Broad-leaved forests in the North-Western Caucasus. I. Order Carpinetalia betuli P. Fukarek 1968
Authors: Shevchenko N. Ye. , Braslavskaya T. Yu. 
Published: January 2022

Braun-Blanquet classification of the piedmont and mountain broad-leaved forests (formed by Carpinus betulus, Quercus petraea/hartwissiana/robur, Fraxinus excelsior) was done basing on 224 relevés collected in 2014–2018 in the North-Western Caucasus (N 43,5–44,8°, E 38,5–41,5°; Fig. 2). DCA-ordination of the data corresponds to their correlation with environment variables (absolute elevation, geographical coordinates, tree canopy density) in Landolt’s ecological scales was carried out.
Suballiance Tamo communis–Carpinenion betuli suball. nov. prov. and new lower syntaxa are proposed (Table 1) within the alliance Crataego–Carpinion caucasicae Passarge 1981. Nomenclature type of the suballiance is the ass. Tamo communis–Carpinetum betuli ass. nov. (Table 3, holotypus is relevé 4: author’s number 83, author N. E. Shevchenko, 19.07.2016, N 44.257°, E 39.760°, 352 m above sea level, slope 5°NE) with three variants: typica ­(Table 3: 1–15), Staphylea colchica (Table 3: 16–27) and Festuca drymeja (Table 3: 28–38). Also, there are ass. Aro macu­lati–Carpinetum betuli (Table 4: 1–15; holotypus is relevé 5: author’s number 427, author N. E. Shevchenko, 12.05.2017, N 44.471°, E 40.516°, 455 m above sea level, slope 6°NE) as well as Abies nordmanniana–Carpinus betulus community (Table 4: 16–27) in new suballiance. Not too common in the North-Western Caucasus are ash (Fraxinus excelsior) forests which have no suffiicient floristic peculiarity and are considered as facies in associations Tamo communis–Carpinetum betuli var. typica and Aro maculati–Carpinetum betuli.
The specificity of the studied forests in comparison with hornbeam and oak forests in the Central Caucasus (Georgia), North Turkey, the Balkans and the Crimea (Passarge, 1981a; Korzhenevskiy, 1982; Didukh, 1996; Korkmaz et al., 2008; Košir et al. 2013; Çoban, Willner, 2019; Novak et al., 2019) is that the North-­Western Caucasus forest flora includes (Table­ 2), besides European species of temperate broad-leaved forests (Acer campestre, Euonymus europaea, Carex sylvatica, Convallaria majalis, Rubus caesius), also southern European species of thermophilous broad-leaved forests (Acer tataricum, Cornus mas, Ligustrum vulgare, Lonicera caprifolium, Tamus communis, Vincetoxicum scandens, Hedera helix, Festuca drymeja), and reduced set of species which are character for Euxinian and ­Caucasian ­forests (Quercus­­ ­hartwissiana, Tilia begoniifolia, Rhododendron luteum, Daphne caucasica, Staphylea colchica, Smilax excelsa, Paris incompleta, Polygonatum orientale, Lathyrus roseus, Campanula alliariifolia, but without Daphne pontica, Epimedium pubigerum, Erica arborea, Ostrya carpinifolia, Salvia forskahlei, Vaccinium arctostaphylos).
DCA-ordination (Fig. 8) showed that the differences in species composition of the broad-leaved ­forest syntaxa are due to both absolute elevation (vector Elev in Fig. 7) and geographic longitude (vector E) of the relevés. So, forests of ass. Aro maculati–Carpinetum betuli and community Abies nordmanniana–Carpinus betulus are situated, in general, at higher positions than forests of ass. Tamo communis–Carpinetum betuli, and the first syntaxon is situated east of the two last ones. Floristic difference between these syntaxa corresponds with parameters assessed by values of Landolt’s scales: soil aeration (vector D), climate continentality (vector K) and light regime (vector L). The lowest α-diversity is in ass. Tamo communis–Carpinetum betuli, and the highest is in the ass. Aro maculati–Carpinetum betuli and community Abies nordmanniana–Carpinus betulus it (Fig. 9).
Associations Carpino betuli–Quercetum petraeae Grebenshchikov et al. 1990 and Rhododendro lutei–Quercetum petraeae Grebenshchikov et al. 1990, earlier described on small sets of relevés in the North-Western Caucasus (Grebenshchikov et al. 1990) within the alliance Carpino betuli–Quercion petraeae Grebenshchikov et al. 1990 (now invalid due to absence of stated holotypus), are very close to the new ass. Tamo communis–Carpinetum betuli. Recently described (also on small sets of data) five associations (Akatova, Ermakov, 2020), within the alliance Crataego–Carpinion caucasicae Passarge 1981, are valid. Therefore, further clarification and optimization of the North-Western Caucasus oak-hornbeam forest classification is required, having in mind the final decision on the alliance/suballiance names and diagnosis. Their belonging to the order (Carpinetalia betuli or Rhododendro pontici–Fagetalia orientalis) is also a debatable question, because researchers working in such forests on Balkans, in the Crimea and the North Turkey have come to different conclusions. The analysis of species with constancy 60–80 % in 224 relevés from the studied area reveals 8 diagnostic ones of the order Carpinetalia betuli vs. only 3 diagnostic ones of the order Rhododendro pontici–Fagetalia orientalis, that allows to assign these oak-hornbeam forests to the first order. The Abies nordmanniana–Carpinus betulus community is intermediate between these two orders but after trees from shade-tolerant fir undergrowth become, the canopy mature will be mixed that is character for forests of the order Rhododendro pontici–Fagetalia orientalis.