A multi-proxy reconstruction of peatland development and regional vegetation changes in subarctic NE Fennoscandia (the Republic of Karelia,Russia) during the Holocene

Abstract

A better understanding of past long-term environmental changes in the subarctic region is crucial for mitigation of the possible negative effects of climate warming in this vulnerable region. This study provides a new multi-proxy reconstruction of regional vegetation changes and peatland development for north-eastern Fennoscandia (Russia) during most of the Holocene. To that purpose, we performed plant macrofossil, pollen, testate amoebae, peat humification, loss on ignition and radiocarbon analyses of the peat deposits from a mire around Vodoprovodnoe Lake (the Kindo Peninsula, the Republic of Karelia). Our data indicate that the peat deposits started accumulating before 9147 ± 182 cal. yr. BP. The vegetation cover in the area was mainly typical for the northern taiga zone, except for the period ~7800–5600 cal. yr. BP, when it generally resembled the middle taiga zone. The vegetation cover and peatland were greatly affected by reoccurring fires, which can be partly related to human activity. These events were associated with an increased proportion of birch in the vegetation cover (as a pioneer species) and/or water level decreases. By 600 cal. yr. BP, the peatland and the surrounding vegetation reached its current state and only minor changes had been recorded since that time. Overall, our results suggest a considerable and unexpected role of fires in the postglacial dynamics of subarctic peatlands.

Keywords

fire North-Eastern Fennoscandia plant macrofossils pollen testate amoebae White Sea Biological Station

Introduction

Arctic and subarctic ecosystems undergo extensive transformations because of considerable climate change, which leads to pronounced changes in land cover, vegetation composition and degradation of permafrost (Gałka et al., 2017a, 2019; Hinzman et al., 2005; van Bogaert et al., 2011). These dynamics in turn cause climate feedbacks, particularly by affecting carbon cycling and release of greenhouse gases (Pearson et al., 2013; Ratcliffe et al., 2017; Schuur et al., 2015). However, the scale of this transformation is not uniform across northern landscapes (Sim et al., 2019).

The environmental and climatic history of Fennoscandia has attracted much attention and is relatively well studied (Nazarova et al., 2020; Stroeven et al., 2016). For Western Fennoscandia, numerous studies have provided quantitative palaeoclimate reconstructions during the Holocene (Meyer-Jacob et al., 2017; Plikk et al., 2019; Poska et al., 2008; Väliranta et al., 2007). However, there is still little information available from Eastern Fennoscandia based upon palaeoecological analyses of lake sediments (Ilyashuk et al., 2013; Solovieva and Jones, 2002; Subetto et al., 2017) and peat deposits (Elina and Yurkovskaya, 1988; Filimonova and Klimanov, 2005; Khotinsky and Klimanov, 1997).

Most of the peat-based palaeoecological reconstructions published for Eastern Fennoscandia are based upon pollen and plant macrofossil analysis (Elina et al., 2010). However, additional proxies can provide important information related to ecosystem development and climate change (Chambers et al., 2012). For instance, testate amoeba assemblages and peat humification are valuable indicators of peatland surface wetness, which reflect the balance between precipitation and evapo-transpiration, and, thus, relate to climate (Mitchell et al., 2008; Payne et al., 2016). Testate amoebae are unicellular eukaryotes, which are very sensitive to hydrological changes, and numerous quantitative transfer functions have been developed to relate water table depth below the peatland surface and the structure of testate amoeba assemblages (Amesbury et al., 2016; Tsyganov et al., 2017). The peat humification method aims to extract humic acids, which are produced during the decomposition process. It is assumed that more highly decomposed peat will produce a darker coloured alkali extract resulting in reduced light transmission (Blackford and Chambers, 1993). Such multi-proxy reconstructions, which include pollen, plant macrofossils, testate amoebae, peat humification analyses and macro-charcoal, have been successfully applied to reconstruct environmental changes, peatland development and vegetation dynamics in other parts of Russia (Novenko et al., 2015, 2016, 2018; Payne et al., 2016; Tsyganov et al., 2019).

Most of the paleoenvironmental studies conducted in North-East Fennoscandia are based upon diatom and pollen assemblages preserved in lake and marine sediment archive deposits. These have been used to reconstruct the geological history and formation of the relief (Kolka et al., 2015; Rainio et al., 1995; Romanenko and Shilova, 2012). Studies based upon pollen and plant macrofossil assemblages from peat deposits are still rare in this region. The Kindo Peninsula (Karelia, Eastern Fennoscandia) is a sparsely populated area and it is assumed that throughout its history anthropogenic impact has been low (Kosheleva and Subetto, 2011), which creates the opportunity to focus upon the natural drivers of long-term peatland and vegetation dynamics. The postglacial development of the southern shore relief of Kandalaksha Bay is characterised by a high rate of shore uplift in the first half of the Holocene, which subsequently decreased at a variable rate (Romanenko and Shilova, 2012). The Kindo Peninsula territory was completely under water after the degradation of the Scandinavian Ice Sheet during the Late Weichselian (Valdai) (~10000–9500 cal. yr. BP) (Demidov et al., 2006; Elina et al., 2010). Then, during the process of glacial isostatic uplift, land areas gradually protruded above sea level, passed the island stage and joined the mainland only at ~1000 cal. yr. BP (Kolka et al., 2015; Korsakova et al., 2016). This ongoing process can be an important non-typical factor in the formation and dynamics of peatland ecosystems.

The aim of the research presented here is to explore the rate and nature of peatland and surrounding vegetation dynamics in the Kindo Peninsula and to reconstruct the environmental changes in the Holocene during the last ~9000 years based upon a multi-proxy approach which included pollen, plant macrofossils, testate amoebae, peat humification, and macro-charcoal analyses.

Study region

The Kindo Peninsula is a projection of the White Sea Karelian coast with an area of ~9 km², located between Velikaya Salma Strait and Kislaya Bay (Figure 1). The climate of the area is humid subarctic, with long winters and short cool summers. The average annual temperature is +0.5°C, the coldest month of the year is February (an average temperature −11°C), the warmest month is July (+13.2°C). The growing season lasts 127 days. Snow cover remains for more than 190 days per year with an average snow depth of 40–60 cm. The annual precipitation is 390–420 mm, while the average evaporation is 150 mm year^-1 (Tchesunov et al., 2008).

Figure 1.

Location of the study site (marked by a star).

The peninsula is formed by an outcrop of the Baltic crystalline shield, which is represented here by the gneiss-amphibolite top of the Rugozerskaya Mountain (103 m above sea level). This area is occupied by round-top ridges up to 100 m wide divided by basins with lakes and mires. Tectonic faults in the relief are in the form of rocky ledges in the slopes with a height of 15–40 m, terraced steps and rifts with a depth of 2–3 m. The depth of unconsolidated sediments reaches several metres at the bottom and the slopes of hollows, whereas they are almost absent on most of the uplands of the peninsula (Olynina and Romanenko, 2007; Tchesunov et al., 2008).

High heterogeneity of the relief results in a complex spatial structure of the vegetation cover. Acidic crystalline bedrocks are poor in nutrient content, which leads to low productivity of terrestrial ecosystems and limited distribution of spruce forests typical for the European taiga zone. Ridges are mostly occupied by lichens only with occasional pine trees, while upper slopes are often covered by sparse pine stands with Ericoid dwarf-shrubs. The rest of the territory is occupied by forests with structure typical for the northern taiga, except for the dominance of Pinus sylvestris L. instead of Picea sp., caused by poor acidic soils. Along the soil moisture gradient, the dominant plants in the shrub and moss layers of pine forests change in the following sequence (from dry to wet): ericoid dwarf-shrubs – dwarf-shrubs with green mosses – dwarf shrubs with Sphagnum mosses. Spruce-dominated forests are generally composed of Picea obovata Ledeb. and P. × fennica (Regel) Kom. and restricted to moist biotopes with rich mineral nutrition (around lake shores and mires, along streams and the sea coast) (Avenarius and Vital, 2006; Vital, 1999). Birch-dominated (Betula spp.) forests occupy felled and burned areas; they typically have an admixture of aspen (Populus tremula L.), pine, spruce and dwarf-shrub-green moss or sedge-Equisetum-dominated lower layers.

About 6% of the Kindo Peninsula is occupied by mires, which are located in lakeside depressions, flat terraces and the lower parts of the ridge slopes. These are generally raised bogs dominated by peat mosses, dwarf-shrubs and cotton-grass (Eriophorum spp.); aapa mires dominated by Sphagnum mosses and dwarf-shrubs on hummock strings and by sedges in minerotrophic lawns; fens dominated by sedges. Marshes with varying degree of salinity are located along the coast (Vital, 1999).

Materials and methods

Study site

Peat deposits for palaeoecological reconstruction were sampled from an ombrotrophic raised bog around Vodoprovodnoe Lake (66.5456°N; 33.1044°E). The mire borders the lake in the northern part forming a floating vegetation mat and extends to the south covering the neighbouring depression. Most of the bog has a developed microrelief, composed of hummocks and hollows. Hummocks are occupied by sparse trees (height 2–8 m) of Pinus sylvestris; dwarf-shrubs (Calluna vulgaris L. Hull, Betula nana L., Empetrum nigrum L. ssp. hermaphroditum (Lange ex Hagerup) Böcher, Ledum palustre L., Andromeda polifolia L., Rubus chamaemorus L.), graminoids Eriophorum vaginatum L. and mosses (Sphagnum capillifolium (Ehrh.) Hedw., S. balticum (Russ.) Russ. ex Hedw., etc.) with an admixture of lichens (Cladonia spp. and Cetraria spp.). Hollows and biotopes with a greater nutrient content (along bog and lake margins) are characterised by the presence of Eriophorum angustifolium L., Carex spp., Trichophorum caespitosum L. and Sphagnum spp.

Peat extraction and sampling

Peat cores were extracted in the central part of the mire (66.543472°N; 33.107611°E) with a Russian corer (50 cm length, 5 cm in diameter) in July 2017. The total depth of the peat deposits was 3.5 m. The underlying layers were not sampled and described. The cores were wrapped in plastic film and aluminium foil, packed into cases and stored at 4^оС. In the laboratory, the peat cores were subsampled for multi-proxy analysis including radiocarbon dating, plant macrofossil analysis, loss on ignition, bulk density, humification degree, pollen and testate amoeba analysis. Samples for the analyses were taken in 2 cm contiguous slices through the upper part (0–170 cm) of the core and 5 cm contiguous slices through the lower part (170–350 cm).

Radiocarbon dating

In order to determine the age of the deposits seven samples were dated using AMS ¹⁴C (Table 1). The analyses were undertaken at the Laboratory of Radiocarbon Dating and Electron Microscopy, Institute of Geography of the Russian Academy of Science (IG RAS). The radiocarbon dates were calibrated using the IntCal13 calibration curve (Reimer et al., 2013). The age-depth model was developed using the Bayesian-based package ‘rbacon’ (Blaauw and Christen, 2018) in the R language environment (R Core Team, 2017).

Table 1.

Radiocarbon dating of the Vodoprovodnoe peat profile.

№	Laboratory code	Depth, cm	Material	Radiocarbon date 14C, BP	Calibrated age range BP, 95% confidence interval (probability)
1	IGAN_AMS5790	44–45	Sphagnum spp. stems	1450 ± 25	1302–1382 (100%)
2	IGAN_AMS5791	84–85	Sphagnum spp. stems	2310 ± 25	2211–2220 (1.9%)2309–2356 (98.1%)
3	IGAN_AMS5792	124–125	Sphagnum spp. stems	3000 ± 25	3077–3095 (6.2%)3105–3249 (90.3%)3306–3322 (3.6%)
4	IGAN_AMS5793	168–170	Sphagnum spp. stems	3595 ± 25	3840–3934 (80.9%)3937–3971 (19.1%)
5	IGAN_AMS5794	260–265	Sphagnum spp. stems	6330 ± 30	7173–7225 (24.5%)7230–7317 (75.5%)
6	IGAN_AMS5795	300–305	Sphagnum spp. stems	7690 ± 30	8417–8541 (100%)
7	IGAN_AMS5796	345–350	Sphagnum spp. stems	8110 ± 30	8997–9094 (92.6%)9098–9122 (7.4 %)

Plant macrofossils

Subsamples (15 cm³) for plant macrofossil analysis were washed and sieved under a warm-water spray using a 0.20-mm mesh sieve. Initially, the entire sample was examined with a stereomicroscope to obtain volume percentages of individual subfossils of vascular plants and mosses. The subfossil carpological remains and vegetative fragments (leaves, rootlets, epidermis) were identified using identification keys (Kats et al., 1977; Mauquoy and van Geel, 2007; Smith, 2004) and were expressed as absolute numbers. The percentage volumes of the different vegetative remains and the Sphagnum sections were estimated to the nearest 5%. The relative proportions of taxonomic groups of Sphagnum, which are of key importance for interpretations, were estimated on the basis of the branch leaves, which were investigated under the microscope on three 22 × 22 mm² cover glasses. The identification of Sphagnum taxa to species level was performed separately on the basis of the stem leaves using specialist keys (Hölzer, 2010; Laine et al., 2018). At several depths, Sphagnum fuscum (Schimp.) H. Klinggr. and Sphagnum rubellum Wilson, S. fuscum/S. russowii Warnst. as well as S. medium Limpr./S. divinum Flatberg and Hassel were reported together due to the difficulty in separating these two species as fossils, due to the similar morphology of their branch leaves and the absence of stem leaves (Gałka et al., 2017b; Hölzer, 2010; Marcisz et al., 2020).

Loss on ignition (LOI), peat bulk density and humification

These peat properties were determined according to the protocols presented by Chambers et al. (2010/2011). Bulk density and LOI were determined from the same sample. First, the sample volume was measured by displacement in a measuring cylinder. Then samples were dried at 100°C overnight and weighed to determine the peat dry weight. After that, the samples were placed in the muffle furnace at 550°C for 4 h and weighed. Another set of samples was used to determine peat humification. The samples were dried at 50°C, weighed (0.2 g) and ground in an agate mortar. Then they were placed into 100 ml of 8% NaOH and heated at 95°C for 1 h. The extract was then diluted to 200 ml with distilled water and filtered (Whatman No. 1). The light absorbance of the filtrate (50 ml diluted to 100 ml with distilled water) was measured with a spectrophotometer (KFK-3-01-‘ZOMZ’, Russia) at a wavelength of 540 nm.

Testate amoeba analysis

Samples were stored in a refrigerator before analysis (Mazei et al., 2015) and prepared in the laboratory following the method based on suspension in water, physical agitation and subsequent settling (Mazei and Chernyshov, 2011).The samples were soaked in distilled water for 24 h, agitated on a flask shaker for 20 min, sieved and washed through a 500-μm mesh to remove coarse material and then left to settle for 24 h. The supernatant was then decanted and the samples were mixed with neutralised formaldehyde and placed into glass vials for storage. One millilitre of the concentrated sample was placed into a Petri dish (5 cm diameter), diluted with deionised water if necessary and inspected at ×200 magnification. All encountered testate amoebae were identified and tallied until a minimum of 150 tests were counted in each sample. Species identification was accomplished using Mazei and Tsyganov (2006) and Tsyganov et al. (2016) as reference sources.

Pollen analysis

The volume of the pollen analysis samples was ~1 cm³ with a sampling interval of 4–5 cm. Before the chemical treatment Lycopodium spores (batch No 10381) were added to each sample to calculate pollen and spore concentrations (Stockmarr, 1971). The chemical treatment of the samples was undertaken using standard procedures (Erdtman, 1960; Moore et al., 1991) and included treatment of 10% KOH in a water bath for 5 min followed by repeated washing with distilled water, dehydration with glacial acetic acid, then acetolysis followed by washing. Counting and identification of spores and pollen was carried out in glycerine temporary slides with a light microscope at ×400 magnification according to the standard method (Moore et al., 1991). Pollen counts comprise up to 500 pollen grains of woody plants, but for some samples with low pollen concentration the pollen sum was limited to 250–300 pollen grains. Pollen and spore atlases (Moore et al., 1991; Reille, 1998, 1999), the collection of permanent pollen preparations at the MSU Department of higher plants, and electronic database (‘Information system of identification of plant objects on the basis of carpological, palynological and anatomical data’) were used as references for pollen and spore identification. The results of pollen analysis were presented as pollen percentages and pollen accumulation rates (PAR).

Data analysis

Data were analysed and plotted using the R language environment (R Core Team, 2017) with the ‘rioja’ (Juggins, 2017) and ‘analogue’ (Simpson and Oksanen, 2016) packages. Results of pollen analyses were calculated and visualised using Tilia 2.1.1 and TGVIEV 2.02 (Grimm, 1990). Statistically significant stratigraphic zones were defined by constrained incremental sum of squares cluster analysis (Grimm, 1987) and a broken-stick model (Bennett, 1996). Peatland surface wetness was reconstructed as water table depth (WTD, cm) using a testate amoeba-based transfer function (reversed weighted averaging model) which was developed for the forest zone of the East-European Plain (Tsyganov et al., 2017).

Results

Radiocarbon dating and peat properties

The results of radiocarbon dating are presented in Table 1 and Figure 2. The age of the deepest layers of the sampled deposits at a depth of 345 cm is 9147 cal. yr. BP (8987–9350 cal. yr. BP). In the lower part of the peat deposits (to the depth 180 cm, 4315 cal. yr. BP), the LOI varied from 97.5 to 99.5% (Figure 3). After that it sharply increased and remained in the range of 99–99.5%. The peat bulk density and humification show similar trends (basically because more decomposed peat is denser) (Figure 3). At depths between 345 and 280 cm (9147–7840 cal. yr. BP) peat bulk density and humification were relatively low which points to the formation of peat deposits during a cold and/or wet period. After that, they increased between 280 and 215 cm depth (7840–5590 cal. yr. BP), indicating a warmer and/or drier period. At the depths of 215–60 cm depth (5590 cal. yr. BP – till present), peat bulk peat density and humification were relatively low, except for short-term peaks between 150–146, 116–112 and at 77, which generally coincides with the presence of macro-charcoal (see plant macrofossil analysis). In the top 60 cm, peat humification generally decreased with two peaks at 41 and 20 cm.

Figure 2.

Age-depth model of the Vodoprovodnoe peat profile.

Figure 3.

Loss on ignition, peat bulk density and peat humification as absorbance of alkali extract for the Vodoprovodnoe peat profile.

Plant macrofossils

Seven zones in the local vegetation development were visually delimited (Figure 4).

Figure 4.

Plant macrofossil diagram of the Vodoprovodnoe peat profile.

PM1 (345–260 cm; 9147–7230 cal. yr. BP) dominated by Sphagnum fuscum, Polytrichum sp., P. strictum Brid. and Hypnales. This combination of dominants is typical for oligotrophic biotopes with rather low moisture, such as hummocks.

PM2 (260–215 cm; 7230–5590 cal. yr. BP) dominated by herbs rootlets, Sphagnum sect. Acutifolia, S. medium/S. divinum, macro-charcoal (260, 240 and 215 cm depth) and fungal remains (sclerotia). This plant community and especially the presence of fungal remains indicate a decrease of water levels which supports the reoccurrence of fire events.

PM3 (215–200 cm; 5590–5038 cal. yr. BP) dominated by S. fuscum, Polytrichum spp. and S. medium/S. divinum, which indicate a return to oligotrophic conditions.

PM4 (200–80 cm; 5038–2230 cal. yr. BP) dominated by S. fuscum/S. rubellum, herb rootlets and Ericaceae rootlets, which indicates oligotrophic conditions. Three macro-charcoal layers were found at the depths of 149, 140 and 115 cm.

PM5 (80–50 cm; 2230–1496 cal. yr. BP) dominated by S. fuscum/S. russowii, herbs rootlets, Ericaceae rootlets. Again, the peatland is more oligotrophic and contains communities, which indicate relatively deep local water tables. Macro-charcoal was identified at the beginning of the zone (77 and 65 cm).

PM6 (50–30 cm; 1496–848 cal. yr. BP) dominated by S. russowii, Ericaceae rootlets and Dicranum undulatum Schrad. ex Brid. This zone is similar to zone 5 with the change of the dominant Sphagnum moss.

PM7 (30–0 cm; 848 cal. yr. BP – till present) S. capillifolium, Ericaceae, Andromeda sp. etc. This Sphagnum species is characteristic for rather dry ombrotrophic conditions. Macrofossil charcoal is present between 20 and 19 cm depths, indicating the occurrence of local fires.

Testate amoebae and WTD reconstruction

Based upon the species composition of the testate amoeba stratigraphy (Figure 5) six zones could be identified.

Figure 5.

Testate amoebae diagram of the Vodoprovodnoe peat profile (only taxa observed in three or more samples with the relative abundance per sample greater than 3% are shown, taxa are ordered by weighted average of the depth/time axis) with the zonation based upon the results of the constrained cluster analysis and reconstructed values of water table depth (WTD, cm).

ТА1 (350–305 cm; 9147–8600 cal. yr. BP). The basal layers of the peat deposit were dominated by xerophilic species (Trigonopyxis arcula Penard, 1912 (up to 40%), Trigonopyxis minuta Schönborn et Peschke, 1988 (up to 60%), Assulina muscorum Greef, 1888 (up to 30%) and Assulina seminulum Leidy, 1879 (up to 10%)).The mean reconstructed WTD is 32.7 ± 4.9 cm (SD).

ТА2 (305–250 cm; 8600–6846 cal. yr. BP). Mostly hydrophilic species Archerella jollyi van Oye, 1956 (its abundance alternates between xerophilic species), however between 270 and 240 cm depth, the abundance of testate amoeba was very low (insufficient for a water table depth reconstruction).

ТА3 (250–110 cm; 6846–2880 cal. yr. BP). Xerophilic T. arcula (up to 80%) dominated in the zone and was observed together with A. muscorum (up to 30%) and Arcella catinus Penard, 1890 (up to 15%). Arcella sp. (up to 40%) with unknown preferences for WTD was also abundant. The mean reconstructed WTD value is 35.8 ± 4.3 cm (SD).

ТА4 (110–80 cm; 2880–2230 cal. yr. BP). The abundance of hydrophilic A. jollyi (up to 80%) peaked at 100 cm and alternates with xerophilic T. arcula (up to 70%) and Bullinularia indica Deflandre, 1953 (up to 20%) which peaked at a depth of 90 cm. This also corresponds to the peat humification. The mean reconstructed WTD is 38.2 ± 4.9 cm (SD).

ТА5 (80–20 cm; 2230–520 cal. yr. BP). A. jollyi (up to 70%) dominated with the presence of some other hydrophilic taxa (Hyalosphenia papilio Leidy, 1879 (up to 30%) and Hyalosphenia elegans Leidy, 1879 (up to 10%)). The xerophilic taxa from the previous zones were rarely observed. The mean reconstructed WTD is 31.0 ± 4.7 cm (SD).

ТА6 (20–0 cm; 520 cal. yr. BP – till present). A. jollyi (up to 60%), dominated, whilst xerophilic species are almost absent. The subdominant species were H. papilio (up to 30%), H. elegans (up to 10%), Physochila griseola Penard, 1911 (up to 10%) and Nebela tincta Awerintzew, 1906 (up to 10%). The mean reconstructed WTD became more variable and decreased to 19.6 ± 14.6 cm (SD).

Pollen analysis

The percentage pollen diagram was divided into four zones based upon the cluster analysis results (Figures 6 and 7).

Figure 6.

Pollen diagram of the Vodoprovodnoe peat profile (additional curves represent ×10 exaggeration of the base curves).

Figure 7.

Pollen accumulation rate (PAR: number of pollen grains cm⁻² yr⁻¹) of the Vodoprovodnoe peat profile.

PZ1 (350–280 cm; 9147–7800 cal. yr. BP). The bottom part of the profile is characterized by high percentages of Pinus (up to 52%) and Betula (up to 32%). Picea and Alnus values were low (~1 and ~3% respectively). The dwarf-shrub and herb taxa were represented by Ericaceae (7%), Rubus chamaemorus L. (2%) pollen, some pollen grains of Rosaceae, Artemisia, Polygonaceae and up to 2% of Poaceae pollen. The spores were mainly represented by Sphagnum (5–108%), Polypodiaceae (<8%) and Lycopodium annotinum L. (<2%). The total PAR was relatively high (the average is 4400 grains cm^-2 yr^-1with a peak of up to 10000 grains cm^-2 yr^-1 between the 315 and 300 cm depth interval), mostly composed of tree taxa pollen and Sphagnum spores.

PZ2 (260–215 cm; 7800–5600 cal. yr. BP). Consists of two subzones:

PZ2a (280–260 cm; 7800–7200 cal. yr. BP). The percentage of Pinus decreased to 32%, while the Betula and Alnus pollen values increased up to 44% and 17% respectively. Single pollen grains of Tilia, Ulmus and Corylus occurred regularly. Ericaceae pollen values increased to 14%. The percentage of Polypodiaceae and L. annotinum spores strongly decreased (<1%), and spores of Sphagnum disappear completely. The total PAR rose to maximal values (an average value of 14000 grains cm^-2 yr^-1), whereas the accumulation rate of spores decreased.

PZ2b (260–215 cm; 7200–5600 cal. yr. BP). Pinus pollen value decreased (5–18%), while the proportion of Betula increased (58%); it dominated throughout the subzone. Picea pollen appeared in reliable amounts (>1%) and rose gradually to 4%. Pollen values of broad-leaved trees and Corylus reached their maximum, however, their percentages were still minor (>1%). The proportion of dwarf-shrub pollen decreased (>6%). The pollen of herbs (Umbelliferae, Asteroideae, Poaceae, Rosaceae) presented regularly. Spores were almost absent except for L. annotinum (3%). The PAR of all taxa declined, especially Pinus pollen.

PZ3 (215–140 cm; 5600–3500 cal. yr. BP). The Pinus (48%) and Picea (6%) pollen values increased simultaneously, while percentages of Betula pollen decreased (24%). The pollen values of broad-leaved trees and Corylus reduced as well. The dwarf-shrub pollen percentages increased, while the occurrence of pollen of herbaceous plants decreased. The spore content significantly rose due to Sphagnum (up to 45%) and L. annotinum (up to 15%). The total accumulation rate of pollen strongly declined (an average 1600 grains cm^-2 yr^-1), especially for Betula and Alnus, conversely the accumulation rate of spores increased (an average of 1000 spores cm^-2 yr^-1).

PZ 4 (140–0 cm; 3500 cal. yr. BP – till present). Pinus pollen was a constant dominant (62%), while the Betula pollen rate decreased (13%). The Picea pollen abundance was around 9%. Pollen values of Alnus slowly decreased to 3%. The pollen values of dwarf-shrubs varied from 5% to 25%. Pollen of herbs appears in negligible values. Sphagnum spores dominated (60%), while spores of other taxa were sparse. The total PAR value was close to the PAR in the previous zone, while the Lycopodium spore accumulation rate dropped.

Discussion

Early stages of mire formation (9150–7800 cal. yr. BP)

According to the reconstructions of Khotinsky (1977) and Elina (1981) and the spore-pollen diagram of our study, the early phases of the mire formation around ~9150 cal. yr. BP correspond to the Boreal stage of the Holocene (Figure 8). Palaeoclimatic reconstructions for Karelia (Klimanov, 1996; Yurkovskaya and Elina, 2009) and the Kola Peninsula (Ilyashuk et al., 2013), show noticeable warming after the colder and drier Preboreal stage. According to the data, published for the Kola Peninsula, the Boreal stage was characterized by a relatively warm climate with mid-July temperatures similar to the present-day values, or even 1°C higher (Ilyashuk et al., 2013). From the model of postglacial development by Romanenko and Shilova (2012), it can be assumed that the Kindo Peninsula territory was an island at that time. According to their data, the age of sapropel deposits of the Vodopovodnoe Lake at a depth of 485 cm is 9512–9362 cal. yr. BP. We report an age of ~9147 cal. yr. BP for the peat deposits at a depth of 345 cm, but located ~300 m away from the sampling position used by Romanenko and Shilova (2012). This might indicate that the mire could develop from the lake, which gradually became isolated from the seashore (Romanenko and Shilova, 2012). A combination of Sphagnum fuscum and Polytrichum sp. macrofossils in the lowest layer of the studied peat deposits (Figure 4) indicates the existence of habitats with relatively low moisture, either the hummocks or the boundary between forest and peatland (Elina et al., 2000). The site might be located in the immediate vicinity of the forest, which is also indicated by relatively significant values of herb pollen and non-Sphagnum spores. Testate amoebae in this period formed Sphagnum-dwelling xerophilous assemblages, which are typical for hummocks (Figure 8, Zone A). Therefore, the lake could have played an important role in development of the mire as a result of terrestrialisation, with simultaneous lateral expansion of the mire and paludification of the adjacent area. The forest vegetation of the island at that time was mainly represented by pine-dominated forest with an admixture of birch and a ground cover of dwarf-shrubs, ferns and herbs. The increase of the total PAR between the 325 and 305 cm depth interval corresponded to the time interval ~8868–8594 cal. yr. BP (Figure 7). This may indicate an active spread of tree stands into the area of dried internal lakes and mires due to rather high temperatures and low precipitation (Yurkovskaya and Elina, 2009), as well as into new terrains raised above sea-levels, due to isostatic uplifting of the peninsula. The latter is typical especially for the abundance of birch, which now occurs along the narrow coastal lines of the peninsula and mire and lake margins (Vital, 1999). However high abrupt peaks of PAR (310 cm, ~8661 cal. yr. BP) may also be a consequence of compaction of peat layers caused by both climatic factors and the local effects of burning which is indicated by macro-charcoal at the same depth (Figure 4). The presence of xerophilic testate amoebae species indicates low mire surface wetness during this period (T. minuta, T. arcula, and A. muscorum). These species are also typical for Sphagnum-dwelling representatives which do not inhabit eutrophic peatlands (Tolonen et al., 1992). Above the charcoal layer at a depth of 310 cm, changes in the macrofossil stratigraphy were minor, Polytrichum occurred in relatively low abundances and Sphagnum fuscum dominated (Figure 4). However, a sharp change to the predominantly hydrophilic species of testate amoebae (A. jollyi) begins in zone TA2 (Figure 5), which is likely due to the high sensitivity of these organisms to hydrological changes. At the same time, the climatic reconstruction published for the area demonstrates the drastic decrease of temperature and increase of precipitation rate (Yurkovskaya and Elina, 2009). This climate change may have resulted in a decreased peat decomposition rate (Figure 3) and total PAR (Figure 7) for the 305–280 cm depth interval.

Figure 8.

Overview of the main developmental stages (A–D) reconstructed based on all proxies from the Vodoprovodnoe peat profile.

A dry eutrophic phase with frequent fires (7800–5600 cal. yr. BP)

The peat-deposits at 280 cm depth (~7800 cal. yr. BP) correspond to the beginning of the Atlantic stage (Figure 8, Zone B), when a sharp increase in average annual temperatures and precipitation occurred (Yurkovskaya and Elina, 2009). At that time, uplift of the Kindo Peninsula territory became more intense (Kolka et al., 2015; Romanenko and Shilova, 2012). During the period of 7800–7200 cal. yr. BP the area was covered by pine and pine-birch forests, which resembled the middle taiga due to denser tree-stands, a better developed shrub layer and herbal ground cover (Figure 6). Spruce, several broad-leaved species (Tilia, Ulmus) and Corylus pollen were detected. The latter ones likely had drifted from their southern territories, due to active invasion of these species into the vegetation at the southern boundaries of the middle taiga subzone (Elina et al., 2000). High PARs (Figure 7) of pine, birch and alder may indicate further spread of forests into the uplifted territory of the peninsula as well as the active overgrowth of the drying wetlands and lakes.

The peat macrofossils subsequently record a similar series of changes. The dominance of herbaceous plant rootlets and Sphagnum sect. Acutifolia is typical for boggy habitats with low moisture or for paludificated forests (Figure 3). Xerophilic testate amoebae became more abundant, but the number of shells found was extremely low, which might be due to the increased decomposition degree and density of the peat layers (Figure 4). This may have been caused by a forest line approaching the peatland and reduced local water table depths, which is confirmed by the presence of fungal remains (Gałka et al., 2020).

During the period from ~7200 to ~5600 cal. yr. BP (260–215 cm), average annual temperatures were quite stable and exceeded modern ones in general by 2^oC, whereas the average annual precipitation corresponded to the modern level (Yurkovskaya and Elina, 2009). This climatic condition may have been favourable for fires, due to the presence of three charcoal layers in the peat deposits at 260, 240 and 215 cm depths (Figure 4). The first charcoal layer (260 cm) likely indicates a powerful fire around 7200 cal. yr. BP, which was probably caused by the first human settlements in the area (Kosheleva and Subetto, 2011). A large amount of ash and a decrease in the amount of pollen compared to the previous period was also noted in the pollen diagrams of many Neolithic settlements of this period in the region (Lobanova, 2009). This fire resulted in drastic changes to the vegetation. In the layers above 260 cm a sharp decrease in pine percentage (<5%) and PAR (Figures 6 and 7) was detected, which probably reflects its disappearance from the tree layer (Seppä and Hicks, 2006), while the PAR of birch strongly increased. Pine-dominated forests probably reduced their cover, while birch and alder were less affected, due to wetter biotopes they grow in. Consequently, burnt areas may have been quickly colonized by birch trees, which formed birch dominated forests with the well-developed ground cover of dwarf-shrubs, herbs and grasses. Similar vegetation changes were observed in the palaeoecological studies of Elina and Lebedeva (1992) and Lavrova et al. (2011) for the White Sea lowland area, ca. 80–100 km south-east of the Kindo Peninsula.

The next strong fire occurred about ~6500 cal. yr. BP (240 cm). The pollen diagrams (Figures 6 and 7) indicate that the fire probably mainly affected the birch-dominated forests, which were widespread during this time, and resulted in a gradual increase of pine abundance. At the same time, a significant increase of spruce pollen was recorded which was also identified by Elina et al. (2000) in the adjacent Karelian territory. Peat deposits during this period were characterized by a high degree of humification and mainly consisted of herbs with a small proportion of Sphagnum sect. Acutifolia and Sphagnum medium/S. divinum, as well as fungal remains (sclerotia) which are usually found in highly decomposed peat (Gałka et al., 2020). A few remnants of the testate amoebae (Trigonopyxis arcula) indicate dry conditions. Given this and the relatively high values of herb pollen, the studied mire area was probably part of a waterlogged forest throughout the Atlantic stage. The charcoal layer at 215 cm depth indicates another fire at about 5600 cal. yr. BP, which is assigned to the late Atlantic stage.

Increasing oligotrophy of the mire (5600–2700 cal. yr. BP)

After the fire around ~5600 cal. yr. BP (215 cm), the studied area probably became a less forested oligotrophic rim of the mire (Figure 8, Zone C). This was indicated by (1) a lower degree of peat layer decomposition (Figure 3), (2) the occurrence of Sphagnum-dwelling xerophilic testate amoebae (Figure 5), (3) an increased abundance of S. fuscum/S. rubellum plant macrofossils and a simultaneous decrease of herb rootlet abundance (Figure 4) and (4) an increase of dwarf-shrub pollen and Sphagnum spores values (Figures 6 and 7). In the surrounding vegetation, pine likely became a forest-forming tree again with an admixture of birch and spruce. The increase in abundance of L. annotinum and ferns spores indicates that the newly formed pine-dominated forests became closer to the northern taiga type in appearance.

The Subboreal stage of the Holocene started at about 4800 cal. yr BP and was characterized by pronounced climate oscillations with strong cooling at the beginning with warming during the middle stage (around 3800 cal. yr. BP), when climatic conditions nearly reached those reported for the Atlantic stage (Yurkovskaya and Elina, 2009). In the later part of the Subboreal stage at~3000 cal. yr. BP the climate stabilized and tended towards a gradual cooling (Yurkovskaya and Elina, 2009; Kosheleva and Subetto, 2011). During the Subboreal stage, the Kindo peninsula still experienced uplift, though at a slower rate, compared to previous stages (Romanenko and Shilova, 2012). The vegetation of the peninsula was represented by pine and spruce-pine forests of the north taiga type with an admixture of birch and dwarf-shrubs dominated ground cover (Figures 6 and 7). In general, the surrounding vegetation was similar to that currently observed and likely underwent minor changes since that time.

However, three charcoal layers in the peat profile (from 149 cm to 115 cm depth) indicate that the peninsula was repeatedly exposed to fires, which might have caused the local changes of the studied mire. At a depth of 140 cm, the peat was dominated by S. fuscum/S. rubellum with Ericaceae and after a fire at about 3500 cal. yr. BP Polytrichum moss appeared in the peat stratigraphy. The low peat decomposition together with the appearance of hydrophilic species of testate amoebae indicates that mire surface wetness was relatively high due to climatic cooling and/or increased precipitation rate.

Increasing surface wetness of the mire (2700 cal. yr. BP – present)

The beginning of the zone is characterized by an unstable hydrological regime associated with fires (Figure 8, Zone D). The strongest fire event at about 2200 cal. yr. BP (75 cm) coincided with a time span of decreased average annual precipitation (Yurkovskaya and Elina, 2009). Hydrophilic testate amoeba assemblages were replaced by xerophilic taxa, and plant macrofossils which indicate the presence of (micro-) habitats with relatively low moisture appeared (S. fuscum/S. russowii, Pohlia sp. and Dicranum undulatum). The moisture level then likely increased, indicated by the appearance of hydrophilic testate amoeba assemblages at around 1500 cal. yr. BP (depth of 50 cm) and the disappearance of herb rootlets.

At ~600 cal. yr. BP the local vegetation composition of the mire site approached the current: macrofossils of Dicranum undulatum disappeared; Sphagnum capillifolium became dominant instead of S. russowii and the occurrence of Ericoid dwarf-shrub macrofossils increased. The same happened to testate amoeba assemblages which were formed mostly by hydrophilic taxa from about 520 cal. yr. BP. This shift was associated with the last determined macro-charcoal layer at this depth.

Conclusions

Overall, the studied mire formed in a lake isolated from the coast line as a result of isostatic glacial shore uplift at about 9000 cal. yr. BP. The development of the mire was associated with simultaneous terrestrialisation of the lake and paludification of the adjacent area. Over most of the studied period the vegetation in the regions remained typical for the northern taiga zone, except for the Atlantic stage (7800–4800 cal. yr. BP), when middle taiga vegetation type was reconstructed. Our data indicate that frequent fires affected the regional vegetation and the mire dynamics, especially during the Atlantic stage of the Holocene. These events were associated with an increased proportion of birch in the vegetation cover (as a pioneer species) and/or water level decreases. This highlights the need to understand the role of natural and human-induced fires under future climate warming scenarios in subarctic regions.

Footnotes

Acknowledgements

We are grateful to A. Tzetlin for the organisation of fieldwork at the White Sea Biological Station. ASh is grateful to V.E. Fedosov for support and encouragement.

Author contributions

YuM, EES, ANT and EDK conceived the study; YuM secured funding; KB, ANT, RJP, ASh, DAV and YuM conducted fieldwork; KB performed testate amoeba analysis; EES and ASh conducted pollen analysis; MG conducted plant macrofossils analysis, NM conducted LOI analysis; YuF conducted peat humification analysis; EZ conducted radiocarbon dating; KB, ANT, EES, ASh, MG and DS conducted data analysis; KB, ANT, DM, EES, Ash and YuM wrote the first draft of the manuscript to which all authors (with the exception of RJP – deceased) contributed with text, comments and ideas. RJP was tragically killed by an avalanche while trying to reach the peak of Nanda Devi (Indian Himalayas) on May 26, 2019 (Mazei et al., 2020).

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was funded by the Russian Science Foundation (19-14-00102) and Russian Foundation for Basic Research (19-05-00377). The work of ASh and EES was supported by Moscow State University Grant for Leading Scientific Schools ‘Depository of the Living Systems’ in frame of the MSU Development Program.

ORCID iDs

Anna Shkurko

Andrey N Tsyganov

Yuri A Mazei

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