Gray alder – Alnus incana (L.) Moench (Betulaceae) in the taiga zone of Western Siberia is a rare species located on the northern and eastern borders of its area. Alnus incana occurs sporadically in the floodplain of the Irtysh [Flora …, 1992; Gordeev, 1999; Kapitonova et al., 2017], the Ob River [Taran et al., 2004], and the valleys of small rivers of its left bank tributaries [Lapshina et al., 2018].

A new association Carici juncellae–Alnetum incanae ass. nov. of treed floodplain swamps, with the participation of gray alder (Fig. 1, 2) was described in the right-bank floodplain of the Irtysh River in its lower reaches (60.73 N; 69.81 E). The association combines rare birch-alder-tussock sedge communities of floodplain swamps that develop under conditions of flooding by river waters and moderately rich ground water supply.

Differentiating species combination: Alnus incana, Carex juncella, C. canescens, Calliergon giganteum, Calliergonella cuspidata, Climacium dendroides, Haplocladium microphyllum, Lewinskya elegans, Pylaisia polyantha (Table 1).

Communities similar in floristic composition were recently described on the territory of the “Malaya Sosva” Nature Reserve under the name of ass. Alno incanae-Betuletum pubescentis Lapshina et al. 2018 [Lapshina et al., 2018]. The main difference of the new association is a taller tree layer, the development of a dense layer of gray alder (up to 70%), the dominance of tussock-forming sedges, and the complete absence of Sphagnum mosses (Table 1), caused by differences in the ecological conditions of habitats (more favorable microclimatic conditions, variability of the moistening and higher richness of peat soils in the floodplain of the Irtysh).

The new association is assigned to the alliance Salici pentandrae–Betulion pubescentis Clausnitzer in Dengler et al. 2004 (order Salici pentandrae-Betuletalia pubescentis Clausnitzer in Dengler et al. 2004) of the class Alnetea glutinosae Tx. 1937. The order and its central alliance combine low-lying eutrophic and mesoeutrophic birch swamps and swampy low forests of Eurasia fed by ground water and river water.

The good condition of gray alder in the communities of floodplain swamps (high projective cover, flowering, fruiting) allows us to expect a wider distribution of communities of this association in Western Siberia under conditions of climate warming.