Relict occurrences of boreal brown-moss quaking rich fens in the Carpathians and adjacent territories
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Quaking rich fens dominated by boreal semi-aquatic brown-mosses such as Scorpidium scorpioides and Calliergon trifarium are extremely rare in the Carpathians. These fens harbour endangered species persisting at few localities in the region. However, their phytosociological classification has not been sufficiently solved yet, because they lack Sphagnum species as well as calcicole species characteristic for the Caricion davallianae alliance. A recent pan-European synthesis on fen vegetation suggests that these fens belong to the Stygio-Caricion limosae alliance (boreal rich fen vegetation). The isolated occurrence of this alliance southward of the boreal zone and outside the Alps is rather exceptional and might represent a relict from an early post-glacial period. In this study, we compared phytosociological data for the Stygio-Caricion limosae alliance between Northern Europe and the Carpathians plus adjacent regions (the Bohemian Massif, the Dinaric Alps) using NMDS and cluster analysis. We found that the species composition of brown-moss quaking rich fens in Central and Southeastern Europe corresponds well with that in Northern Europe, confirming their assignment to Stygio-Caricion limosae. We further reconstructed the potential past distribution of the alliance in Czech Republic and Slovakia using available floristic and macrofossil data. Macrofossil data suggest that this vegetation type had been much more common in Central Europe and that today it persists only in ancient fens, showing the long-term stability of environmental conditions. The main causes of its present-day rarity are Middle-Holocene woodland phases in fens and recent water table decreases caused by anthropogenic deterioration of the water regime in the landscape.
KeywordsClassification Macrofossil Mires Plant communities Vegetation Wetlands
The Carpathian mountains are an important biogeographical interface in Europe where northern (boreal, arcto-alpine) and southern (temperate, Mediterranean) biogeographical elements meet as a consequence of a specific distribution of glacial and postglacial refugia (Novák 1954; Mráz and Ronikier 2016; Jamrichová et al. 2017). The region harbours a wide array of vegetation types that reflect both climatic and edaphic variability, including different types of fens (Dítě et al. 2018). Nowadays, only few fens act as refugia of rare boreal and boreo-continental species such as Calliergon trifarium, Carex chordorrhiza, C. limosa, Juncus triglumis, Meesia triquetra, Scheuchzeria palustris and Scorpidium scorpioides (Rybníček and Rybníčková 1965, 1972; Coldea 1990; Migra and Mičeta 1997; Janovicová 1998; Migra and Šoltés 1998; Dítě and Pukajová 2002a,b; Dítě and Kubandová 2005; Dítě and Šoltés 2010; Dítě et al. 2017). Such fens occupy sites with a high water level and moderately high concentrations of dissolved carbonates. Due to these conditions, they can be characterized as ‘quaking rich fens’. However, the phytosociological classification of these floristically-unique habitats remains unresolved.
The vegetation of these fens is mostly composed of brown mosses and, unlike other types of rich fens, frequently lacks Sphagnum species. Furthermore, calcium-demanding species typical of the extremely rich fens of the Caricion davallianae alliance (e.g. Blysmus compressus, Carex davalliana, Philonotis calcarea, Primula farinosa) are generally absent. In the Carpathians such vegetation type has been previously assigned either to the Caricion lasiocarpae alliance (Rybníček et al. 1984; Coldea 1990; Coldea et al. 1997; Hájek and Háberová 2001), which is, however, an ambiguous name (Mucina et al. 2016), or to the Caricion davallianae and Sphagno warnstorfii-Tomentypnion nitentis alliances (Dítě et al. 2007). In Scandinavia similar vegetation has been classified as the Caricion lasiocarpae (Dierssen 1996) or the Stygio-Caricion limosae alliance (Nordhagen 1943; Dahl 1956; Moen et al. 2012).
A recent pan-European synthesis and classification of fen vegetation based on large set of vegetation-plot data and formalized classification approach (Peterka et al. 2017) has classified brown-moss quaking rich fens in the Carpathians (Slovakia, southern Poland, Romania) into the Stygio-Caricion limosae alliance as originally described by Nordhagen (1943). This alliance is widespread in the boreal zone of Europe, but it has never been distinguished in temperate Europe before that synthesis. Peterka et al. (2017) detected relatively frequent occurrence of the alliance in the Alps and few isolated occurrences in the Carpathians, the Dinaric Alps and, with certain compositional dissimilarity, in the Bohemian Massif (Czech Republic). All these isolated fens of the Stygio-Caricion limosae alliance harboured regionally rare species – boreal and arcto-boreal fen elements (Rybníček 1966; Rybníček and Rybníčková 1972; Birks and Walters 1973; Erzberger and Papp 2007; Štechová et al. 2010; Šoltés et al. 2015; Dítě et al. 2018). In temperate Europe these species are considered to be glacial or postglacial relicts, i.e. organisms that were common and widely distributed in glacial and postglacial times, but which retreated during interglacials to restricted areas with habitat or climatic conditions analogous to those of their original glacial habitats (Pearson 1965; Frahm 2012; Dítě et al. 2018). Since the vegetation of the Stygio-Caricion limosae alliance contains a high number of species considered as glacial relicts, we hypothesize its wider distribution during the Late Glacial and Early Holocene. This hypothesis could be tested by means of mapping macrofossil records, especially those of bryophytes, which are usually well preserved in the sediments and may be identified at the species level (Janssens 1983). A certain picture of the potential historical distribution of the alliance can be provided also using the floristic data.
A focus on vegetation classification uncertainties may seem a purely academic exercise, but vegetation classification, which has a long tradition in Europe, has recently gain practical importance by underlying habitat typology that acts as a basic tool for nature conservation at both national and pan-European scales (De Cáceres et al. 2015; Chytrý et al. 2016). Thus, disharmony in the classification concepts and approaches may complicate effective habitat protection, especially if the target vegetation might represent a relict type that developed under presently non-existing climatic or edaphic conditions or both.
In this study, we therefore aimed (i) to evaluate whether the vegetation of brown-moss quaking rich fens in the Carpathians and adjacent areas differs from that of the Stygio-Caricion limosae alliance from Northern Europe, whence the alliance had been described, and (ii) to map the potential historical distribution of the alliance in Central Europe (Czech Republic, Slovakia) using both the macrofossil records and floristic data confronting recent and historical distributions and suggest the reasons behind the decline of these exceptional fens in temperate Europe.
Material and methods
What is the Stygio-Caricion limosae alliance?
The Stygio-Caricion limosae alliance includes the vegetation of rich fens (sensu Sjörs 1952; Malmer 1986; Hájek et al. 2006) in topogenic, strongly waterlogged wetlands with peat accumulation. Stands are characterized by a well-developed moss layer consisting mainly of ‘brown mosses’, i.e. non-sphagnaceous weft-forming mosses (see also Udd et al. 2015). Most typical and often dominating bryophytes are Calliergon trifarium and Scorpidium scorpioides. They can be accompanied by other brown mosses such as Calliergon giganteum, Campylium stellatum s.l., Cinclidium stygium, Drepanocladus exannulatus and D. revolvens agg. Sphagnum species occur rather sporadically. The herb layer is composed of sedges widespread in boreal and sub-arctic areas such as Carex chordorrhiza, C. diandra, C. lasiocarpa, C. limosa and C. livida, other species of the Cyperaceae family (Eriophorum angustifolium, Scirpus cespitosus, S. hudsonianus) and herbs of oligotrophic to mesotrophic aquatic and semi-aquatic habitats (Menyanthes trifoliata, Pedicularis palustris, Potentilla palustris). The occurrence of bladderworts (Utricularia spp.) is also typical for the alliance because of the high water level.
The Stygio-Caricion limosae alliance was described by Nordhagen (1943) from the central part of southern Norway. The locus classicus is located in Sikilsdalen Valley, in the east of the Jotunheimen Mts. Nordhagen (1943) distinguished four associations within the alliance: Stygio-Caricetum chordorrhizae [Amblystegio scorpioidis-Caricetum chordorrhizae Osvald 1925], Stygio-Caricetum limosae [Amblystegio scorpioidis-Caricetum limosae Osvald 1923], Stygio-Caricetum lasiocarpae and Stygio-Eriophoretum polystachyi. The alliance has been recognized also by other Scandinavian phytosociologists (Dahl 1956, 1987; Sjörs et al. 1965; Singsaas 1989; Moen et al. 2012).
The nomenclature was harmonized following Tutin et al. (1968, 1969, 1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993) for vascular plants and Frey et al. (2006) for bryophytes. Closely related taxa were merged (for details see Table S1 in the Electronic supplementary material). Nomenclature of vegetation units generally follows Mucina et al. (2016). In other cases, the author citation is given with the first reference.
Our study is focused on the vegetation of the Carpathians and adjacent territories (the Dinaric Alps and the Bohemian Massif), with detailed focus on the former Czechoslovakia from where macrofossil and floristic records are available. For comparison between the study area (Fig. 1) and Northern Europe, we used the dataset of vegetation-plot records (phytosociological relevés) compiled for the pan-European synthesis and classification of fen vegetation (Peterka et al. 2017; see the next section).
The dataset contains plots from the European Vegetation Archive (EVA; Chytrý et al. 2016), from private database of the authors of this study and several unpublished plots gathered by the late Kamil Rybníček. For this study we selected plots that may belong to the Stygio-Caricion limosae alliance as defined in Peterka et al. (2017). We selected not only ‘core plots’ (i.e. the relevés fully matching the formal definition; terminology adopted from Peterka et al. 2017), but also ‘non-core plots’ (i.e. relevés not matching the formal definition but showing a high degree of compositional similarity to core plots). For a detailed description of the methodology, see Peterka et al. (2017). Within our study area, core plots cover three localities in the Western Carpathians (two near the village of Beňadovo and the Puchmajerovej jazierko lake in Slovakia, and one at the Polana Biały Potok locality in Poland), one locality in the Eastern Carpathians (fen near Știol lake in the Rodna Mts, Romania) and one locality in the Dinaric Alps (Barno jezero lake in the Durmitor Mts, Montenegro; Table S2, Figures A–D). Within these areas, the non-core plots cover ≈ 30 localities (Table S3).
Vegetation data processing
To assess the similarity of plots from another point of view and the internal variability of Stygio-Caricion limosae, non-hierarchical K-means cluster analysis was applied. The number of clusters was subjectively set to six, since several preliminary classifications with different parameters proved that more clusters would lack clear ecological or geographical interpretation. The species cover values were square-root transformed and the classification algorithm was repeated 20 times. The diagnostic species of each particular cluster (henceforth referred to as a group) were determined using the phi-coefficient (Chytrý et al. 2002) with all clusters standardized to an equal size. Species with fidelity to a particular group with a phi > 0.3 were regarded as diagnostic. The significance of fidelity was verified using Fisher’s exact tests (P < 0.001). The resulting groups from the K-means clustering were further visualized in the NMDS ordination diagram (Table S4, Fig. E).
Palaeoecological and floristic data
To reveal the potential historical distribution of Stygio-Caricion limosae, we used macrofossil and floristic data and restricted the analysis only to the Czech and Slovak Republics due to the lack of available data from other regions.
The macrofossil data were obtained from the Macrofossil Database of the Czech and Slovak Republics (http://www.sci.muni.cz/botany/mirecol/paleo). Most of them have been published in Rybníček and Rybníčková (1968), Jankovská (1970, 1980), Rybníčková and Rybníček (1972), Hájková et al. (2015a), and Gálová et al. (2016). From these sources, we extracted data on Calliergon trifarium, Carex chordorhiza, C. lasiocarpa, C. limosa, Cinclidium stygium and Scorpidium scorpioides, i.e. the diagnostic species of the Stygio-Caricion limosae alliance (cf. Peterka et al. 2017) with the exception of the very common species Menyanthes trifoliata. Individual localities were considered a potential fossil record of the Stygio-Caricion limosae alliance if (i) any of the above-listed diagnostic moss species strongly dominated the peat sample (i.e. the peat was almost completely built of remains of the species) or (ii) at least two diagnostic species co-occurred in the sample. Note that each macrofossil record in our dataset comes from the peat sediment (i.e. fossil is deposited exactly at the place where the recorded species grew). In addition, macrofossil data captures very small plots, mostly cores ≤ 5 cm in diameter. Thus, the recorded species co-occured at a very small spatial scale. The age of fossil samples is reported in calibrated years before present (i.e. before the year 1950) and it is based either directly on the 14C date of the layer in which the target species was found, or it is extrapolated from the depth-age model.
Floristic data for vascular plants were adopted from Kaplan et al. (2016) for C. chordorrhiza in the Czech Republic, Řepka and Grulich (2014) for C. lasiocarpa and C. limosa in the Czech Republic, and Dítě and Pukajová (2002a,b) and D. Dítě (unpublished data) for the same sedges in Slovakia. Floristic data for bryophytes were taken from a review of all published records, herbarium records and own unpublished records (Hájková et al., in prep.) on which P.H., T.Š., D.D., T.P., Z.P. and M.H. participated. Widely and ambiguously localized records were not considered either for vascular plants or for bryophytes in any country. Individual localities were assigned as potential historical record of the Stygio-Caricion limosae alliance if at least one diagnostic, and usually dominant, moss species (either Calliergon trifarium or Scorpidium scorpioides) co-occured with at least one diagnostic species of sedge (Carex chordorrhiza, C. lasiocarpa, C. limosa).
Comparison of species composition of brown-moss quaking rich fens between regions
The position of plots in the NMDS diagram (Fig. 2) shows that species composition of core plots of Stygio-Caricion limosae from the Western Carpathians (Slovakia, southern Poland) is similar to the species composition of core plots from Northern Europe. As regards the locality at Barno jezero lake (Durmitor Mts), one core plot gathered at the locality appeared to be similar to plots from Northern Europe whereas another plot displayed a slightly marginal position within the cluster of core plots together with the core plots from the Eastern Carpathians (Romania). The non-core plots from the entire Carpathians, the Bohemian Massif and the Durmitor Mts overlap generally with the non-core plots from Northern Europe in the NMDS scatterplot. Hence, NMDS analysis revealed that plots identified as vegetation of Stygio-Caricion limosae sampled in the Carpathians and adjacent territories generally do not differ in species composition from those sampled in Northern Europe. Analogous results were obtained by the K-means clustering (Table S4), in which the plots from Central and Southeastern Europe were grouped together with those from Northern Europe. The resulting groups of vegetation plots corresponded to vegetation types, roughly at the level of phytosociological associations, rather than to geographical regions. For syntaxonomical synopsis of the Stygio-Caricion limosae alliance in the Carpathians and adjacent territories, see Table S5.
Potential historical distribution of boreal brown-moss quaking rich fens in the Czech and Slovak Republics
Potential historical localities of brown-moss quaking rich fens, as indicated by floristic data, largely overlap with localities of vegetation plots of the Stygio-Caricion limosae alliance (see Fig. 3 and Table S6). Only few potential historical sites where the target vegetation has not been recorded are somewhat isolated geographically (e.g. Vidnavské mokřiny in Opava region and several sites on the Labe River floodplain). In most cases, they have not been investigated palaeoecologically. Other potential historical sites detected exclusively on the basis of floristic data are located in the Třeboň Basin and the Žďárské vrchy Hills, though nearby the phytosociologically approved localities.
Our study revealed that the Carpathian brown-moss quaking rich fens with boreal species do not differ from the vegetation of the Stygio-Caricion limosae alliance in Northern Europe. Hence, the name Stygio-Caricion limosae should be newly used even for the Carpathian communities. On the one hand, previous vegetation surveys on the territory of the Carpathians (Rybníček et al. 1984; Coldea 1990; Coldea et al. 1997; Hájek and Háberová 2001; Dítě et al. 2007) did not recognize the Stygio-Caricion limosae alliance. On the other hand, at the association level, Czech and Slovak phytosociologists distinguished Amblystegio scorpioidis-Caricetum chordorrhizae and Amblystegio scorpioidis-Caricetum limosae, which were recognized by Nordhagen (1943) within the description of the Stygio-Caricion limosae alliance. Therefore, a compositional similarity of several Carpathian brown-moss quaking rich fens to the analogous boreal vegetation had been previously recognized, though it remained obscured by applying the widely defined, and ambiguously interpreted phytosociological unit – the Caricion lasiocarpae alliance (Rybníček et al. 1984; Hájek and Háberová 2001; for details see Table S4). The ambiguous status of Caricion lasiocarpae stems partly from broad interpretation of this phytosociological unit and partly from its various interpretations in different vegetation surveys (see Mucina et al. 2016 for an overview). Fen vegetation from the Rodna Mts in the Eastern Carpathians was described as a separate association Swertio perennis-Caricetum chordorrhizae and assigned, again, to the Caricion lasiocarpae alliance (Coldea 1990; Coldea et al. 1997). Classifications of brown-moss quaking rich fens in the Carpathians were obviously complicated due to the low number of existing localities and, consequently, due to the low amount of phytosociological data. We believe that direct comparisons of vegetation plots with material from northern Europe brought a sufficient evidence for their classification within the Stygio-Caricion limosae alliance.
Historical distribution and reasons for the decline of brown-moss quaking rich fens in Central Europe
Although being increasingly endangered across Europe (Janssen et al. 2016), brown-moss quaking rich fens are still quite common in the boreal and subarctic zone of Europe (e.g. Moen et al. 2012; Joosten et al. 2017) whereas they occur very sporadically in the temperate Europe. However, these fens might have been more common here in the past and their distribution might have resembled the present-day distribution in Northern Europe. Evidence from macrofossil and partially the floristic data presented in this study supports the hypothesis for their wider potential historical occurrence.
We are aware that both macrofossil and floristic data may overestimate the historical distribution of brown-moss quaking rich fens because they indicate only the presences, but not the absences, of indicator species. The floristic data further do not provide evidence that the recorded species indeed co-occurred within the local plant community. Using floristic data, we can conclude that the brown-moss quaking rich fens might had been present in the locality, but we cannot certainly determine the time period of species co-occurrence.
At first glance, the number of potential Late Glacial and Early Holocene localities identified by palaeoecological and floristic data is equal to the number of existing recent localities identified by phytosociological data. At the second glance, though, considering (i) that the area explored by phytosociologists is intrinsically much larger than the area investigated by palaeoecological cores and (ii) that palaeoecologically-proved sites are mostly located in lowlands where fens have been much destroyed as compared to highlands and mountains, the difference in the number of historical and recent sites illustrates clearly a decline of brown-moss rich quaking fens in temperate Europe during the Holocene.
Diagnostic species of Stygio-Caricion limosae have been found in peat profiles in the Western Carpathians even at localities where they have not been recorded recently (Magyari et al. 1999; Hájková et al. 2015a; Hájková et al. 2012, 2017; Gálová et al. 2016). Layers composed of Calliergon trifarium and Scorpidium scorpioides have been found, for example, in Liptovská kotlina basin (recently calcareous fens of Caricion davallianae alliance; Hájková et al. 2015a), in the Upper-Hron Basin (recently again Caricion davallianae) and in the Malé Karpaty Mts (recently Sphagnum-dominated birch carr; Gálová et al. 2016). The same layers have been found in the neighbouring lowland river floodplains such as the Borská nížina lowland (Hájková et al. 2015b), Lower-Morava river valley (Rybníčková and Rybníček 1972), Upper-Morava river valley (Hájková et al. 2017) and Ostrava Basin at the Czech-Polish border (Fejfar et al. 1955). Several localities were detected also in the Bohemian Massif (for details see Table S7). Most macrofossils data come from the period 14,000–9,000 calibrated years BP, i.e. before the middle-Holocene expansion of closed forest (Pokorný et al. 2015; Hájek et al. 2016). Location of macrofossil records, indicating the potential occurrence of the Stygio-Caricion limosae alliance, suggests that this vegetation type had occurred in the lowlands and in mountain basins. These landscapes seem to be geomorphologically suitable for the development of topogenic quaking fens, especially when climate moistened and permafrost melted at the Pleistocene-Holocene transition. The brown-moss quaking rich fens might therefore had been widely distributed in the Central European landscape in that period, representing an initial successional stage of peatland development. In the Middle Holocene, the climate changes and consequent woodland expansion caused a reduction of these fens (Hájková et al. 2015a). As mentioned above, several palaeoecological studies proved also the replacement by calcareous fens of the Caricion davallianae alliance or Sphagnum-dominated fens. This process might be driven not only by raising the fen surface above the water level (Hájková et al. 2015a), but partially also by other factors. One of them could be general increase of phosphorus limitation during the Holocene prior to the Industrial Era (Kuneš et al. 2011), which gradually favoured the Caricion davallianae species that are adapted to phosphorus-limited wetlands (Pawlikowski et al. 2013). Another potential cause of decline is decreasing continentality over the course of the Holocene, because higher summer precipitation-to-evaporation ratio promotes spread of Sphagnum species over brown-moss fens (Vicherová et al. 2017). We believe that the persistence of the Stygio-Caricion limosae vegetation at scattered sites during the Holocene was conditioned by locally stable ecological conditions such as high and stable water level which blocks the succession towards Sphagnum and/or wooded fens. On such sites, brown-moss quaking rich fens could have survived up to the second half of 20th century. Thereafter they probably disappeared at many localities due to modern human activities, i.e. drainage for agricultural purposes, eutrophication or abandonment (Růžička 1987; Navrátilová et al. 2017).
Both the Middle-Holocene forest optimum and fen deterioration in modern agricultural landscapes contributed to decline of all fen types. Brown-moss rich fens however seem to be more sensitive to successional changes as compared to other fen habitats (Hájek et al. 2015; Janssen et al. 2016; Navrátilová et al. 2017). Even small water level decline or slight nutrient addition may cause transformation of a brown-moss rich fen to a depauperate fen vegetation. Hájek et al. (2015) have recently demonstrated changes in the moss layer in Czech fens manifested by the replacement of brown-mosses of pristine fens by Sphagnum species or competitively strong brown-mosses such as Calliergonella cuspidata. Analogous changes have been documented in the Netherlands (Kooijman et al. 1994; Paulissen et al. 2014) and even in Fennoscandia (Juutinen 2011; Rehell and Virtanen 2016). Kooijman (2012) partially explained these successional changes by macronutrient input into fen ecosystems that result in acidification. Vicherová et al. (2015) experimentally demonstrated the triggering effect of increased potassium concentration, which alleviates the detrimental effect of calcium on calcifuge sphagna and thus facilitates their expansion. The effect of increasing macronutrient input acts synergistically with water table decline that can occur with ongoing succession, but also as a consequence of the hydrological deterioration of entire landscapes. Udd et al. (2016) demonstrated the higher competition capability of Sphagnum species in brown-moss fens under drier conditions. Mälson and Rydin (2007) further showed that the combined effect of surface desiccation and the increased cover of vascular plants were a possible cause of the reduction of Scorpidium scorpioides populations. The effects of higher macronutrient availability (caused by atmospheric deposition, local eutrophication or abandonment of formerly mown fens), higher productivity of herb layer and increasing water uptake by vascular plants plus peat desiccation cannot be therefore easily separated from each other. The important point is that, even though seemingly negligible, a decline of the calcium-enriched water table by a few centimetres may have crucial consequences for community structure of brown-moss quaking rich fens because it may allow Sphagnum species to outcompete brown mosses (Granath et al. 2010; Vicherová et al. 2015). The persistence of brown-moss quaking rich fens up to present times in the Central European landscape is therefore a rare phenomenon which is apparently determined by a complex interplay of (i) high and stable water level, (ii) limited overall macronutrient input and (iii) Holocene continuity. The latter was well documented at the localities Puchmajerovej jazierko lake and Poľana Biały Potok (Table S2).
The species composition of brown-moss quaking rich fens of the Carpathians and surrounding territories corresponds to the communities of the Stygio-Caricion limosae alliance in boreal Europe. In Central and Southeastern Europe brown-moss quaking rich fens represent a relict phenomenon from the Late Glacial and Early Holocene periods whose disappearance accelerated because of the wide-scale eutrophication and disruption of water regime of the Central European landscape that took place in the second half of the 20th century. Besides other general threats (conversion to hay meadows, forest establishment, fertilizer application in the surrounding landscape), they can be easily replaced by Sphagnum-dominated fens due to autogenic succession. The persisting brown-moss quaking rich fens should be therefore treated as the sites of priority conservation interest in Central and Southeastern Europe.
The research was funded by the Czech Science Foundation (Centre of Excellence Pladias; 14-36079G) and Masaryk University (MUNI/A/1301/2016). The research of PH was partially supported by Czech Academy of Sciences (RVO 67985939). We are grateful to Kamil Rybníček (†) for providing unpublished vegetation plots. Our unpublished data from Romania were collected with the help of Veronika Horsáková and Michal Horsák. Many thanks go to Eva Hettenbergerová for management of the Macrofossil Database of the Czech Republic and Slovakia and for general support during the preparation of the manuscript. The co-ordinating editor and two anonymous reviewers provided important comments on the first draft of the paper.
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