Nature Conservation Research: Заповедная наука (Apr 2021)
Vegetation dynamics in the Kostomuksha State Nature Reserve (Russia) and surroundings against changes in the natural environment during the Holocene
Abstract
In the current situation of climate instability, research on the natural trends of vegetation development against global climate change is of high relevance, especially in areas once covered by the latest glaciation (Weichselian), and not altered substantially by human activities afterwards. The Kostomuksha State Nature Reserve, which is part of the Russian-Finnish Nature Reserve and the Green Belt of Fennoscandia, is a model area of well-preserved natural north-taiga pine (Pinus sylvestris) forest. To re-construct the history of its formation in post-glacial time, stratigraphic, radiocarbon, and palaeobotanical (determination of pollen, spores, plant macrofossils, Pediastrum algae) studies of lacustrine-paludal deposits were carried out in two sections of the mire Mezhselgovoe. The general trends and local features of vegetation development on dry land, and of the paludification process in the study area were investigated by comparing data derived by the author anew or previously from the cores taken from the Härkösuo and Isosuo mires, situated in the Finnish part of the Russian-Finnish Nature Reserve with published palaeogeographic material from nearby areas. The vegetation reconstructions cover the period from ice retreat from the study area in Preboreal time until present. Concurrent changes in the natural environment (including palaeoclimate) are specified, and the geomorphology and human impact in the territory are taken into account. The reconstructions are based on 25 radiocarbon datings. Deglaciation of the Russian-Finnish Nature Reserve area started around 9700 radiocarbon-dated years before present (14С BP) / 11 160 calibrated years before present (cal BP) and ended not later than 9500 ± 70 14C BP (10 830 ± 160 cal BP). As ice was retreating, the area got occupied by Artemisia-Chenopodium-forbs-dominated periglacial steppe, tundra (dwarf birch and dwarf shrub-true moss, willow and sedge-grass types), and birch-dominated forest-tundra communities. The first category occupied a relatively minor proportion of the study area. By the end of the Boreal period, spaces occupied by tundra vegetation also shrank. The most widespread by then were open birch woodlands. They were replaced by sparse birch and pine-birch forests, which remained widely represented through most of the Boreal period near the mires of Mezhselgovoe, Härkösuo (9110 ± 80 14C BP / 10 300 ± 100 cal BP), and Isosuo (8370 ± 60 14C BP / 9370 ± 80 cal BP). A notable increase in the proportion of Pinus in the forests was observed from 8750 ± 50 14C BP (9750 ± 120 cal BP) to a maximum in Late Boreal time, as evidenced by the dates 8240 ± 60 14C BP (9220 ± 100 cal BP) and 8090 ± 200 14C BP (9000 ± 260 cal BP). Pine and birch-pine forests prevailed throughout the Atlantic period. In the warmer and more humid climate of that time, forests acquired a middle-taiga appearance, and the environment was favourable for the expansion of Alnus and, possibly, Ulmus. Picea started spreading in the study area not later than 5700 14C BP (6500 cal BP), and since 5440 ± 40 14C BP (6240 ± 50 cal BP) its role in the plant cover has been substantial. The expansion of spruce forests reached its maximum in Subboreal time, especially after 3290 ± 70 14C BP (3520 ± 80 cal BP), when they co-dominated together with pine forests. Global cooling early in Subatlantic time caused the plant cover of the study area and its southerner neighbours to lose nemoral species, and sparsed out pine and spruce forests so that they acquired north-taiga features. The proportion of Picea in the forests decreased, especially in the last 800 (700) years. At present, pine forests dominate there. Alongside the dominant vegetation, birch and alder-birch post-fire communities have occurred there since Boreal time, while alder communities since Atlantic time. The forests affected by wild (lightning-ignited) fires have almost fully recovered by now. They mostly remained pristine in the absence of significant anthropogenic impacts. Data indicate that mire formation in some wet depressions started in Preboreal time (9500 ± 70 14C BP / 10 830 ± 160 cal BP). More thorough warming of shallowed palaeolakes in Boreal and Atlantic times promoted sapropel formation, overgrowing with wetland vegetation, and a shift to the telmatic development pathway. The detailed reconstruction of the formation history of contemporary north-taiga pine forests in the context of palaeoclimatic changes in the Holocene provided in this article bears information on how they may change in the future under different climate scenarios. Of particular interest are data on the time of spruce migration to the Russian-Finnish Nature Reserve area, which update and add detail to existing cartographic models of spruce distribution in northwest Russia and the eastern Baltic Shield. The resultant set of palynological and radiocarbon (20 dates) data can be used for correlations with palaeogeographical material from other regions in the efforts to reconstruct vegetation dynamics over time and space.
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