Taran G.S. (1994b) Floodplain Ephemeretum of Middle Ob – a New Class for Siberia, Isoëto-Nanojuncetea Br.-Bl. et Tx. 1943 on the Northern Border of Expansion

Taran G.S. (1994b) Floodplain Ephemeretum of Middle Ob – a New Class for Siberia, Isoëto-Nanojuncetea Br.-Bl. et Tx. 1943 on the Northern Border of Expansion // Sib. J. of Ecol. No.6, 578-582

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In 1986-1991 we explored the Ob floodplain within the Aleksandrovskoye district of the Tomsk region. The Aleksandrovskoye fragment of the Ob river is situated in the middle taiga subzone, between the mouths of the Tym and Vakh rivers, large tributaries of the Ob. Its southern and northern extreme points have the coordinates of 59º28´ north, 79º33´ east and 60º43´ north, 78º05´ east.

In the process of studying the vegetation cover of Aleksandrovskoye floodplain, original communities short-living bank annual plants were discovered which in the West European syntaxonomical literature are ascribed to the class Isoëto-Nanojuncetea Br.-Bl. et Tx. 1943, or the class of small rush vegetation. Appearance of such communities is usually observed in dry years. Sprouting of ephemers begins after denudation of banks – on the average, in the middle of July, and continues almost till the end of the vegetation season. In the vicinities of the settlement of Aleksandrovskoye, development of ephemers in each concrete site of low-water bank is completed in 10-11 weeks after its denudation from water. A peculiarity of the ephemeretum of the Aleksandrovskoye floodplain is its certain floristic poverty. This is caused by the fact that these cenoses exist on the northern border of the areal of the small rush vegetation class. These are the most northern of the thoroughly studied habitats of this class (60º24´– 60º35´ north).

The communities of the Aleksandrovskoye floodplain belong to the association Cypero-Limoselletum (Oberd. 1957) Korneck 1960 which is quite usual in West Europe [1-10]. In the European part of Russia, it has been found in the middle reaches of the Volga [11]. There have been also reports of its being found in Siberia. So, M.D. Spiridonov [12, 13] made descriptions of two pioneer communities of willow sprouts in the floodplain of Irtysh, which may be ascribed to willow facies (salicosum triandro-viminalis) of the association. However, the researchers’ attention is more often attracted by communities of the needle spike-rush facies (Cypero-Limoselletum eleocharitosum acicularis) which, as a rule, are identified by them with the association Eleocharitetum acicularis (Baumann 1911) Koch 1926 of the class Littorelletea. Such cenoses are found on the left tributary of the Ob, the Baksa river [14], and in the middle reaches of the Lena river [15]. In Yakutia, the expansion of the association Cypero-Limoselletum northwards is possible to the upper reaches of the Yana river (67º40´ north), where mudwort is found “on clayey banks of lakes and channels in river floodplains, sometimes very abundantly” [16, p. 94].

In middle reaches of the Ob river, the Cypero-Limoselletum is represented by two new subassociations whose detailed characteristics are contained in our deposited article [17]. Since it is planned to publish a complete classification of plant communities of the Aleksandrovskoye floodplain as a whole single paper, here we are going to content ourself with preliminary information – a synoptic table, nomenclature types of subassociations (see Table), and a brief characterization of the latter.

The little sorrel subassociation (C.-L. rumicetosum ucranici Taran 1994) unites East European-Siberian communites with Rumex ucranicus and Bidens radiata spread in floodplains of sufficiently large rivers within the taiga zone. Diagnostic species are Rumex ucranicus, Bidens radiata, Marchantia alpestris. In the Aleksandrovskoye floodplain, these communites are found along the main bed of the Ob river in low parts of islands and large banks. The subassociation is represented here by two variants: typical that of mossgrass (var. Coleanthus subtilis). The latter is indicated by diagnostic species of the subassociation C.-L. coleanthetosum from whose communities diaspores are carried out by littoral flows. At the same time, in islands remote from littoral flows, the mossgrass is rare.

The physiognomical diversity of little sorrel subassociation in the Aleksandrovskoye floodplain is limited to five facies: mudwort (limosellosum aquaticae), physcomitrellosum patentis, willow (salicosum triandro-viminalis), little sorrel (rumicosum ucranici), mossgrass (coleanthosum subtilis).

The subassociation C.-L. coleanthetosum Taran 1994 unites mossgrass–water starwort communites in shors near the mouths of Ob and Irtysh tributaries within the limits of the taiga zone. Diagnostic species are Coleanthus subtilis, Polygonum volchovense Tzvel., Eleocharis acicularis (loc.), Botrydium granulatum (loc.). The leading species is Coleanthus subtilis. Polygonum volchovense has been found as a mossgrass satellite on Volkhov and Bolshoi Salym rivers [21, 22]. Eleocharis acicularis è Botrydium granulatum are auxiliary species underlining the ecological differences of habitats of the little sorrel and mossgrass subassociations.

In the middle reaches of the Ob, the subassociation is represented first of all by mossgrass–waters starwort (Callirtiche verna) communities. These are attached to backwater zones that arise in the lower reaches of the Ob’s tributaries due to protracted spring-summer floods. Stagnation of tributaries’ waters causes, on the one hand, an abundant sedimentation of silt fractions of alluvium and, on the other hand, late drying of the formed silt banks, which hinders the development of closed grass communities of perennial species. On more or less large tributaries, the lower part of the backwater zone can have the form of a kind of internal delta – a mouth shor [23]. Therein, the main bed of the tributary is divided into a large number of channels and temporary small flows separated by vast areas of naked banks. The latter spread over the the whole width of the tributary’s floodplain terrace. Mouth shors of tributaries of the middle reaches of the Ob river represent hydrologically almost annually dried ponds. They are primary habitats of mossgrass communities and natural prototypes of fish ponds of Central Europe where the mossgrass is also found [24-26]. Very characteristic of tributaries of the Amur river in its lower reaches to which another large fragment of this species is attached, are similar hydro-geomorphological formations – dam lakes at river mouths [27].

The subassociation C.-L. coleanthetosum was described from Laryogan in whose lower reaches there is a well-formed mouth shor, 4.5 km long and about 0.5 km width. The physiognomical diversity of Laryogan communities is limited to 4 facies: typical (callitricho vernae-coleanthosum), water starwort (callitrichosum vernae), cudweed (gnaphaliosum sibirici), and needle spike-rush.

With respect to area, absolutely prevalent are communities of the typical and water starwort facies spread on the main surface of the shor banks, where in dry years they occupy about 50 hectares. The mossgrass population density is here about 450 spec/m². Since 100 million seeds are formed on 1 hectare at the density of 5 spec/m² [25], it seems that in the Latyogan shor about 450 billion diaspores are formed in dry years. Analysis of hydrological and meteorogical data taking into account the altitudinal position of communities of the mossgrass variant little sorrel subassociation has demonstrated that for the period of 1936-1991, the probability of annual appearance of mossgrass and its reaching the fruit bearing stage was about 55 %.

The needle spike-rush facies replaces successionally and topologically the typical one as the bank surface become higher, forming a more or less pronounced microzone. The cudweed facies is spread above the mouth shor, upstream Laryogan. As the surface of mouth banks becomes higher, the cudweed communities are replaced by those of willow sprouts (Salix viminalis, S. dasyclados) – syntaxonomical fragments of Cypero-Limoselletum.

Since the mossgrass has been entered into the Red Book of Russia [28], the Laryogan shor is the only reliably discovered habitat where the species is reproduced steadily and in mass, it is necessary to declare it to be a monument of nature or a botanical forbid territory.

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