Diversity of broad-leaved and pine–broad-leaved forests on the eastern border of their distribution

As a result of long-term research carried out in the Southern Ural region, extensive information on the species richness and phytosociological diversity has been obtained for the broad-leaved forests belonging to the alliance Aconito lycoctoni–Tilion cordatae Solomeshch et Grigoriev in Willner et al. 2016 (order Carpinetalia betuli P. Fukarek 1968, class Carpino-Fagetea sylvaticae Jakucs ex Passarge 1968). The study is based on the analysis of 787 relevés made between 1989 and 2019. Relevés and their further analysis were performed according to the Braun-Blanquet aproach (Braun-Blanquet, 1964; Westhoff, Maarel, 1978). Two suballiances, 5 associations, 14 subassociations, 12 variants, and 1 facies were described in the alliance Aconito-Tilion. The combinations of diagnostic species were determined for each syntaxa. Nomenclatural types for new syntaxa are given in phytocoenotic tables and in the text. Сommunities of the alliance Aconito-Tilion are distributed meridionally from the southern taiga subzone (southern border of Perm Krai and Sverdlovsk Region), where they border with boreal forests. They are replaced by thermophilous oak forests of the alliance Lathyro pisiformis–Quercion roboris Solomeshch et Grigoriev in Willner et al. 2015 in the southern edge of the Ural Mountains and the Ural River basin of Orenburg Region (Fig. 1). In the latitudinal direction, forests of the alliance Aconito-Tilion are distributed in the forest-steppe zone of Bashkir Urals, Bugulma-Belebey Upland and foothills of western macroslope of Ural Mountains. Eastwards, they border hemiboreal light-coniferous–small-leaved herbaceous forests of the order Chamaecytiso ruthenici–Pinetalia sylvestris Solomeshch et Ermakov in Ermakov et al. 2000, class Brachypodio pinnati–Betuletea pendulae Ermakov, Korolyuk et Lashchinsky 1991. In the west, the communities of the alliance Aconito-Tilion are replaced by mesophytic broad-leaved forests of the­ ­alliance Querco roboris–Tilion cordatae Solomeshch et Laivinņš ex Bulokhov et Solomeshch in Bulokhov et Semenishchenkov 2015. According to floristic and structural-physiognomic characters, two suballiances were distinguished within this alliance. Suballiance Aconito lycoctoni–Tilienion cordatae suball. nov. combines broad-leaved forests typical for the region. Suballiance Tilio cordatae–Pinenion sylvestris suball. nov. includes pine–broad-leaved forests which represent ecotone communities in the transition stripe between European temperate broad-leaved forests of the class Carpino-Fagetea and Siberian hemiboreal light-coniferous–small-leaved herbaceous forests of the class Brachypodio-Betuletea. Suballiance Aconito-Tilienion (holotypus: Stachyo sylvaticae–Tilietum cordatae ass. Martynenko et al. 2005) includes broad-leaved forests growing near the eastern border of their range. In these forests, the main dominants of the tree layer are Tilia cordata, Ulmus glabra and Acer platanoides. Co-dominants of herb layer are shade-tolerant broad herb species — Asarum europaeum, Aegopodium podagraria, Dryopteris filix-mas, Galium odoratum, Pulmonaria obscura, Viola mirabilis, etc., as well as Ural and Siberian tall-herb species such as Aconitum lycoctonum, Crepis sibirica, Bupleurum longifolium, Heracleum sibiricum, Cacalia hastata, Cicerbita uralensis. The suballiance is represented by two associations: Brachypodio pinnate–Tilietum cordatae Grigoriev ex Martynenko et al. 2005 and Stachyo sylvaticae–Tilietum cordatae Martynenko et al. 2005. Within these associations, four new subassociations were described: Brachypodio pinnate–Tilietum cordatae pulmonarietosum mollis subass. nov. (Table 1, columns 2, 3; Table 2, rel. 1–30), Stachyo sylvaticae–Tilietum cordatae alliarietosum petiolatae subass. nov. (Table 1, column 12; Table 2, rel. 31–46), S. s.–T. c. violetosum hirtae Grigoriev ex subass. nov. (Table 1, column 11), S. s.–T. c. pulmonarietosum mollis Khaziakhmetov, Solomeshch, Grigoriev et Muldashev ex Shirokikh, Martynenko, Baisheva, Fedorov, Muldashev et Naumova 2021 subass. nov. (Table 1, column 13). Suballiance Tilio-Pinenion (holotypus: Tilio cordatae–Pinetum sylvestris ass. nov.) combines mixed pine–broad-leaved forests in the Southern Urals and the eastern edge of the Russian Plain with the dominance of Pinus sylvestris in the tree layer and broad-leaved tree species in the lower one (Acer platanoides, Quercus robur, Tilia cordata, Ulmus glabra). There are both species typical for European broad-leaved forests and Siberian hemiboreal light-coniferous herbaceous forests in the composition of these forests. On the Ufa and Zilair Plateaus and in the hilly terrains of the central part of the Southern Urals, these forests are distributed mainly in contact zone with hemiboreal forests of the order Chamaecytiso-Pi­netalia, and, less often with dark-coniferous–broad-leaved forests of the class Asaro europaei–Abietetea sibiricae Ermakov, Mucina et Zhitlukhina in Willner et al. 2016 (Fig. 1). Additionally, small massifs of these forests occur in the Fore-Ural region and Bugulma-Belebey Upland. Three associations with a number of smaller syntaxonomic units are described within this suballiance (Table 3). The results of the classification are confirmed by cluster analysis (Fig. 2). Ass. Tilio cordatae–Pinetum sylvestris ass. nov. (Table 3, columns 1–3, Table 4, 5) represents the most typical communities of the suballiance Tilio-Pine­nion. Four subassociations are described within the association: T. c.–P. s. typicum subass. nov. (Table 3, columns 1–2; Table 4), T. c.–P. s. caricetosum pilosae subass. nov. (Table 3, column 3; Table 5, rel. 21–33), T. c.–P. s. cerastietosum pauciflori subass. nov. (Table 3, columns 4–5; Table 6), T. c.–P. s. galietosum odorati (Martynenko et Zhigunov in Martynenko et al. 2005) stat. nov. (Table 3, column 6; Table 6, rel. 44–57). Ass. Carici arnellii–Pinetum sylvestris Solomeshch et Martynenko ass. nov. (Table 3, column 8; Table 7) combines the most mesophytic communities of the suballiance, which grow in valleys of mountain rivers on rich grey forest soils with periodic short-term waterlogging. Ass. Euonymo verrucosae–Pinetum sylvestris Martynenko et al. 2007 includes the most xerophytic communities of the suballiance, distributed on steep southern slopes of hills and on the Ufa Plateau. The similarities and differences between investigated broad-leaved, pine–broad-leaved and hemiboreal forests are reflected in the ordination diagram (Fig. 3). The pine–broad-leaved forests of the suballiance Tilio-Pinenion occupy intermediate position between the communities of the suballiance Aconito-Tilienion and the hemiboreal forests of the order Chamaecytiso-Pinetalia. Therefore, it was difficult to clarify the place of the first suballiance in the system of higher units. Two syntaxonomic decisions were possible: the suballiance can be considered as: — the extreme western variant of the alliance Trollio europaea–Pinion sylvestris Fedorov ex Ermakov et al. 2000 within the order Chamaecytiso-Pinetalia (hemiboreal light-coniferous Siberian herbaceous fo­rests); — a part of the alliance Aconito-Tilion of the order Carpinetalia betuli (broad-leaved forests of the Southern Urals). Both points of view have sufficiently strong evidence, but the authors have settled on the second decision which is supported by the fact that after felling hemiboreal forests of the order Chamaecytiso-Pinetalia are replaced by secondary birch or aspen forests, whereas the pine–broad-leaved forests of the suballiance Tilio-Pinenion — by dense broad-leaved forests with nemoral species. In addition, the lower tree layer of Tilio-Pinenion forests is formed by tree species typical for Urals broad-leaved forests — Acer platanoides, Quercus robur, Tilia cordata, and Ulmus glabra. In the Southern Ural all these species, besides linden, grow near the eastern border of their range which indicates that communities of the suballiance Tilio-Pinenion should be assigned to European forests of the order Carpinetalia betuli rather than to Siberian forests of the order Chamaecytiso-Pinetalia.