Еловые леса Ижорской возвышенности (Ленинградская область): типология и современное состояние

Abstract

The article describes the results of field geobotanical and landscape studies in spruce (Picea abies) forests of the eastern and southeastern parts of Izhora upland (Leningrad Region), carried out in 2016. This upland is a plateau with prevailing absolute heights of 100–150 m. The carbonate rocks of the middle Ordovician represent the upper part of the geological basis. Everywhere the bedrock is overlain by the moraine of 1–5 m thickness. The moraine has high content of carbonates on a larger area. Features of the geological structure of the territory determined the widespread fertile (for the taiga zone) sod-carbonate soils (rendzinas) and richer vegetation than in the surrounding southern taiga landscapes. According to geobotanical subdivision (Geobotanicheskoye..., 1989), the vegetation of Izhora upland accords to the extrazonal subtaiga okrug of Ordovic plateau, which is located in the southern taiga subzone. The data on the current spruce forests of Izhora upland are very few (Nitsenko, 1960; Boch et al., 1992; Sambuk, Smagin, 1992). The results of our research allowed us to supplement information on spruce forests and their current state in the territory with a thousand-year period of agricultural use. The characteristic of five groups of spruce forest associations, including the full species composition of communities, their layer structure and quantitative characteristic of species participation in each layer is presented (table). The approaches of ecological-phytocenotic classification were used for grouping of the available relevés, as well as dominants of all layers of communities and determinant species were taken into consideration. Plant communities of Piceeta brachipodiosa group of associations (rel. 1–4) occur on the tops and gentle slopes of low hills composed of calcareous loamy moraine rich in limestone fragments. Along with spruce always present in the tree layer is Pinus sylvestris; broad-leaved species are not found, small-leaved species are rare. The broad-leaved species are absent in young grows. The shrub layer in such forests is usually not thick. The grass Brachypodium pinnatum is a dominant of herb layer; Convallaria majalis, Rubus saxatilis and nemoral herb species (Viola mirabilis, Hepatica nobilis, Aegopodium podagraria) are co-dominants. The projective cover degree of mosses ranges from 30 to 85 %. Rhytidiadelphus triquetrus and Hylocomium splendens predominate in moss layer. Plant communities of Piceeta convallariosa group of associations (rel. 5–8) occur in similar sites. Besides spruce and pine, the tree layer includes small-leaved species (Populus tremula, Betula pendula). Acer platanoides and Quercus robur grow occasionally in young grows. The shrub layer is usually closed and rich in species: Lonicera xylosteum, Sorbus aucuparia, Padus avium, Daphne mezereum, Ribes alpinum and Viburnum opulus. Convallaria majalis is a dominant and Rubus saxatilis is a co-dominant of herb layer. In addition to these species Calamagrostis arundinacea, Geranium sylvatica and nemoral species are constant, but not abundant in communities. Moss cover dominated by Rhytidiadelphus triquetrus is well developed. Piceeta nemoriherbosa group of associations (rel. 9–15) is the most common type of communities in the Izhora upland. They are distributed mainly on plains, hills and ridges composed of carbonate moraine. Besides spruce, there are pine, rarely aspen and birch in the first tree canopy and Acer platanoides in the second one, as well as in young grows. The dense shrub layer is character for this type. This group of associations can be divided according the composition of shrubs and moss abundance: spruce nemoral herb-moss forests with Lonicera xylosteum and spruce nemoral herb with Corylus avellana. Nemoral species are the dominants in herb layer. Hepatica nobilis and Aegopodium podagraria are the most constant and abundant among them. Viola mirabilis, Lathyrus vernus, Carex digitata, Paris quadrifolia, Pulmonaria obscura are present with less constancy. Continuous moss cover is absent in spruce forests with Corylus avellana and its projective cover degree reaches 50–85 % in those with Lonicera xylosteum. The moss layer is dominated by Rhytidiadelphus triquetrus. Plant communities of Piceeta oxalidosa group of associations (rel. 16–20) occur on plain areas and low ridges with different composition of soil-forming rocks: both carbonate and non-carbonate morainic loam and cobble-less loam. Tree layer is formed by spruce only. The dense shrub layer, mainly of Lonicera xylosteum, is typical for this type. Oxalis acetosella is dominant of the herb-dwarf shrub layer. There are also nemoral (Hepatica nobilis, Aegopodium podagraria, Paris quadrifolia, Anemonoides nemorosa) and boreal species (Luzula pilosa, Gymnocarpium dryopteris, Maianthemum bifolium, Rubus saxatilis) in communities. The proportion of these two groups of species is different, but can be roughly equal. Vaccinium myrtillus was met only in these communities. Well-developed moss cover is typical for the majority of the communities. Plant communities of Piceeta aegopodiosa group of associations (rel. 21–24) were described on plains and gently sloping hillsides with different soil-forming rocks, including the once cultivated and used for agricultural purposes. Aspen is always present in the tree layer, birch and grey alder are in the majority of the communities; the latter is character species only for this type of spruce forests. Aegopodium podagraria is a dominant in herb layer. Moss layer is well formed; its projective cover degree reaches 70–85 %. Spruce nemoral herb forests prevail in the studied area. Their specific features, in comparison to similar subtaiga communities, are smaller abundance or even absence of broad-leaved trees in the tree layer, the absence of some characteristic species in the herb and moss layers (e. g., Asarum europaeum, Carex pilosa, Rhodobryum roseum), and well-formed moss cover dominated by boreal species of mosses (Rhytidiadelphus triquetrus and Hylocomium splendens). Author’s and available literature data show that there are no broad-leaved-coniferous forests in the current vegetation cover of the Izhora upland and the richest mixed (with broad-leaved tree species) spruce forests are extremely rare. In addition to prevailing spruce nemoral herb forests there are also spruce wood sorrel forests, more character for southern taiga. Nemoral herb spruce forests is a kind of transition between south-taiga and subtaiga forests: a small share of broad-leaved trees and developed moss cover brings them closer to the first types, while abundance of nemoral species in the herb layer and developed shrub layer to the second ones. According to some climatic parameters, the Izhora upland differs from the surrounding landscapes: here is more severe winter, the amount of active temperatures is 100–200° lower and, accordingly, the vegetation period is shortened. Despite this, more southern types of plant communities are common here, which is most likely due to the richness of the soil, but not to climate. Thus, the zonal vegetation status of this area can be defined as a calciphytic variant of the southern taiga forests rather than as an extra-zonal subtaiga forest, as presented in the existing schemes of geobotanical subdivision (Rastitelnost..., 1980; Geobotanicheskoye..., 1989).