Spruce forests of the Koptu River valley (the Republic of Tuva) as a relict of Late Pleistocene vegetation

Spruce forests of the Tuva Republic are not sufficiently described in literature. Spruce forests occupy small areas, but they are characterized by high species diversity. Enclaves of forest communities outside the forest belt are particularly interesting. The actual problem concerns species composition which forms plant communities existing under environmental and climate conditions that are not favorable for it. The aim of the research is to describe the communities of the valley dark coniferous forests in the model river valley in Central Tuva, to assess their diversity, to analyze species composition and to identify the main factors determining it. In the course of the fieldwork in 2020, we examined coniferous forests along the Koptu river, on the southern slope of the Academic Obruchev range (See Figures). In this part of the range, the foothills are occupied by steppe vegetation; forest-steppe landscapes prevail at altitudes of 800-1000 m; above there is the taiga belt, the upper border of the forest is about 1900 m above sea level. Two key areas were investigated; one of them is located in the middle course of the river within the taiga belt (51.90- 51.92°N; 95.45-95.48°E, 1400-1500 m above sea level), the other is in the lower course of the river in the forest-steppe belt (51.55-51.70°N; 95.34-95.45°E, 700-900 m above sea level). In total, 11 relevés were made on plots of 20×20 m by the standard method. We established that the forests are rich in species: 98 taxa were registered (See Table). It could be expected that coniferous forests in the river valley within the forest-steppe belt are a continuation of the above located communities, a continuation of communities from the taiga belt. However, our materials have demonstrated that it is not so. We revealed that, despite the physiognomic similarity, the dark coniferous communities growing in the river valley in its lower course differ significantly from those growing within the taiga belt (See Table). Larix sibirica is abundant everywhere; but in the taiga belt Pinus sibirica dominates in the valley communities, and Picea obovata prevailes in the river valley in the forest-steppe belt. Apart from it, the communities of these two key sites differ significantly in their species composition. Analysis of the ecology of differentiating species shows that the difference is due to a complex of factors. Four groups of species can be distinguished in the investigated forests: I. Taiga species which are only encountered in forests of the upper part and are absent below (Aegopodium alpestre, Carex iljinii, Cerastium pauciflorum, Vaccinium vitis-idaea, V. uliginosum). II. Meadow species, symmetrically, only occur in the forests of the lower part and are absent in the taiga belt (Galium boreale, Lathyrus pratensis, Rubus saxatilis and Vicia cracca, Achillea asiatica, Adonis sibirica, Artemisia laciniata, Iris ruthenica). The two groups mentioned above reflect the difference in altitude as well as the influence of the zonal environment. III. A series of species in the downstream valley forests reflect a waterlogging process, which takes place there due to the altitudinal gradient declining and surface flattening (Angelica tenuifolia, Caltha palustris, Carex cespitosa, C. dioica, Equisetum scirpoides, Delphinium crassifolium, Galium uliginosum, Lathyrus palustris, Parnassia palustris, Ribes procumbens; among mosses – Calliergonella cuspidata, Helodium blandowii, Timmia megapolitana, Tomentypnum nitens). IV. A number of species with mainly taiga or Arctic-Alpine distribution were recorded only in larch-spruce forests in the river valley in the forest-steppe belt and they are absent in the forests of the upper key area (Carex alba, Bistorta vivipara, Carex sedakowii, Orthilia obtusata, Pedicularis sceptrum-carolinum, Saussurea alpina, Schizachne callosa). For these species, it is impossible to explain their presence in the forest-steppe belt by modern ecological conditions. Their presence in the valley spruce forests has historical reasons and could be caused by the conditions of the Late Pleistocene cooling. So, we can draw a conclusion that the massif of valley spruce forests in the lower part of the Koptu river is a relict community formed in the Late Pleistocene periglacial environment on permafrost soils. In present conditions, it is in unstable equilibrium. The massif is the habitat of a number of rare species. In particular, the Orchidaceae taxa were found here, four of which are included in the Red Book of the Republic of Tuva (Cypripedium calceolus, Dactylorhiza sibirica, Epipogium aphyllum, Neottianthe cucullata). The valley forest of the lower part of the Koptu river can be nominated as a high conservation value forest and recommended to be included in the list of the Important Plant Areas of the Altai-Sayan region. The massif is under the strong anthropogenic pressure; in order to avoid extinction, it needs to be protected.