Resilience of plant and testate amoeba communities after climatic and anthropogenic disturbances in a Baltic bog in Northern Poland: Implications for ecological restoration

Abstract

This study explores the history of the development of Sphagnum communities in an ombrotrophic peatland – Bagno Kusowo – over the past 650 years, based on high-resolution plant macrofossil and testate amoebae analysis. Our research provided information related to the length of peatland existence and the characteristics of its natural/pristine state before the most recent human impacts. Changes in the Sphagnum communities before human impact could have resulted from climate cooling during the ‘Little Ice Age’ (LIA). In this cold and unstable hydrological period, among vascular plants, Eriophorum vaginatum and Baeothryon caespitosum dominated in the peatland vegetation. Peat-forming Sphagnum communities survived the drainage conducted during the 20th century at the Bagno Kusowo bog. We provide three important messages through this study: (1) testate amoebae reflect similar hydrological trends in two peat cores despite considerable microhabitat variability, (2) average long-term water level 10 cm below the surface should be a target for active bog conservation and (3) sites like Bagno Kusowo are extremely important to preserve the remains of pristine biodiversity (including genetic diversity of plants and protists) that was completely removed from most of the raised bogs in Europe due to human activities, for example, drainage.

Keywords

Baltic raised bog human impact ‘Little Ice Age’nature conservation plant macrofossils pollen testate amoebae

Introduction

Ombrotrophic peatlands depend on the water that is derived directly from the atmosphere, and therefore, they are particularly sensitive to hydroclimatic change. In an ombrotrophic peatland, specific Sphagnum species in plant communities dominate under conditions of a stable and high water level (Schouten, 2002). In Europe, numerous modern habitats of various Sphagnum species have been described based on pH, water depth, co-occurring species, competition, growth and productivity (e.g. Dierssen, 2001; Gunnarsson et al., 2004; Hölzer, 2010; Ingerpuu and Vellak, 2013; Laine et al., 2011; Rydin, 1997; Wojtuń et al., 2013). However, knowledge of the long-term ecology of Sphagnum and its habitat is still far from satisfactory. Ombrotrophic peatland development was substantially affected by climate changes in the Holocene (Barber et al., 2004; Gałka et al., 2015, 2016; Lamentowicz et al., 2015; Mauquoy and Barber, 1999; Tuittila et al., 2007; Van der Linden and Van Geel, 2006). Furthermore, disturbances due to human activities increased in the past several centuries, which led to a huge deposition of minerals and nitrogen on peatland surfaces (Dudová et al., 2012; Ireland et al., 2014; McClymont et al., 2008; Swindles et al., 2015). This led to a decline in the Sphagnum peat-forming species (Hughes et al., 2008). It has also been emphasised that autogenic processes are involved in the development of Sphagnum peatlands (Charman, 2002; Kulczyński, 1949; Osvald, 1923). Recent palaeoecological studies conducted in Finland and Patagonia have revealed that patterns in ombrotrophic peatland development are very complex (Loisel and Yu, 2013; Tuittila et al., 2007), and this might be related to autogenic factors. A similar conclusion was reached by Swindles et al. (2012), who analysed ecohydrological feedbacks in ombrotrophic peatlands in NE Europe.

Insights into the effect of past climate changes on Sphagnum populations are of key importance for modelling future vegetation change. Multi-proxy palaeoecological research on peatlands, particularly involving analyses of fossil organisms (biotic proxies), is important to provide a long-term perspective for peatland conservation and their restoration (Gorham and Rochefort, 2003; Gorham et al., 2001; Lamentowicz et al., 2011). Plant macrofossils and pollen analysis are commonly used in palaeoecological studies to reconstruct the history of peatlands (Feurdean et al., 2015; Gałka, 2014; Novenko et al., 2009; Van der Linden and Van Geel, 2006). Furthermore, testate amoebae (TA) are a sensitive group of protists, which are used as a quantitative proxy for past hydrological changes connected with climate or human activities (Blundell et al., 2008; Charman, 2007; Laggoun-Défarge et al., 2008; Lamentowicz et al., 2013; Markel et al., 2010; Payne, 2010; Schoning et al., 2005; Swindles et al., in press). Indeed, TA respond dynamically to various disturbances including drainage and peat exploitation. Moreover, their communities can be used to track a recent tendency of a bog towards recovery (Mitchell et al., 2008). We assume that with the above-mentioned proxies (especially plant remains and TA), peatland resilience and potential for restoration could be assessed.

In the past 200 years, almost all bogs in Europe have been disturbed (Joosten and Clarke, 2002; Minayeva and Sirin, 2009). In Poland, many of the Baltic raised bogs are under legal protection, most often as nature reserves, and they have been announced as Natura 2000 sites (Herbichowa et al., 2007). Furthermore, in recent years, some restoration efforts have been made within the framework of Life EU projects (Herbichowa et al., 2007; Pawlaczyk et al., 2005). The projects were carried out without high-resolution multi-proxy palaeoecological perspective providing reference conditions, despite information about the target plant communities as reference conditions for restoring peatlands is really needed. Knowledge of past dynamics is important when seeking to predict future changes and avoid pitfalls in the management of natural resources, which is essential for projects to carry out an effective restoration (Bennion et al., 2011; Bunting and Whitehouse, 2008; Seddon et al., 2014; Willis et al., 2010). Peatlands are exceptional ecosystems accumulating atmospheric carbon (Dise et al., 2011; Gorham, 1991). Being very sensitive ecosystems, they are very vulnerable to any disturbances and extremely difficult to restore especially in the present climatic scenario. The better the knowledge about their past dynamics, the better the restoration strategy will be (McCarroll et al., 2015). However, the common notion exists that the neo-ecological approach is enough to appropriately manage with the future of peatlands.

Palaeoecological results presented in this study are part of a wider study of the development of Baltic raised bogs in Northern Poland (Gałka, 2011). The palaeoecological studies conducted in Northern Poland covered four peatlands: Stążki (Gałka et al., 2013b), Mechacz Wielki (Gałka et al., 2013a), Gązwa (Gałka et al., 2015; Gałka and Lamentowicz, 2014) and Bagno Kusowo (Gałka et al., 2014; Lamentowicz et al., 2015). Results of this study were preceded by the analysis of the first core, which has already been published (Gałka et al., 2014). Replication of coring is a reliable method of validation for inferences (Charman et al., 1999; Hendon et al., 2001). Analysis of at least two cores is expected to improve previous interpretations. Nevertheless, data from replicated multi-proxy and high-resolution palaeoecological studies of at least two cores are very rare for the southern Baltic region. Such a strategy has so far been applied only in the case of two Baltic raised bogs: Stążki (Lamentowicz et al., 2011) and Gązwa (Gałka et al., 2015; Gałka and Lamentowicz, 2014), where two parallel peat cores covering whole peat layer were analysed.

This study is partly related to the former profile to improve and validate our published results to broaden the past perspective for the active nature protection. Focussing on the long-term dynamics of Sphagnum-dominated peatland, we aim at (1) reconstructing peatland development dynamics to provide an important past perspective for nature protection and restoration and (2) assessing the impact of climate and human-mediated drainage, as well as the levels of recovery using complementary information from TA and plant macrofossils.

Site location

Bagno Kusowo bog is located in Northern Poland (Figure 1) in the glacial area formed by the activity of the last Scandinavian ice sheet, which retreated from this region approximately 15,000 yr BP (Marks, 2002). The bog is a nature reserve and is part of the ‘Lake Szczecineckie’ Special Area of Conservation (SAC) Natura 2000 (PLH 320009).

Figure 1.

Setting of the study site.

The studied peat bog area of 318.82 ha fills a former lake basin located on a moraine plateau. The average altitude is in the range of 150–160 m a.s.l. The highest elevation – Polska Góra – near the peat bog is 202 m. Mixed and riverside forests as well as fields and meadows surround the raised bog. In the forests, Fagus sylvatica and Pinus sylvestris are the dominant species. In the wetter areas, Alnus glutinosa and Salix spp. occur. In the south-east, the peat bog is adjacent to Lake Wielatowo. The water level in the lake stands at 143 m a.s.l. The analysed site is in a transitional climate with a significant influence of oceanic air masses. The total annual precipitation reaches 650 mm. The average temperature in July is 17°C, whereas in January it is −3°C (Woś, 1999). Bagno Kusowo peat bog was drained in the early 1960s for peat extraction. The peat exploitation occurred in the southern part of the bog, where currently peat pits are undergoing restoration. Despite the construction of ditches, the northern part of the bog is waterlogged and has retained some of its original character. Only this part possesses open treeless surfaces where sparse stands of dwarf pines grow. The species found here include Sphagnum magellanicum, Sphagnum cuspidatum, Sphagnum capillifolium, Baeothryon caespitosum, Eriophorum vaginatum, Drosera rotundifolia and Oxycoccus palustris. S. magellanicum and E. vaginatum are the dominant species growing in the lawn microforms. Hummock-hollow microforms are not common and cover only small patches of the bog. There are also natural small ponds in the northern part of the bog. Based on the drillings conducted by the authors, the maximum peat deposit in the three open spaces of the northern part of the bog is 795 cm, although there might be places with thicker peat layers.

Materials and methods

Coring and subsampling

A core with a length of 100 cm and a width of 10 × 10 cm² was collected in the northern part of the Bagno Kusowo bog using a Wardenaar sampler (Wardenaar, 1986; Figure 1). In the laboratory, a cuboid of 5 × 5 × 100 cm³ was removed and divided into 1-cm layers.

Chronology

Four accelerator mass spectrometry (AMS) radiocarbon dates were performed at the Poznań Radiocarbon Laboratory, and these were used to generate a chronology for the peat profile. For the dating analyses, Sphagnum stems were selected (30–40) from the sample with a thickness of 1 cm and a volume of approximately 25 cm³ (Table 1). The calibration of the radiocarbon dates and the construction of the age–depth were performed with OxCal 4.1 software (Bronk-Ramsey, 2009) and the IntCal13 curve (Reimer et al., 2013) applying a P_Sequence function with a k parameter of 1 cm⁻¹ and 1 cm resolution. The most distinct changes in the peat composition, which might be signals of changes in the peat accumulation rate, were introduced using the ‘boundary’ command. One boundary was established at depth 30 cm according to the hydrological shift identified by TA. The calibrated dates are expressed as years AD and BC.

Table 1.

Radiocarbon datings from Bagno Kusowo, core II. Sphagnum stems were used as dated material in all cases.

Depth (cm)	Laboratory no.	Age ¹⁴C date	Calibrated range 95.4%	Dated material
10–11	Poz-47342	111.94 ± 0.39 pMC	1692–1918	Stems of Sphagnum
41–42	Poz-47343	145 ± 25 BP	1668–1946	Stems of Sphagnum
78–79	Poz-47344	350 ± 30 BP	1457–1635	Stems of Sphagnum
99–100	Poz-47341	620 ± 30 BP	1292–1400	Stems of Sphagnum

pMC: ‘percent Modern Carbon’, with modern or present defined as year 1950.

Plant macrofossils

Plant macrofossils were analysed at 1-cm intervals in contiguous samples with volume 25 cm³. The samples were washed and sieved under a warm water current over 0.25-mm mesh screens. Initially, the whole sample was analysed with the use of a stereoscopic microscope. In this way, the percentage of the individual fossils of vascular plants was determined, and the carpological (fossil) remnants and vegetative fragments (leaves, rootlets and epidermis) were identified using the available identification keys (Mauquoy and Van Geel, 2007). The volume percentages of the different vegetative remains and Sphagnum sections were estimated in increasing steps of 5%. The relative proportions of the taxonomic sections of Sphagnum were estimated under microscope on the basis of the branch leaves on three 32 × 32-mm cover glasses. Therefore, the identification of the Sphagnum at species level was performed separately based on the stem leaves using specialist keys (Hölzer, 2010; Laine et al., 2011) and recent materials. Moss nomenclature follows Ochyra et al. (2003) and vascular plant nomenclature follows Mirek et al. (2002). The volume proportion of amorphous organic matter was estimated in steps of 25% during sieving, and this serves as a proxy of peat decomposition (Gałka et al., 2013b). The results, presented in the form of diagrams of plant macroremains, were prepared in C2 graphics programme (Juggins, 2003). Sphagnum fuscum and Sphagnum rubellum, as well as S. rubellum and S. capillifolium, have been reported together due to the difficulty in differentiating them in the fossil state, particularly in the case of a lack of stem leaves (Gałka et al., 2013b, 2014; Hölzer, 2010; Tuittila et al., 2007).

TA

TA were analysed in subsamples of 6 cm³ taken at 1-cm intervals. The samples were prepared by sieving and back-sieving (Booth et al., 2010). The TA were analysed at a 200–400× magnification, with a minimum of 150 tests per sample whenever possible. The identification was performed at the highest possible taxonomical resolution based on the available literature, which includes Grospietsch (1958), Mazei and Tsyganov (2006) and Ogden and Hedley (1980). Quantitative reconstruction of water-level depth changes based upon changes in the composition of TA taxa was carried out by C2 software (Juggins, 2003), using the training set (consisting 123 samples) developed for Northern Poland by Lamentowicz and Mitchell (2005) and Lamentowicz et al. (2008). A weighted averaging model was applied to reconstruct the past depth to the water level.

Pollen

Samples for pollen analysis were taken from every 4 cm and analysed at the Adam Mickiewicz University in Poznań palaeoecology laboratory. The 1-cm³ sediment samples were prepared following standard methods (Berglund and Ralska-Jasiewiczowa, 1986), including acetolysis, for 3 min. The samples were imbedded in pure glycerine and stained with safranin. An average of ca. 400 terrestrial pollen grains was analysed. The pollen percentages were calculated based on the sum of the trees and shrubs (arboreal pollen (AP)) and herbs (non-arboreal pollen (NAP), except wetland and aquatic plants). Pollen was identified using specialist keys and atlases, particularly Beug (2004). The pollen diagram was plotted using the Tilia/Tilia-Graph software package (Grimm, 1991, 1992).

Results

Lithology and chronology

The analysed 100-cm-long core was predominantly composed of Sphagnum and E. vaginatum. The bottom section (100–60 cm) was dominated by S. fuscum and S. rubellum. From 89 to 83 cm, the amorphous organic matter reached ca. 25%. The upper section was dominated by peat, predominantly formed of S. rubellum, S. capillifolium and S. magellanicum. The presence of Ericaceae rootlets was up to 15%. Between 45 and 38 cm, the abundance of E. vaginatum increased, and between 38 and 31 cm that of S. balticum increased. Based on AMS dating, an age–depth model was prepared (Figure 2). The mean rate of peat accumulation over the entire period was 1.51 mm/yr.

Figure 2.

Age–depth model of the peat profile in Bagno Kusowo bog, core II.

Plant macrofossils

Five phases of the development of the local vegetation were visually delimited (Figure 3):

KBII-ma-1 (AD 1350–1520). Domination of S. rubellum, S. fuscum and E. vaginatum along with the presence of S. balticum in the bottom part of the zone; appearance of S. cuspidatum around AD 1420 and 1520.

KBII-ma-2 (AD 1520–1680). Domination of S. rubellum and S. fuscum up to AD 1630; an increase in the occurrence of S. cuspidatum to 40% around AD 1650; appearance of S. magellanicum; numerous carpological findings of Calluna vulgaris, B. caespitosum and Andromeda polifolia; the first presence of P. sylvestris.

KBII-ma-3 (AD 1680–1860). Domination of S. rubellum, S. capillifolium and E. vaginatum; continuous presence of S. balticum and S. magellanicum.

KBII-ma-4 (AD 1860–1940). Presence of four Sphagnum species, including S. balticum, S. magellanicum, S. rubellum and S. capillifolium and very low representation of macrofossils of vascular plants.

KBII-ma-5 (AD 1940–2010). Presence of S. rubellum and S. capillifolium (dominance in the bottom part) and S. magellanicum (dominant species in the upper part); numerous C. vulgaris remains; appearance of O. palustris and Betula pubescens.

Figure 3.

Percentage plant macrofossils diagram presenting local vegetation development in Bagno Kusowo bog.

TA

Five TA zones were visually determined, representing different hydrological states of the bog (Figure 4):

KBII-am-1 (AD 1350–1480). Decreasing abundance in Archerella flavum (from 40% to <20%) and increasing representation of Arcella discoides to ca. 80%.

KBII-am-2 (AD 1480–1640). Amphitrema wrightianum reaches a peak in abundance with A. flavum; considerable decrease in A. discoides; gradual increase in Cryptodifflugia oviformis; a peak in Assulina muscorum.

KBII-am-3 (AD 1640–1800). Decline and posterior increase in A. flavum with simultaneous increase in C. oviformis; the top of the zone is a transition from wet to dry habitat conditions.

KBII-am-4 (AD 1800–1970). Substantial increase and domination of C. oviformis; regular fluctuations of A. discoides and complete disappearance of A. flavum.

KBII-am-5 (AD 1970–2010). Reappearance of A. flavum and drop in C. oviformis; increase in the presence of Hyalosphenia elegans and Physochila griseola.

Figure 4.

Percentage testate amoebae diagram presenting water table changes in Bagno Kusowo bog.

Pollen

Three local pollen assemblage zones were visually distinguished (Figure 5):

KBII-po-I (AD 1380–1530). Presence of deciduous forests near the peatland, including Fagus, Quercus, Carpinus and Alnus; increase in the proportion of Ericaceae around AD 1480; low presence of pollen of cereals and agricultural weeds such as Triticum, Secale and Centaurea cyanus.

KBII-po-2 (AD 1530–1950). A decrease in deciduous trees, most evidently Carpinus and Corylus; increased presence of pollen of species such as Triticum, Secale, Fagopyrum, C. cyanus and Plantago lanceolata; an increase in the contribution of Ericaceae pollen between ca. AD 1830 and 1920 (Calluna and Vaccinium type); a rapid increase in Sphagnum spores around AD 1720.

KBIII-po-3 (AD 1950–2010). An increase in pollen from coniferous trees Pinus and Picea; a decrease in the contribution of cereals and plants indicative of human activity; low occurrence of Sphagnum spores.

Figure 5.

Percentage pollen diagram presenting vegetation changes and human activity in the surrounding of Bagno Kusowo bog.

Discussion

Palaeohydrology, climate and land-use change

Both Bagno Kusowo peat profiles recorded two water-level decreases, which may correspond to periods of climate cooling (Figure 6). The first substantial water-level decrease occurred during the Maunder Minimum (AD 1645–1715) and the second occurred in the Dalton Minimum (AD 1790–1830). A successive decrease in the water level in core II continued from AD 1480 (i.e. it commenced in the period of the earlier climate cooling in the Spörer Minimum). A water-level decrease and fluctuations in the peatland triggered the development of vascular plants, for example, C. vulgaris, S. fuscum, S. rubellum and S. capillifolium grew together with B. caespitosum. Among TA, C. oviformis, which is an indicator of the ground water decrease (Lamentowicz and Mitchell, 2005), showed a gradual increase during this period. The appearance of macrofossils of B. caespitosum in peat consisted of S. fuscum and S. rubellum in the periods of water-level decrease, and fluctuation over the last millennium has also been documented in other peatlands in Northern Poland (Lamentowicz et al., 2008, 2009, 2011). However, Bragazza (2006) suggested that B. caespitosum prefers a wetter part of the water-level gradient in alpine conditions. In comparison with the same study, E. vaginatum occurred in much wider section of the hydrological gradient, which suggests that this species might be a good indicator of water-level fluctuations. Therefore, we can interpret the occurrence of B. caespitosum microfossils as an indicator of water-level disturbance in the history of Kusowo peatland development. B. caespitosum occurs in wetter habitat than C. vulgaris; however, in oceanic climate, such as in Scotland, it grows often with C. vulgaris (Ingram, 1964). In Central Ireland, B. caespitosum was found on the drier areas of the pool and hummock complex (Tansley, 1939). Furthermore, Osvald (1949) described British bog community “Calluna–Scirpus caespitosus–Rhacomitrium Sociations,” which occupied the most exposed dry areas of blanket bog. C. vulgaris currently occurs in Baltic raised bogs, especially on the dry hummocks and extensive areas of drained peatlands; therefore, it can be regarded as an indicator of hydrological disturbances (Herbichowa, 1998; Wołejko et al., 2005).

Figure 6.

Comparison of chosen taxa from three data sets: pollen, plant macrofossils and testate amoebae with the quantitative reconstruction of depth-to-water table (DWT).

S. fuscum, S. rubellum and S. capillifolium are species occurring in a similar habitat; mostly relatively, they not only form dry hummocks but also form moss carpet (Hölzer, 2010; Laine et al., 2011; Rydin and McDonald, 1985; Smith, 2004). Furthermore, E. vaginatum (dominating during the second dry phase) may be regarded as an indicator of water-level decrease/instability in an ombrotrophic peatland, as confirmed by modern studies (Lavoie et al., 2005; Silvan et al., 2004), and fossil vegetation (Gałka et al., 2013b, 2014; Herbichowa, 1998). E. vaginatum can currently grow in a peatland with a seasonal water-level drop of up to as much as 72 cm below the peatland surface (Bragazza and Gerdol, 1996). Although E. vaginatum occurs in ombrotrophic peatlands, it is not a dominant plant in well-hydrated peatlands with stable hydrology (Lavoie et al., 2005). In the case of the two analysed cores, macrofossils of E. vaginatum were found in periods of climate cooling and water-level decrease in the Bagno Kusowo peatland (Figure 6). The increase in E. vaginatum macrofossils in drier periods was also documented in Tăul Muced bog in Eastern Carpathian Mountains (Gałka et al., 2016). Moreover, regarding TA, the presence of A. discoides (that is often found together with macrofossils of E. vaginatum) suggests hydrological instability (Lamentowicz et al., 2009). This species is often found together with E. vaginatum in bog peat profiles from Northern Poland (e.g. Marcisz et al., 2015). However, in this study, both species co-occurred mostly until ca. AD 1550. Later, E. vaginatum reappeared (ca. AD 1800–1850), but A. discoides did not respond so clearly. C. oviformis could likely become dominant during this period, as an effect of a climatic change towards drought rather than hydrological instability.

A possible explanation of changes in Sphagnum communities observed in this study might be related to the climatic shifts that took place during the last millennium. The Mediaeval Warm Period of the 9th–13th centuries AD was followed by a period of climate cooling called the ‘Little Ice Age’ (LIA; 15th–19th centuries AD; Büntgen et al., 2011). Climate change caused hydrological disturbances and changes in vegetation in ombrotrophic peatlands, which has been documented in various parts of Europe (De Vleeschouwer et al., 2009; Feurdean et al., 2015; Gałka and Lamentowicz, 2014; Gałka et al., 2014; Mauquoy et al., 2002, 2004, 2008; Swindles et al., 2007).

In addition to the effect of climate on changes in the vegetation of the Bagno Kusowo peatland, the impact of human activity should also be considered. During the lowest water levels in the Bagno Kusowo peatland (approximately between AD 1680 and 1820), substantial deforestation occurred in the catchment area (Figure 6). Deciduous forests dominated by beech, hornbeam and oak were cut down and transformed into cultivated fields. This increase in human activity is manifested by an increase in the human indicator curve. At this time, the amount of pollen grains (Secale and Triticum) and plants typical for pastures increased (P. lanceolata and Rumex acetosa-acetosella; Figure 6). The anthropogenic transformation of the catchment area during the past several centuries likely resulted in hydrological instability in the peatland and dust deposition on the peatland surface (Ireland et al., 2014; Swindles et al., 2015). The issue of human activity in the period AD 1400–1800 and its impact on vegetation were described in more detail in the context of the analyses of the first previous core (Gałka et al., 2014).

In the case of the two remaining analysed Baltic raised bogs located in north Poland, Słowińskie Błota (Lamentowicz et al., 2009) and Stążki (Lamentowicz et al., 2011), a decrease in water level has also been observed in periods of cold climate. This decrease most likely caused the expansion of E. vaginatum and the reduction of S. fuscum abundance. A ground-water-level decrease between AD 1550 and 1750 has also been documented in the montane Tăul Muced peatland in north Romania (Feurdean et al., 2015). Hiatus in peat deposit in NE Czech Republic was documented between AD 1320 and 1954, possibly connected to extreme droughts (Dudová et al., 2012). Contrasting situation occurred in North-Western Europe and Scandinavia. In Great Britain (Barber et al., 2003; Charman et al., 2006), Ireland (Swindles et al., 2010), Denmark (Mauquoy et al., 2008), Finland (Väliranta et al., 2007) and Sweden (Andersson and Schonig, 2010), an increase in climate humidity has been documented during the LIA. The water level increased in these peatlands, corresponding to the development of communities of Sphagnum from the section Cuspidata and TA-based reconstruction.

The differences in moisture pattern during LIA between NE Europe and CE Europe may be due to the fluctuations in large-scale atmospheric circulation, which had different influences on peatland development (Feurdean et al., 2015). The dissimilarity of the peatlands palaeohydrology in Europe may be explained by the position of the jet stream in CE Europe to Carpathians during the LIA (dry/unstable) and NW Europe (wet). However, this interpretation should be better explored in the future with multiple fine-resolution reconstructions. Air mass changes and shifts in the southerly jet stream as the opposite response (to dryness) of ombrotrophic peatland in Tierra del Fuego during LIA were invoked by Chambers et al. (2014). Furthermore, the differences in moisture changes during LIA between a moist arid Central China and a dry East China may be clarified by the presence of westerly jet stream (Fallah et al., 2016).

S. cuspidatum reappeared in Bagno Kusowo at the core II ca. AD 1600. This may be interpreted as a water-level increase. S. cuspidatum is a species that occurs in the wettest parts of ombrotrophic peatlands, growing in hollows (Hölzer, 2010; Laine et al., 2011). An increase in water level ca. AD 1600 also documents an increase in the occurrence of A. wrightianum (Figure 4), indicative of very wet habitats (Lamentowicz and Mitchell, 2005) and often one of the dominant species in peat developed by S. cuspidatum (Gałka et al., 2014; Mauquoy et al., 2008; Sillasoo et al., 2007). Disappearance of this species ca. 1650 in coring site might be related to the onset of the Maunder Minimum or autogenic change of peatland development (Morris et al., 2015). The presence of S. cuspidatum ca. AD 1600 and 1650 in Bagno Kusowo may indicate short-term increasing moisture in East Central Europe. In Hungary, in peatland Sirok Nyirjes, the development of Sphagnum communities was documented ca. AD 1600, where S. cuspidatum played a main role (Jakab and Sümegi, 2011). However, this species did not occur in the former core, and this suggests a diversity of peatland microforms that changed differently in space and time on the peatland surface. Consequently, despite this difference, both cores stored the hydrological signal that was preserved in TA communities.

Ca. AD 1800, the water level was decreasing, as evidenced in the case of the two analysed cores of the Bagno Kusowo bog (Figure 6), with the reappearance of S. balticum in core I and increase in core II. According to TA-based reconstruction, the water level was between 15 and 25 cm below the peatland surface. Currently, S. balticum occurs frequently in well-hydrated Baltic raised bogs (Laine et al., 2011; Rydin and McDonald, 1985), where the water level ranges between 0 and 25 cm. S. balticum is most frequently found in habitats with a water level of approximately 5 cm (Väliranta et al., 2007). This species is frequently recorded in peat cores sampled in Baltic raised bogs (Gałka et al., 2015; Sillasoo et al., 2007; Väliranta et al., 2012). It is considered to be an indicator of wet shifts in ombrotrophic peatlands (Sillasoo et al., 2007; Väliranta et al., 2007). The change in climatic conditions towards increased water level since ca. AD 1850 was also documented in Tăul Muced bog (Feurdean et al., 2015). However, in opposition to our data and from those of Tăul Muced in Western Europe and Scandinavia, drier climatic conditions were indicated (Charman et al., 2006; Mauquoy et al., 2008; Väliranta et al., 2007; Van der Knaap et al., 2011). The different pattern of the moisture conditions in East Central Europe during the past centuries and millennia (e.g. Feurdean et al., 2015; Gałka et al., 2013b, 2014) shows that further research is needed to understand the differences. However, the S. balticum occurred in small percentage in both cores, and therefore, it cannot be a strong quantitative indicator of very wet habitat dominated by S. fuscum.

The results of TA indicate increased water level (ca. 3–10 cm below surface in two cores) after AD 1950, which suggest a resilience of peatland after anthropogenic disturbances. This is in contrast to the observation of increased C. vulgaris macrofossils that usually indicate dry habitat. The differences in the proxies (TA and plant macrofossils) which sometimes indicate opposite moisture conditions were fully explained by Väliranta et al. (2012) during reconstructing peatland water level using transfer functions for plant macrofossils and TA from two boreal peatland sites in Finland and Estonia. In Kusowo core II, however, C. vulgaris macrofossils percentage was very small during that period – much larger in core I which again suggests a diversity of microforms and importance of sampling spot that cannot be extrapolated onto entire peat surface. It is actually surprising how well both TA quantitative reconstruction trends are linked, despite differences in past plant composition. Therefore, our first important message is that TA reflect long-term hydrological change, despite microhabitat variability.

Drainage, resilience and nature protection

The analysis of two cores showed a recovery of the peatland after 1950s from the disturbance caused by peat exploitation and drainage. The water level fluctuated between 5 and 10 cm after AD 1950 in comparison with the lowest recorded values reaching over 30 cm ca. AD 1820 (Figure 5).

Disappearance or low abundance of S. balticum and A. flavum (mixotrophic TA, dominating prior to AD 1800) ca. AD 1920 suggests that drainage had an impact on the whole peatland ecosystem even in non-drained northern part of the peatland.

The relative abundance of A. flavum has been increasing for the past several decades, which indicates ecosystem recovery along with the present water-level depth of ca. 10 cm. Actually this water-level position (that is also reflected in TA functional traits) can be a clue for the good state of ombrotrophic bogs (Lamentowicz et al., 2015). This is the second important message – for active nature conservation, water level of 10 cm depth should be a target in peatlands’ management.

Good preservation of ombrotrophic plant communities of the Bagno Kusowo dominated by Sphagnum may be due to exceptional hydrology as well as may be due to a not-extensive drainage in the northern part of the bog.

Hydrological disturbances since AD 1800 caused by digging drainage ditches are represented by the predominance of vascular plants. After AD 1950, E. vaginatum and C. vulgaris abundantly increased at the sampling sites. These two species are currently common in the northern part of the Bagno Kusowo bog, but they are not so abundantly found as in other sites with much more disturbed hydrology such as Słowińskie Błoto (Lamentowicz et al., 2009) or Stążki (Lamentowicz et al., 2008). Although the results did not document lower wetness for TA in coring site II, we documented disappearance of S. balticum and lower abundance of S. magellanicum ca. AD 1950, which may be due to recent warming trend (Breeuwer et al., 2010) or due to increasing nitrogen that interacts with warming (Limpens et al., 2011).

Hydrological disturbances have also led to the P. sylvestris afforestation in S and N parts of the peatland where P. sylvestris occurs in a dwarf form. The most common Sphagnum species in the Bagno Kusowo bog are now S. magellanicum, S. fallax and S. rubellum. Therefore, Sphagnum population of the northern bog area typical of peat-forming ombrotrophic bog survived the drainage and peat exploitation. A similar pattern is present in the Mechacz Wielki bog (area of 146 ha) located in North-East Poland, which was also subjected to drainage practises in the past. The drainage of this bog did not result in the disappearance of Sphagnum species typical for Baltic bogs such as S. fuscum, S. rubellum and S. cuspidatum (unpublished data). Unfortunately, there are few examples of peatlands where Sphagnum species typical of Baltic raised bogs survived drainage practices. Smaller peatlands were damaged and ombrotrophic species disappeared (Herbichowa et al., 2007; Pawlaczyk et al., 2005). Similar situation has taken place in Słowińskie Błoto bog (Lamentowicz et al., 2009).

In contrast, the drainage of the Gązwa bog (204.76 ha), for instance, resulted in the disappearance of the populations of S. fuscum, S. rubellum and S. magellanicum, which have been replaced with assemblages dominated by S. angustifolium and S. fallax according to two peat profiles (Gałka and Lamentowicz, 2014; Gałka et al., 2015). The third important message that comes from our work is that sites such as Kusowo and Mechacz are extremely important to preserve the remains of pristine biodiversity (including genetic diversity of plants and protists) that was completely removed from the most of raised bogs in Europe.

High-resolution palaeoecological studies are rare but crucial for a good understanding of the present ecosystem (Willis and Bhagwat, 2010). Such studies provide important information on the long-term dynamics of Sphagnum-dominated peatlands (Chambers et al., 2013). Modern ecological surveys are important but are not sufficient to assess the state and resilience of bogs. Their long-term developmental history in terms of their species composition and response to the past land-use and climate change can be identified using a range of palaeoecological techniques. Many sites are regarded as close to pristine; however, detailed studies show the extent of the damage that was done usually 60–100 years ago. An important example is Linje mire (5.8 ha), which was exploited and drained 100 years ago (Marcisz et al., 2015). The hydrology of this site was considerably disturbed and all former Sphagnum species (S. magellanicum and S. fuscum) were completely replaced with monospecific patches of S. fallax. The site is protected as a nature reserve and as Natura 2000 site being microrefugium of Betula nana. However, it should be strongly emphasised that the Sphagnum cover (formerly dominated by S. magellanicum) was completely exchanged. In reality, sites still possessing the partly pristine vegetation are much more important for nature conservation than those after severe secondary succession. In conclusion, more effort should be taken to sustain and restore sites closer to natural conditions.

Human-mediated disturbances promote long-term impact on peatlands through different activities such as forest exploitation, intensive clear cutting and drainage and peat exploitation (Lamers et al., 2015). As a result, most of the peatlands have transformed ecosystems that are not impossible to return to their pristine condition. Several sites in Northern Poland have been restored through ditch damming (Pawlaczyk et al., 2005). Currently, the conservation priority is to assess the effects/success of restoration on hydrology and developmental trends of Baltic bogs. Palaeoecological studies of Bagno Kusowo (this study), Gązwa (Gałka et al., 2015) and Linje (Marcisz et al., 2015) show how different the pathways of Sphagnum communities can be from a persistent degraded state to relatively healthy assemblages dominated by S. fallax (secondary community) or S. magellanicum–S. cuspidatum community close to the pristine conditions. However, our study reinforces the notion that macrofossil analyses can be supported to indicate ecosystem conservation stage, pointing to the key complementary role of protists–TA as key indicators of hydrology and food-web structure in Sphagnum (Jassey et al., 2013).

Conclusion

Drainage activities during the 20th century have caused irreversible damage to many raised peat bogs in the south Baltic zone. Good preservation of ombrotrophic plant assemblages of the Bagno Kusowo dominated by Sphagnum may be due to the exceptional hydrology as well as the size of the peatland and the thickness of the peat layer of approximately 8 m, making the peatland difficult to drain.

Palaeoecological analyses, particularly plant macrofossil and TA analyses, constitute an important tool for the determination of present ecosystem conservation stage and reference (target) conditions for the restoration of ombrotrophic peatlands. High-resolution multi-proxy analyses enable to reconstruct climate and drainage impact on vegetation, particularly the ombrotrophic Sphagnum communities constituting well-preserved raised bogs.

Footnotes

Acknowledgements

We are indebted to Adam Hölzer for help in the identification of fossil of species Sphagnum.

Funding

This study was financed through a grant from the National Science Centre (NCN) that was awarded for the project Multi-proxy study of the Baltic bogs in N Poland with the aim to provide reference conditions for active nature protection (no. NN305062240) and a grant from Switzerland through Swiss contribution to the enlarged European Union (PSPB-013/2010) and from National Science Centre (NCN; no. 2015/17/B/ST10/01656).

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