic of the class Festuco-Brometea. The meadow mesophytes dominate in subtaiga steppe meadows absolutely; meadow-steppe species are rare. They are marked with a lack of diagnostic species of alliance Galatellion biflorae. They are ascribed to the class Molinio–Arrhenatheretea, order Galietalia veri and alliance Trifolion montani as the communities Filipendula stepposa–Cenolophium denudatum and Astragalus danicus–Potentilla anserinа.
Forest steppe meadows of alliance Aconito barbati–Vicion unijugae (order Carici macrourae–Crepidetalia sibiricae, class Molinio-Arrhenatheretea) are usual for northern, subtaiga subzone of the Tobol and the Irtysh watershed. Diagnostic species list of the communities belonging to the order Carici macrourae–Crepidetalia sibiricae on West-Siberian plain is impoverished in comparison with the uplands and the mountains of South Siberia wh ere this order was described by Ermakov et al. (1999). We have revealed 2 new associations of alliance Aconito barbati–Vicion unijugae: Aegopodio podagrariae–Brachypodietum pinnati and Dracocephalo ruyschiani–Heracleetum sibiricae. In subtaiga subzone communities of these associations occupy the small well drained habitats between mesophytic forests and arable lands. The first association unites the xerophytic forest meadows with Aegopodium podagraria dominance, occurring only on right bank of the Ishim River. The second one includes forest meadows of subtaiga widely spread on the Tobol and the Irtysh watershed. Association Dracocephalo ruyschiani–Heracleetum sibiricae comprises two subassociations: D. r.–H. s. typicum and D. r.–H. s.molinietosum caeruleae. One of diagnostic species of the last mentioned subassociation is Molinia caerulea that is a common species of boreal wet meadows of Europe. The eastern border of this species distribution is on the left riverside of the Ishim River.
Meadows of alliance Festucion pratensis of order Arrhenatheretalia are characteristic only for subtaiga, they belong to two associations. Meadows of subassociation Cirsio setosi–Phleetum pratensis pastinacetosum sylvestris are widespread. They are wide spread over the abandoned arable lands and rather monotonous by their floristic composition. The floristic peculiarity of plant communities is abundance of diagnostic species of classes Artemisietea и Chenopodietea. Meadows of Agrostio tenuis–Festucetum pratensis Yamalov 2005 occur on sandy soils; they present one of the first stages of pine forest demutation. Some of species are not characteristic for typical subassociation described by S. M. Yamalov (2005), therefore the meadows of this association on the Tobol and the Irtysh watershed are referred to a new subassociation A. t.–F. p. senecionetosum jacobaea.
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