Phytocoenotical positions of hypoarctic plant species in boreal-forest zone communities of European Russia

The aim of this study is that of revealing changes in phytocoenotical positions of hypoarctic, hypoarctic-montane, and arctic-boreal vascular plant species in different subzones of the boreal-forest zone in comparison with those in southern hypoarctic tundras where such species prevail. For this purpose, we use 14 model species registered in the set of 4995 releves made in European Russia in 1996-2018 (See Fig. 1). The set was classified following the dominant-determinant approach to vegetation. We distinguish 16 vegetation units for the southern hypoarctic tundra subzone (See Table 1) and the same number of units for the boreal-forest zone with several subzones and belts (See Table 2), 5 crowberry heath, peatmoss bog, and matgrass meadow units similar in both cases. Changes in species positions are traced following these units combined in rows for analogous habitats in different subzones. Many species of the hypoarctic fraction of a flora occur in mires (which is well-known) but also successfully penetrate under the coniferous forest canopy in the northern-boreal subzone. But middle- and southern-boreal forest communities serve as refugia for these species no more. The latter survived only in bogs and fens, some of them (Nardus stricta, Bistorta vivipara) also in secondary matgrass meadows. Most of the hypoarctic species (in the broad sense of the term) are oceanic in the study area. Such are Nardus stricta, Rubus chamaemorus, Empetrum hermaphrodi-tum, Chamaepericlymenum suecicum, Saussurea alpina, etc. This fact is proved by the statistically testified values of Spearman rank correlation rs between species cover (%) and Conrad continentality index (See Table 3). Prevalence of the oceanic species correspond to both the zonal characters of the Hypoarctic climate as a whole (cold but damp) and the meridional characters of its European sector. Plants neutral in respect to continentality (Eriophorum vaginatum, Betula nana in bogs, etc.) often dominate but are never represented by so many species. The continental plants (E. russeolum, Saxi-fraga aestivalis) occur more close to the Urals in the boreal-forest zone (See Table 3). Oceanic hypoarctic species are most typical for oligotrophic communities on acidic soil, including those with developed peat or raw humus deposits. But they also occur in communities on rich soil or carbonate rock, although never so abundant. Oligotrophic hypoarctic species are represented by the three major floristic elements, namely the tundra-bog, the hollow, and the heath-redwood ones. The tundra-bog plants (Betula nana, Rubus chamaemorus, Eriophorum vaginatum, etc.; the “cortege” of Betu-la nana) occur on peatmoss bog ridges and in bogged Siberian spruce- and Scots pine-peatmoss forests. This element is seemingly one of the most ancient among the hypoarctic plants in the boreal-forest zone, as its representatives reach the southern-boreal bogs in their distribution. Vast ranges, Angara-land origins, usually also the neutral response to continentality changes are typical for such plants, to the exception of the suboceanic Rubus chamaemorus. The hollow element (Eriophorum angustifolium, E. russeolum, Trichophorum cespitosum, Carex rotundata) is close to the tundra-bog one in species distribution and age of formation. But hygrophytes prevail among its species. Oceanic species of the heath-redwood element (Empetrum hermaphroditum, Cham-aepericlymenum suecicum, etc.; the “cortege” of Empetrum hermaphroditum) probably originated in maritime heaths of the Subarctic oceanic sectors in the Late Tertiary. They migrated to periglacial and mountain tundras in the Pleistocene and then to the northern-boreal coniferous forests of feathermoss types in the Holocene. Empetrum hermaphrodi-tum becomes sub-dominant in the dwarfshrub layer of both pine and spruce northern-boreal forests. Chamaepericlymenum suecicum occurs in feathermoss-spruce forests of the northernmost-boreal belt but only in peatmoss-spruce ones in the southern belt of the subzone. In the middle-boreal subzone, the first species becomes rare and restricted to treeless rocky or maritime habitats, and the second one is known from its relict Little Ice Age findings. Species of the heath-redwood element are also closely associated with the supra-littoral hypoarctic ones (the “cortege” of Leymus arenarius) in their present-day distribution in maritime habitats. Mesotrophic and mesoeutrophic hypoarctic species, both the oceanic and the continental ones, form the poorly known willow-taiga element. These are Viola biflora, Saus-surea alpina, also Alchemilla glomerulans s.l. and Epilobium hornemannii in Fennoscandia, and Saxifraga aestivalis in the Cis-Urals and Urals. Such species occur in subniveal meadow tundras and willow-herb thickets dominated by Salix lanata in the tundra zone. But they turn on to growing under the canopy of elfin mountain birch woodlands and, more southwards, riverine tall-herb and herb-peatmoss spruce forests in the northernmost-boreal belt. These types of spruce forests also represent the coenotic niche of these plants in the southern belt of the northern-boreal subzone. But spring fens become their only refugia in the middle- and southern-boreal subzones where spruce forests are mainly lacking the hypoarctic elements (See Tables 1, 2). The hypoarctic species subdivision into ecological-phytocoenotical elements and sub-elements in the boreal-forest zone is dictated by history of genesis and migration of these species to no less extent than by their ecology. The article contains 1 Figure, 3 Tables and 69 References. The authors are grateful to Dr. S.Yu. Popov (M.V. Lomonosov Moscow State University) for the provision of his unpublished releves, and to Dr. V.Yu. Neshataeva, Dr. S.S. Kholod, Dr. O.G. Baranova (Komarov Botanical Institute RAS) and Dr. V.M. Bubyreva (St. Petersburg State University) for their most valuable comments. The Authors declare no conflict of interest.

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