Autogenic and allogenic factors affecting development of a floating Sphagnum -dominated peat mat in a karst pond basin

Abstract

Lateral expansion of floating vegetation mats over the surface of aquatic ecosystems (terrestrialization) is one of the ways of peatland development. This process was commonly studied in kettle-hole lakes, whereas karst ponds have received less attention. We used a suite of palaeoecological analyses at Karstovoe mire (Mordovia, Russia) to reconstruct the formation of a floating Sphagnum-dominated peat mat over the karst pond. The results show that the floating peat mat had covered the central part of the pond by ca. AD 1600. Remains of Scirpus sp. and Calamagrostis sp. in the basal layers indicate that these plants might form a framework on which Sphagnum mosses and sedges were established. The terrestrialization could be triggered by the ‘Medieval Warm Period’ (AD 950–1250) as droughts reduce water levels and allow the pioneering plants to colonize exposed bottom sediments on the margins of lakes. Later, the development of the mire was mainly driven by autogenic factors that could be explained by the relatively stable hydrological regime in freely floating or poorly attached vegetation mats. In the mid 19th century, the surface wetness of the mire started to decline that can be related to both increased human activity associated with fires and to a greater thickness of the mat so that autogenic and allogenic effects were difficult to disentangle. In less than a century after that, the fen transformed to a pioneer raised mire. Our results show complex and context-dependent effect of autogenic and allogenic factors on the development of floating peat mats.

Keywords

last millennium mire Mordovia multi-proxy peat terrestrialization

Introduction

Karst ponds are small lentic ecosystems formed in local ground subsidence caused by the surface dissolution of water-soluble rocks such as limestone, gypsum and dolomite (Monroe, 1970). These aquatic ecosystems are widespread throughout the karst regions and as other types of ponds and lakes they can develop into mires by combined effects of lateral peatland expansion over the water surface (terrestrialization) and sediment accumulation at the bottom (infilling) (Rydin and Jeglum, 2006). The characteristics of karst ponds which could make them especially prone to terrestrialization include (1) deep, steep-sided, clay-lined bottom, (2) lack of surface inlets or outlets, (3) water-level fluctuations, (4) weakly minerotrophic waters and mat-forming plant species and (5) location in climatic conditions favourable for peat accumulation (Wilcox and Simonin, 1988). The terrestrialization of ponds results not only in abrupt changes in structure and functioning of aquatic ecosystems but also can act as atmospheric carbon sink by accumulating peat (Payette et al., 2004). However, the mechanisms which control terrestrialization and infilling of karst pond ecosystems have received little attention.

According to the classical model of terrestrialization in lentic ecosystems (Whittaker, 1975), a vegetation mat forms along the perimeter of a lake or pond. The pioneer mat-forming species frequently include Carex lasiocarpa, Menyanthes trifoliata, Myrica gale, Typha sp., Scirpus sp. and Chamaedaphne calyculata (Swan and Gill, 1970). The shoots, roots and rhizomes of these species contain specialized aerenchyma tissue (Kausch et al., 1981) which makes them less dense than water. Therefore, their remains provide a semi-buoyant and hydrologically stable matrix suitable for Sphagnum colonization and rapid vertical accumulation of peat (Charman, 2002; Warner et al., 1989). Methane bubbles trapped in the peat as a result of anaerobic decomposition are also involved in the buoyancy of peat substrates (Smolders et al., 2002). Sometimes portions of the mat may break loose, as a result of damming and wave action, and become free-floating rafts (Rydin and Jeglum, 2006). In karst ponds located in forested areas, a semi-buoyant matrix can be formed by floating plants, leaves and tree branches which are subsequently colonized by green mosses (Volkova, 2010). Another way of floating mat formation is when the sedge peat deposited at the bottom of the depression during low water periods floats after a rise in water level providing the substrate for Sphagnum colonization under less mineralized conditions (Wilcox and Simonin, 1988).

The succession of the established peatland vegetation is usually directed towards more closed vegetation, formation of peat mats, drier conditions, increasing organic matter and decreasing ash content (Rydin and Jeglum, 2006). The final stage of the succession is largely determined by local hydrological and climatic conditions and may represent a fen, swamp forest or a bog. Terrestrialization of lentic ecosystems has been generally viewed as a gradual, climate-independent process (Kratz and DeWitt, 1986). However, this autogenic developmental model had been questioned as recent research in North America (Booth et al., 2016; Ireland and Booth, 2011) revealed episodic and non-linear expansion of floating peat mats that was likely driven by hydrological variability (either climate or human-dependent). Such patterns have also been observed for the mires located in Europe (Drzymulska et al., 2013; Kowalewski and Zurek, 2011; Lamentowicz et al., 2007; Słowiński et al., 2016). An important successional phase in mire development is the fen–bog transition (FBT) which is characterized by the replacement of nutrient-rich minerotrophic fen by ombrotrophic bog communities developing in more acidic conditions (Granath et al., 2010). This transition has widely been explored via plant macrofossil and testate amoeba analysis at several sites across Europe and North America (Elliott et al., 2012; Hughes and Barber, 2003, 2004; Langdon et al., 2003). Two common ways of FBT are generally distinguished: ‘dry-pioneer’ and a ‘wet-pioneer’ (Hughes and Barber, 2004). The dry-pioneer route is defined by deep or highly fluctuating water tables (Hughes, 2000). The ‘wet-pioneer’ route is associated with near-surface water levels, so that transition to ombrotrophy is thus achieved through the rapid accumulation of peat. The type of FBT can be driven by autogenic (Clymo, 1991) and/or allogenic controls, such as effective precipitation (Hilbert et al., 2000). However, it is unclear whether similar mechanisms and dynamics characterize mire development in karst regions located in mixed forest zones. Understanding these mechanisms is particularly important at the southern margins of peatland distribution, because these peatlands may be especially vulnerable to ongoing and future climate changes.

Most of the works on floating peat mat formation were performed on lake-peatland systems forming in kettle-holes (Ireland and Booth, 2011; Kratz and DeWitt, 1986; Lamentowicz et al., 2007, 2008, 2015), whereas karst ponds and peatlands were much less studied. This paper attempts to reconstruct the development of a floating Sphagnum-dominated peat mat in a karst pond using multi-proxy palaeoecological analysis and to determine autogenic and allogenic drivers of its development. To this aim, we sampled peat deposits from the floating peat mat in the karst pond Karstovoe located in the mixed forest zone of the European part of Russia (the Republic of Mordovia). The obtained material was used to reconstruct chronology, vegetation succession (plant macrofossil analysis), surface wetness (peat humification and testate amoeba analyses), climate and human impacts (pollen, non-pollen palynomorph (NPP), micro- and macrocharcoal analyses). The special attention is given to the description of the main stages in the formation of the floating peat mat in the context of the landscape and climate conditions taking into account the potential human impacts.

Study area

The study was performed in the Mordovia State Nature Reserve located in the central part of the East European Plain (the Republic of Mordovia, Russia) (Figure 1). The area is situated between the forest zone of the Meshchera Lowlands and the forest-steppe of the Volga Upland (Mil’kov and Gvozdetsky, 1986). The area represents the floodplain of the Moksha River (103–110 m a.s.l.) and an undulating plain with moraine and fluvioglacial deposits (110–132 m a.s.l.) (Novenko et al., 2018). Numerous mires develop in small depressions formed by karst processes (Grishutkin, 2013). The climate is temperate and moderately continental with relatively cold winters (the mean January temperature: −10.5°C) and warm summers (the mean July temperature: +20.3°C) (Temnikov weather station, 20 km southwest of the study site, since 1886; http://www.ecad.eu). The mean annual temperature is +4.7°C. Annual precipitation is about 520 mm with more than a half of it falling as rain in summer and autumn while the rest falling as snow in winter and spring (Bayanov, 2015). The vegetation of the study area is dominated by mixed pine-deciduous forests with Pinus sylvestris L., Quercus robur L. and Tilia cordata Mill (Tereshkin and Tereshkina, 2006). Picea abies L., Ulmus glabra Huds., Acer platanoides L. and Fraxinus excelsior L. are present as an admixture. Oak-lime forests mainly occur on moraine and limestone deposits which are characterized by fertile soils. Secondary forests are mainly formed by Betula pendula Roth. and Populus tremula L. in the areas of former clear-cutting.

Figure 1.

(a) Location of the study site within the Eastern Europe and (b) the position of the Karstovoe mire.

The study site is a small mire (~20 m in diameter) Karstovoe (54.7757°N, 043.433°E). The mire is formed in a round deep (more than 10 m) karst depression and represents a floating Sphagnum-dominated peat mat (about 1 m thick) covering the entire surface of the basin so that no open water is present. The overstorey of the peat mat is dominated by Betula pubescens Ehrh. (density is about 0.1, h = 5–7 m) with rarely occurring low P. sylvestis and P. abies trees. The shrub layer is mainly formed by Ledum palustre L., Chamaedaphne calyculata (L.) Moench, Vaccinium uliginosum L., Vaccinium oxycoccos L. and Andromeda polifolia L. The field layer is poorly developed and mainly consists of Eriophorum vaginatum L. with rarely occurred Carex pseudocyperus L., Carex limosa L. and Drosera rotundifolia L. The moss layer is formed by Sphagnum sp. A moat swamp dominated by Carex vesicaria L. encircles the floating peat mat. The mire is surrounded by a P. sylvestris forest with P. abies, T. cordata and A. platanoides in the understory. The shrub layer of the forest is formed by Vaccinium myrtillus L. and V. uliginosum. The herb layer is dominated by Molinia caerulea (L.) Moench.

Materials and methods

Field sampling

Five peat cores (each 1 m long) were extracted from the central part of the floating peat mat with a Russian corer (5 cm in diameter, 50 cm long) in August 2015. The cores were wrapped in plastic and aluminium foil, placed in boxes and stored at 4°C before further analysis. In laboratory, the cores were subsampled for multi-proxy palaeoecological analysis. Each sample was stored in a ziplock plastic bag.

Radiocarbon dating

The chronology of the cores was determined with Accelerator Mass Spectrometer radiocarbon dating (hereafter AMS ¹⁴C dating) of four peat samples (Table 1). The analysis was performed in the Laboratory of Radiocarbon Dating and Electron Microscopy, the 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) and the age–depth model was developed using Bayesian-based package ‘rbacon’ (Blaauw and Christen, 2018) in the R language environment (R Core Team, 2017). We used the programme default settings for all priors except for accumulation rate, which was set to 5 yr cm⁻¹. The dates are quoted in the text as the 2-sigma (2σ) calibrated range AD.

Table 1.

Radiocarbon dates of the peat core from the Karstovoe mire.

Laboratory code	Depth (cm)	Material	Radiocarbon date (¹⁴C yr BP)
IGAN-AMS-5599	22–24	Sphagnum stems	100 ± 20
IGAN-AMS-5600	46–48	Sphagnum stems	130 ± 20
IGAN-AMS-5601	72–74	Sphagnum stems	200 ± 20
IGAN-AMS-5602	96–98	Sphagnum stems	300 ± 20

Plant macrofossil analysis

Plant macrofossils were analysed at 5-cm interval in contiguous subsamples of approximately 5–10 cm³. The samples were disaggregated with water and washed through a 0.25-mm mesh sieve. The plant remains were identified using a dissecting microscope at 200× magnification following Dombrovskaya et al. (1959) and Katz et al. (1977) and represented as a percentage of the total number of identified remains.

Loss on ignition and peat humification

Subsamples for loss on ignition (LOI) were taken in 4-cm contiguous slices through the entire peat depth. LOI was determined following the procedures suggested by Dean (1974) (combusting 50 g of the samples at 550°C for 5 h). Peat humification was analysed in 2-cm contiguous subsamples from the top (0–50 cm) section of the core and in 3-cm contiguous subsamples from the bottom (50–100 cm) section using the standard alkali-extraction and colorimetry method (Chambers et al., 2010–2011). The samples were dried (50°C), weighted (0.2 g) and ground in an agate mortar. After that they were placed in 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 540-nm wavelength. Greater values of light absorbance indicate greater peat humification and vice versa.

Testate amoeba analysis

Subsamples for testate amoeba analysis were taken in the same way as for peat humification analysis. The preparation of the samples was done 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 decanted off and the samples were mixed with neutralized formaldehyde and placed in glass vials for storage. One millilitre of the concentrated sample was placed in a Petri dish (5 cm diameter), diluted with deionized 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. The obtained data were analysed and plotted using in the R language environment (R Core Team, 2017) with the packages ‘rioja’ (Juggins, 2017) and ‘analogue’ (Simpson and Oksanen, 2016). 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) that was specifically developed for the forest zone of European Russia (Tsyganov et al., 2017).

Pollen analysis, NPPs and microcharcoal

Subsamples for analysis of pollen, microcharcoal and NPP were contiguously taken every 2 cm in the top (0–50 cm) and every 3 cm in the bottom (50–100) section of the cores. The preparation followed Moore et al. (1991): heating in 10% KOH for 10 min and acetolysis in a water bath for 5 min. Relative pollen abundance was calculated based on total terrestrial pollen, which included arboreal pollen (AP) and non-arboreal pollen (NAP); spores excluded. A minimum of 500 pollen grains (AP + NAP) per sample were identified and counted at 400× magnification with a light microscope (Motic BA210, Spain) following Beug (2004). To determine concentrations, Lycopodium tablets were added to each sample (Stockmarr, 1971). Microscopic charcoal concentrations were assessed using the point-count methodology (Clark, 1982). Plant (P. sylvestris stomata), animal (Habrotrocha angusticollis) and fungal (Podospora sp., Delitschia sp., Sordaria sp., Sporomiella sp., Gelasinospora sp., Helicoon pluriseptatum Beverw., Geoglossum sphagnophilum Ehrenb., etc.) NPP were identified following Van Geel (1978) and Kuhry (1985). The diagrams were plotted using the Tilia/Tilia-Graph software package (Grimm, 1990).

Macrocharcoal

Macrocharcoal particles (>125 µm) were analysed in contiguous 3 cm samples following the standard protocol (Mooney and Tinner, 2011). Peat samples (1 cm³) were placed in bleach (100 mL of 10% solution of NaOCl in water) at room temperature for 24 h. Then, the samples were gently washed through a sieve with the mesh size of 125 µm. All materials within the sieve were then carefully transferred to a labelled Petri dishes and all charcoal particles were tallied under a dissecting microscope at the magnification of 40×. The obtained counts were used to calculate the accumulation rates of charcoal particles (cm⁻² yr⁻¹).

Results

Age–depth model, plant macrofossils and LOI

The results of the radiocarbon dating and the age–depth model are presented in Table 1 and Figure 2. The age of the bottom layers of the peat mat can be estimated as AD 1570. The plant macrofossil analysis (Figure 3) indicates that the peat deposits could be divided in two zones: PM1 (100–15 cm, AD 1570–1940) and PM2 (15–0 cm, AD 1940–present). The PM1 zone was formed by mesotrophic peat with plant remains dominated by Sphagnum fallax, Sphagnum obtusum, Sphagnum (sect. Sphagnum) and herbs (Calamagrostis sp. and Carex sp.). The PM2 zone was characterized by the presence of oligotrophic peat, which mainly consisted of Sphagnum angustifolium, Sphagnum magellanicum and S. fallax. The entire peat section was characterized by relatively high values of LOI which varied from 88% to 98% (Figure 3).

Figure 2.

Age–depth model for the floating Sphagnum-dominated peat mat in the Karstovoe mire.

Figure 3.

Percentages of plant macrofossil distribution and loss on ignition (%) along the floating Sphagnum-dominated peat mat in the Karstovoe mire.

Testate amoebae and peat humification

The analysis revealed 50 testate amoeba taxa (Figure 4; Table S1, Supplementary Materials, available online). The most abundant species were Archerella flavum (16% of the total shell count), Hyalosphenia papilio (15%), Nebela militaris (10%), Heleopera petricola (9%), Hyalosphenia elegans (8%), Assulina muscorum (6%), Cyclopyxis eurystoma (6%) and Trigonopyxis arcula (5%). All these species and Nebela tincta were common and were observed in more than 80% of the samples. Eight species (Arcella discoides, Arcella rotundata, Cyclopyxis puteus, Euglypha compressa glabra, Euglypha cristata decora, Gibbocarina galeata, Placocista jurassica and Plagiopyxis declivis) were encountered in one sample only (maximal relative abundance per sample was less than 1.3%). Overall, the assemblages were dominated by Sphagnum-dwelling hydrophilous testate amoebae.

Figure 4.

Testate amoeba diagram showing the most common testate amoeba taxa (occurred in two or more samples with the relative abundance ⩾2% at least in one sample), the zonation based on constrained incremental sum of squares, reconstructed water table depths (cm) and peat humification determined as light absorbance. The taxa are ordered by weighted average of the depth/time axis. Solid black line for each taxon means multiplication by factor 10.

Basing on the species composition of testate amoeba assemblages, three zones can be distinguished in the peat deposits (Figure 4). Zone TA1 (99–36 cm, AD 1570–1840) was characterized by the dominance of typical Sphagnum-dwelling hydrophilous species A. flavum (25% of total count in the zone), H. papilio (26%), H. petricola (11%) and H. elegans (10%). Xerophilous species were also found but were less abundant N. militaris (5%), A. muscorum (4%) and T. arcula (4%). At the beginning of the zone, Argynnia dentistoma and Physochilla griseola were constantly present. Reconstructed WTDs varied in the range of 4–14 cm. Zone TA2 (36–15 cm, AD 1840–1940) could be considered as transitional because species from the zones TA1 and TA3 were present. In this zone, the hydrophilous species were generally replaced by xerophilous ones. The zone was characterized by H. petricola (5%), N. militaris (5%), A. muscorum (4%), C. eurystoma (4%), H. elegans (4%), A. flavum (3%), H. papilio (3%) and T. arcula (3%). The surface wetness decreased (reconstructed WTDs 9–24 cm). The zone TA3 (15–0 cm, AD 1940–present) was mostly dominated by xerophilous Sphagnum-dwelling species: N. militaris (25%), Hyalosphenia subflava (12%), C. eurystoma (9%), A. muscorum (7%), Corythion dubium (7%), T. arcula (7%) and Trinema complanatum (6%). Surface wetness continued decreasing (reconstructed WTDs: 19–28 cm). The changes in peat humification (Figure 4) corresponded well to the reconstructed WTDs overall confirming gradual decrease in the surface wetness during the development of the floating peat mat.

Pollen, spores and NPPs

Pollen assemblages were characterized by high percentages of tree pollen (80–97%), especially Betula, Pinus and Picea (Figure 5). The percentage of Quercus pollen varied from 1% to 15%, whereas pollen of other broadleaf trees (Tilia, Ulmus, Fraxinus and Acer) permanently occurred in smaller quantities (1–2%). The proportion of Alnus and Corylus pollen did not exceed 10%. Pollen of shrubs, such as Salix, Sorbus, Rhamnus and Viburnum, was detected. Among NAP, plants indicating anthropogenic activity commonly occurred. Cerealia formed a continuous curve; pollen of cultivated plants such as Fagopyrum esculentum and Cannabis was recorded. Pollen of weed (Centaurea cyanus), plants typical for ruderal communities (Chenopodiaceae, Artemisia, Plantago major/media, Polygonum aviculare-type, Ranunculus-acris-type and Rumex acetosella-type), pastures and meadows (Poaceae, Caryophyllaceae, Asteraceae, Cichoriaceae, Fabaceae and Rosaceae) were frequent. The permanent components of pollen assemblages were spores of Polypodiaceae, Lycopodium annotinum and L. clavatum. Spores of Equesetum sp., Hupperzia selago and Pteridium aquelinum occurred sporadically. Pollen and spores of wetland plants were represented by Cyperaceae, Ericales, Sparganium sp., Menyanthes trifoliata and Sphagnum mosses.

Figure 5.

(a) Percentage diagrams of terrestrial pollen and spores and (b) wetland plants with non-pollen palynomorphs from the peat deposits of the Karstovoe mire.

According to the changes in the pollen assemblages and NPPs, the diagram was divided into four local pollen assemblage zones (LPAZ). Zone LPAZ 1 (100–63 cm, AD 1570–1730) was characterized by high proportion of AP (95–97%) with predominance of Betula and Pinus (20–40%) and smaller quantities of Quercus (10–15%) and Picea (10–25%). Spermatophores of copepods were found at the depths 70, 72 and 100 cm that indicated the presence of contemporary open water (Van Geel, 1978). In addition, maximal abundances of the fungus Geoglossum sphagnophilum Ehrenb. (Van Geel, 1978, Type 77A) were detected at the depths of 96–100 cm (up to 100% in comparison with the total sum of AP + NAP). In other parts of the peat deposits, it occurred sporadically. This fungus generally grows among Sphagnum mosses. A fruiting-body of HdV 8B was present in peat at the depths of 100–74 cm, 70–78 cm and 36–30 cm, reaching the maximal abundance between 70 and 63 cm. As reported by Van Geel (1978), this species was generally found growing superficially on several plants. After that (LPAZ 2, 63–43 cm, AD 1730–1815) the proportion of P. sylvestris pollen increased (up to 60%) that coincided with increased concentration of its stomata (at the depths of 52–24 cm). This might indicate an increase of Scots pine in forest stands and its appearance in the local mire vegetation. The abundance of Sphagnum spores reached maximal values that could point to an intensive growth of the floating vegetation mat.

At the beginning of the LPAZ 3 (subzone LPAZ 3a, 43–33 cm, AD 1815–1855), Quercus pollen reached maximal values (12–17%) and decreased afterwards, whereas the pollen of plants indicating anthropogenic activity (Cerealia, Rumex, Plantago, Artemisia, Chenopodiaceae) became more abundant. This could be related to the beginning of forest clearance. At the same time, a high peak (at 38 cm, around AD 1835) of Cyperaceae pollen (up to 80% of AP + NAP) was detected. This corresponded to the increased abundances of Habrotrocha angusticollis (Murray) (at the depths 48–26 cm) which is the characteristics for wet phases of wetland development (Shumilovskikh et al., 2015). In the subzone LPAZ 3b (33–22 cm, AD 1855–1905), the proportion of Poaceae, Artemisia, Cerealia and other anthropogenic indicators increased. In the local mire vegetation, the proportion of Ericales increased, whereas Cyperaceae and Sphagnum were low. This subzone was also characterized by the presence of Gelasinospora sp. (HdV 1) which is typical for highly decomposed peat, formed under dry, oligotrophic conditions and with the occurrence of charcoal (Van Geel, 1978). The top zone LPAZ 4 (22–0 cm, AD 1905–present) was characterized by decreasing concentrations of Picea and broadleaf tree pollen, which was compensated by increasing Pinus pollen, that could be associated with an expansion of birch-pine forests after the clearance (the establishment of the nature reserve). Low abundances of Ericales, Cyperaceae and Sphagnum were detected.

Among other NPPs (Figure 5b), small quantities of the fungus Helicoon pluriseptatum Beverw. (Van Geel, 1978: HdV 30) were observed at the depths of 92–22 cm. This fungus is known from peatlands on birch leaves, pine and spruce needles and cones (Yeloff et al., 2007). A number of microscopic fungi such as Podospora sp., Delitschia sp., Sordaria sp. and Sporomiella sp. were relatively abundant at the depths of 82–72 cm and 22–34 cm (LPAZ 3) that corresponded to the increase in pollen values of anthropogenic indicators. Species of these genera are obligatory coprophilous, occurring on the dung of domestic livestock as well as wild herbivores; however, except Sporomiella sp., they also grow on other kinds of decaying organic material (Krug et al., 2004). The NPP-type HdV 13 was recorded from the depth of 86 cm and upwards with the highest values at the depths of 74–70 cm and 33–18 cm. The taxonomic attribution of this type is problematic (Kuhry, 1997) but following Shumilovskikh et al. (2015), we named it Desmidiospora. It is typical for oligotrophic Sphagnum peat, usually for relatively dry phases of peatland development (Shumilovskikh et al., 2015; Van Geel, 1978).

Micro- and macrocharcoal

Analysis of charcoal accumulation rates (Figure 6) indicates relatively low background fire activity at the regional and local levels until the beginning of the 18th century. After that, regional fire activity increased reaching its maximum at the end of the century (concentration of micro-charcoal: 789 × 10³ particles cm⁻³, accumulation rate: 30 × 10³ particles cm⁻¹ yr⁻¹), whereas fire intensity at the local level remained low. This can be related to expanding human colonization, which however did not considerably affect the area around the mire. Starting from the 19th century, two clear local fires can be distinguished around AD 1845 and 1906 (macrocharcoal concentration: 210 and 270 particles cm⁻³, accumulation rates: 17.5 and 22.5 particles cm⁻¹ yr⁻¹, respectively). A smaller peak in macrocharcoal accumulation rates in the top part was probably related to the recent fires in the area in AD 2010.

Figure 6.

(a) Micro- and (b) macrocharcoal accumulation rates in the floating Sphagnum-dominated peat mat of the Karstovoe mire.

Discussion

Onset of terrestrialization in the karst pond (prior to ca. AD 1570)

The obtained results show that the Sphagnum-dominated peat mat had covered the central part of the basin by the end of the 16th century. The basal peat layers were mainly formed by Sphagnum mosses with the participation of Scirpus sp., Calamagrostis sp., Carex rostrata and other herbal remains, whereas no leaves, branches or sedge peat remains were detected. The relatively high proportion of Sphagnum mosses point to the fact that these layers were most likely formed some time after the onset of terrestrialization. However, the remains of typical pioneering mat-forming plants (i.e. Scirpus sp. and Carex sp.) may indicate a classical lateral (centripetal) encroachment of original vegetation mats from the margins of the karst pond (Kratz and DeWitt, 1986; Rydin and Jeglum, 2006; Warner et al., 1989). This mechanism of terrestrialization has been widely observed for other types of depressional ecosystems, including kettle-hole and thermokarst lakes (Drzymulska et al., 2013; Ireland and Booth, 2011; Kowalewski and Zurek, 2011; Milecka et al., 2017; Parsekian et al., 2011).

The development of the floating peat mat could be triggered by the ‘Medieval Warm Period’ (AD 950–1250) which was clearly demonstrated for the region by the previous multi-proxy climate reconstructions (Novenko et al., 2018). According to the allogenic model of peatland development in kettle lakes (Ireland and Booth, 2011), prolonged drought events result in decreased water levels and exposed organic-rich lake sediments which are readily colonized by pioneering plants (especially sedges). Subsequent water-level rise causes this semi-buoyant, loosely rooted mat of vegetation to detach from the poorly consolidated lake sediments and float creating the substrate for further Sphagnum colonization. Moreover, this model suggests rapid expansion of peat mat in response to extreme water-level fluctuations (Drzymulska et al., 2013; Ireland and Booth, 2011; Kowalewski and Zurek, 2011). The estimated rates of lateral expansion (centripetally encroachment) of vegetation mats vary considerably from ~1–5 cm yr⁻¹ (estimates from low-resolution retrospective studies) to 0.7–3.5 m yr⁻¹ (estimates from the basal peat radiocarbon dates; Korhola, 1992). It has been also shown that human settlement can trigger peatland expansion (Warner et al., 1989) through deforestation and water-level variability associated with that. However, in case with the studied karst pond, this way can be largely excluded, because, despite the signal of human presence at the pollen diagram, the area adjacent to the peatland remained unaffected. Overall, our data indicate that karst ponds may follow classical lateral (centripetal) encroachment of original vegetation mats from the margins according to the allogenic model.

Minerotrophic stage of peat mat (AD 1570–1840)

The later development of the floating Sphagnum-dominated peat mat took place during the ‘Little Ice Age’ (LIA, AD 1400–1700; Mann et al., 2009). Our pollen data show that the vegetation around the mire at that time was characterized by the spruce-pine and pine-birch forests with the participation broadleaved trees (Quercus, Ulmus and Tilia). The area remained poorly populated by people. According to the charcoal data, the fire intensity was low. The LIA was detected for the study area by Novenko et al. (2018) as a phase of increased concentrations of Picea pollen and decreased micro-charcoal accumulation rates. Pollen-based climatic reconstructions for the Upper Volga region (north from the study site) (Klimanov et al., 1995; Novenko et al., 2014) showed that the mean annual temperatures during the LIA were 2°C lower than at present, whereas the precipitation level was 50–100 mm yr⁻¹ higher. Overall, the period was characterized by greater frequency of extreme weather events (winter frosts, summer droughts, intensive rainfalls, etc.) (Moreno-Chamarro et al., 2017). The review of the pollen data for the forest zone of the East European Plain (Novenko, 2016) demonstrated that these climate changes affected the structure of forest vegetation and favoured the development of spruce, whereas broadleaved trees were reduced. These effects were especially pronounced at the northern border of the forest zone and at the upper limits of forest distribution in mountains (Wanner et al., 2008). However, in the forest zone itself was less affected so that only the proportions of the forest-forming trees varied without considerable shifts in the landscape structure at the regional scale.

The end of the LIA was associated with increased winter temperatures, extension of the growing season and reduced precipitation in the whole Eastern Europe, however the warming was not uniform (Klimenko et al., 2012). This coincided to the establishment of the Sarov Monastery in close vicinity of studied mire at AD 1706 and the beginning of forest usage by monks (Yanina, 2010). During AD 1780–1810, warming became more intensive (Novenko et al., 2016) that was followed by greater human impacts (as shown by the increased proportions of cultural plants and by the replacement of broadleaved trees and spruce by pine-birch forests in our pollen diagram). However, despite these changes, the Sphagnum-dominated peat mat in the karst pond remained relatively stable and represented a mesotrophic mire which mainly formed by Sphagnum (sect. Sphagnum) and Calamagrostis sp. Our testate amoeba-based reconstruction of WTDs showed that the surface wetness of the mire was high that can indicate its close contact to the pond water. At the same time, the obtained pollen data demonstrate an intensive growth of the floating vegetation mat (maximal proportions of Sphagnum spores). This supports the idea that, once initiated, floating mats induce autogenic changes in ecosystem characteristics (increasing acidification, favouring development of specific vegetation assemblages, etc.) (Kowalewski and Zurek, 2011) providing that environmental conditions do not reach threshold values to shift the balance (Granath et al., 2010). The resilience of mire could be is explained by freely floating or poorly attached vegetation mats that results in high surface wetness which is relatively independent from the water table fluctuations (Gaudig et al., 2006).

‘Fen–Bog Transition’ and pioneer raised mire (ca. AD 1840–present)

According to our pollen data, the anthropogenic pressure in the area continued to increase and was mostly related to the forest clearance and agricultural land use. This resulted in greater fire intensity (as shown by macrocharcoal accumulation rates) with the most destructive fires around AD 1850. The historical data (Yanina, 2010) indicate that these fires destroyed not only a part of the forest stands but also shallow mires in the area. At the same time, surface wetness of the mire started to decline gradually that was clearly demonstrated by testate amoeba-based reconstruction. These changes can be considered as an onset of FBT because by the mid 20th century (AD 1940), WTDs decreased considerably and the vegetation of the floating Sphagnum-dominated peat mat became oligotrophic that altogether can point to increasing isolation of the surface of the mire from the minerotrophic waters of the basin (i.e. ombrotrophication). Overall, ombrotrophication could be generally achieved via vertical accumulation of the peat mass (autogenic) or a lowering of the water table (allogenic) (Granath et al., 2010; Hughes and Barber, 2003). The allogenic human-induced changes in the study area could potentially alter the hydrology of the catchment leading to flashy runoff and prolonged dry periods (Hughes and Barber, 2003). Previous studies have demonstrated that changes in precipitation or the hydrology in the mire surroundings are vital for the rich FBT (Hilbert et al., 2000; Svensson, 1988), presumably due to reduction in flooding which seems to prevent bog sphagna establishment (Granath et al., 2010). However, in our case, the decreasing surface wetness could be also related to autogenic factors, that is, spatially constrained growth of floating peat mat so that the surface began rising above the water level. Overall, the development of the floating peat mat during this period could be driven by both autogenic and allogenic factors which might be difficult to disentangle.

Our data show that FBT transition lasted less than 100 years and was associated with increased proportions of typical ombrotrophic sphagna (S. angustifolium and S. magellanicum) (Laine et al., 2011; Väliranta et al., 2017) with the presence of E. vaginatum. In favourable conditions, the FTB can be rapid once the effects of ground waters were reduced (Granath et al., 2010; Loisel and Yu, 2013). Even gradual reduction in WTD due to peat accumulation (autogenic process) will eventually manifest a rapid change (catastrophic shift) when the critical threshold is reached, that is, when bog sphagna become competitive (Granath et al., 2010). Once established, Sphagnum mosses change the ecosystem to their advantage by acidifying the habitat and creating water-logged, nutrient-poor environments that impede other species (Van Breemen, 1995). The appearance of E. vaginatum in the studied mire may further assist the ombrotrophication process (Väliranta et al., 2017). E. vaginatum has been shown to act as an ‘ecosystem engineer’ that changes the habitat conditions in a way they are more suitable for the establishment of oligotrophic–ombrotrophic Sphagnum mosses (Hughes and Dumayne-Peaty, 2002). The long and aerenchymatous roots of E. vaginatum allow large WTD fluctuations to be tolerated (Tuittila et al., 2007). Moreover, the highly resistant remains of E. vaginatum promote peat accumulation and consequent ombrotrophication (Coulson and Butterfield, 1978). Overall, the FBT in the studied mire was relatively rapid that supports the idea about non-linear, steplike behaviour peatland ecosystems at this stage.

Conclusion

The studied floating Sphagnum-dominated peat mat in the karst pond is an example of a progressive succession of peat ecosystems which normally advances towards decreasing base saturation and increasing acidity. Our data indicate that the onset of floating peat mat formation might be triggered by allogenic factors, that is, drought associated with the decreased water levels in the karst pond. The further development of the floating mat was mostly driven by autogenic factors until the mid 19th century. The FBT which took place shortly after that could be a result of a complex interaction of autogenic (peat accumulation) and allogenic factors (climate and human impact). Overall, the study demonstrates that karst pond ecosystems in the mixed forest zone can be potentially sensitive to peatland expansion and the subsequent succession will be driven by interaction of both autogenic and allogenic factors.

Supplemental Material

Supplemental_Material – Supplemental material for Autogenic and allogenic factors affecting development of a floating Sphagnum-dominated peat mat in a karst pond basin

Supplemental material, Supplemental_Material for Autogenic and allogenic factors affecting development of a floating Sphagnum-dominated peat mat in a karst pond basin by Andrey N Tsyganov, Dmitry A Kupriyanov, Kirill V Babeshko, Tamara V Borisova, Viktor A Chernyshov, Elena M Volkova, Daria A Chekova, Yuri A Mazei and Elena Yu Novenko in The Holocene

Footnotes

Acknowledgements

The authors are thankful to Alexander Ruchin, Elena Vargot and Oleg Grishutkin for the help in organization of the field works on the territory of the nature reserve.

Funding

This work was supported by the Russian Science Foundation (grant no. 16-17-10045).

ORCID iD

Andrey N Tsyganov

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