The vegetation of rich fens (Sphagno warnstorfii- Tomentypnion nitentis) at the southeastern margins of their European range

Question: Rich fens of the Sphagno warnstorfii-Tomentypnion nitentis alliance require a specific combination of base richness and climate to occur. Their rarity at the southeastern margins of their European range has previously prevented rigorous vegetation classification. We asked how many associations may be delimited here and whether some of them are restricted to the high Balkan Mountains showing high endemicity. Study area: Entire territories of Bulgaria and Romania. Methods: We compiled all available vegetation-plot records, including some hitherto unprocessed data. We classified them by both divisive (modified TWINSPAN) and agglomerative (beta-flexible clustering) numerical classification method, with OPTIMCLASS1 applied to set the number of clusters. A semi-supervised approach (k-means) was additionally applied to confirm the classification of Southern-Carpathian (Romania) rich fens, where some Balkan taxa occur. Differences in base richness and elevation were tested by one-way ANOVA with Tukey’s pairwise test. Results: Three associations were delimited and all three occur in Bulgaria, from where only one association had been previously reported. Two associations characterised by Sphagnum contortum and Balkan and Southern-European species occur in Bulgaria, but not in Romania, one at lower elevations around 1,200 m, and one at higher elevations around 2,000 m where pH is lower. One lower-elevation (around 1,300 m) association with S. warnstorfii and S. teres is shared between Romania, Bulgaria and Central Europe. Conclusions: We have described a new high-mountain association, with two subassociations that differ by successional stage and dominant peat moss species (S. contortum and S. warnstorfii, respectively). These subassociations could be reconsidered when more data from other Balkan countries are available. Rich fens in southeastern Europe are rare, have a diverse vegetation, and are deserving of the further attention of nature conservation authorities and vegetation scientists. Taxonomic reference: The nomenclature was harmonized following The Euro+Med PlantBase (Euro+Med 2021) for vascular plants and Hill et al. (2006) for bryophytes, except of Angelica pancicii that is accepted as a separate taxon in Bulgaria (Andreev et al. 1992; Delipavlov et al. 2003). Critical taxa, not always reliably differentiated in the field and in literary sources, were merged to aggregates: Alchemilla vulgaris agg. (all Alchemilla species), Anthoxanthum odoratum agg. (A. alpinum, A. odoratum), Molinia caerulea agg. (M. arundinacea subsp. arundinacea, M. arundinacea subsp. freyi, M. caerulea), Palustriella commutata agg. (P. commutata, P. falcata), Plagiomnium affine agg. (P. affine, P. elatum, P. ellipticum), Sphagnum palustre agg. (S. centrale, S. palustre). Syntaxonomic reference: Peterka et al. (2017) for alliances. Copyright Michal Hájek et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Vegetation Classification and Survey 2: 177–190 doi: 10.3897/VCS/2021/69118 RESEARCH PAPER International Association for Vegetation Science (IAVS) Michal Hájek et al.: Bulgarian and Romanian rich fens 178


Introduction
Rich fens, a habitat in which acidicole and calcicole species both occur, are one of the most important wetland habitats in terms of biodiversity conservation, being increasingly endangered across Europe (Janssen et al. 2016;Chytrý et al. 2019;Singh et al. 2019). They are usually formed by calcium-tolerant peat moss species, non-sphagnaceous brown mosses and both calcicole and acidicole vascular plants (Du Rietz 1925;Rydin et al. 2013;Peterka et al. 2014;Singh et al. 2019), unlike other mire types where either peat mosses with acidicole vascular plants or brown mosses with calcicole vascular plants dominate. The coexistence of different species groups is caused not only by the intermediate pH and calcium levels, but also low nutrient availability and specific climate conditions such as total precipitation and the number of hot days (Hájek et al. 2021a). Calcium-tolerant peat mosses found in fens fed by moderately calcium-rich water, require either a stable water level of a narrow pH and calcium range (semi-aquatic species such as Sphagnum contortum), or the ability to escape from calcium-rich groundwater by forming hummocks (S. warnstorfii). To survive on calcium-rich groundwater any Sphagnum requires a constantly humid climate that facilitates a downward transport of toxic calcium from photosynthesizing capitula (Vicherová et al. 2017). If a summer dry period occurs, calcium moves upwards due to evapotranspiration and its high concentration in capitula can be lethal ). This mechanism explains why calcium-tolerant peat mosses barely colonise calcium-rich fens in areas experiencing dry summers, such as the submediterranean-subcontinental regions of the Balkan Peninsula (Hájek et al. 2008a. In extremely seasonal climates, calcium-tolerant peat mosses do not occur at all (Naqinezhad et al. 2021). A balance between the two major functional groups of mire mosses, peat mosses and brown mosses, may be disrupted not only by a change in climate, but also by increasing nutrient availability that supports the expansion of some calcium-tolerant species of peat moss such as Sphagnum teres Vicherová et al. 2015), or declines in water table that allow calcifuge peat mosses to avoid carbonate-rich groundwater and spread over the fen surface (van Diggelen et al. 2006;Granath et al. 2010;Kooijman 2012). The spread of calcifuge peat mosses can be associated with the loss of some endangered vascular plants, whose seedlings or offsets cannot compete with fast-growing acidicole peat mosses (Singh et al. 2019). The high level of endangerment and a sensitivity to environmental and climatic changes focuses the attention of plant ecologists and vegetation scientists on rich fens, especially at the margin of their range. Assessments of rich fens are, however, complicated by insufficient attention on their classification. The vegetation of rich fens, combining different functional groups of mire plants, have previously been neglected in traditional phytosociology, and descriptions of such vegetation are missing from several countries. In the current European-scale overviews, the rich fens have been clearly delimited at the levels of the vegetation alliance Sphagno warnstorfii-Tomentypnion nitentis (Mucina et al. 2016;Peterka et al. 2017) and the EUNIS habitats (https://eunis.eea.europa.eu/habitats.jsp; Chytrý et al. 2020). According to the synthesis of Peterka et al. (2017), they widely occur in northern Europe, the Baltic region, and Central-European mountains and highlands (the Alps, the Western Carpathians, Bohemian Massif). South and southeast of these mountains, rich fens are extremely rare, with the edge of the range in the Eastern and Southern Carpathians in Romania (see also Hájek et al. 2021b) and isolated islands in the Eastern Balkans, specifically in south-west Bulgaria (Hájek et al. 2008a;Peterka et al. 2017). Due to their rarity, the alliance Sphagno warnstorfii-Tomentypnion nitentis has not been distinguished in vegetation surveys from the Bulgarian high mountains (Roussakova 2000;Hájek et al. 2005;Hájková et al. 2006) and only one association has been reported from lower elevations (Hájek et al. 2008a). This low-elevation association, Geo coccinei-Sphagnetum contorti Hájek et al. 2008, is characterised by the semi-aquatic calcium-tolerant peat moss Sphagnum contortum and lawn-forming S. teres, coexisting with some endangered brown mosses (Hamatocaulis vernicosus), calcicole vascular plants (Eriophorum latifolium) and several species of wet grasslands. Although hummock-forming S. warnstorfii does occur in Bulgaria (Natcheva and Ganeva 2005;Hájková and Hájek 2013), its rarity at low elevations has prevented distinguishing a separate association. In high elevations, fens with S. warnstorfii contain some Balkan endemics which has resulted in their classification within the Cirsio heterotrichi-Caricetum nigrae (Soó 1957) Hájek et al. 2005 and Primulo exiguae-Caricetum echinatae Roussakova 2000 associations, previously classified to the Caricion fuscae alliance (Roussakova 2000;Hájková et al. 2006), later re-arranged to Narthecion scardici (Peterka et al. 2017). The synthesis of Peterka et al. (2017), however, showed that some high-mountain plots with S. warnstorfii from Bulgaria are closer to Sphagno warnstorfii-Tomentypnion nitentis than to Narthecion scardici.
In Romania, a neighbouring country also at the edge of the range for calcium-tolerant peat mosses and fen specialists (Horsáková et al. 2018), the Sphagno warnstorfii-Tomentypnion nitentis communities have also been rarely recorded (Hájek et al. 2021b). Most of them have been classified to the Sphagno warnstorfii-Eriophoretum latifolii Rybníček 1974 association, described from the Czech Republic (Rybníček 1974), while a single relevé has been classified as the Menyantho trifoliatae-Sphagnetum teretis Warén 1926 association characterised by tall sedges of boreal distribution. The high-mountain communities in the Southern Carpathians have been classified within the Sphagno warnstorfii-Eriophoretum latifolii, although they contain some Balkan species (Gymnadenia frivaldii, Dactylorhiza cordigera, Plantago gentianoides) and may show some similarities with Bulgarian high mountain species.
In this study we ask whether some associations with S. warnstorfii can be distinguished from Bulgaria, along with the previously reported Geo coccinei-Sphagnetum contorti, and whether Southern Carpathian rich fens may belong to the same association as Bulgarian ones. The output from our study is a classification of Bulgarian and Romanian rich fens at the association level.

Data set
To answer our two questions, we merged the existing limited datasets from previous studies (Romanian, Bulgarian high-mountain and Bulgarian low-elevation) into one, and added new original data from the Vitosha Mts (Bulgaria) sampled in 2006, after the Hájková et al. (2006) paper was published. We followed a habitat classification system for fens in which rich fens are delimited from extremely rich fens and calcareous fens by the presence of Sphagnum species (Malmer 1986;Hájek et al. 2006;Chytrý et al. 2020). We therefore only kept records with at least a 1% (Braun-Blanquet cover code 1) cover of Sphagnum species. The resulting dataset (70 relevés; Figure 1) is quite small considering that the geographical survey area covers two countries, but the dataset includes nearly all the rich fens known to occur in Bulgaria and the majority of rich fens that occur in Romania. An advantage of our data set is a unified sampling protocol and unified effort to identify bryophytes. Two co-authors (M.H., P.H.) participated in the sampling of all relevés, and two other co-authors (I.A., D.S.) participated in sampling a number of relevés in both countries and I.G. and D.D. in Romania. Sampling took place between 2001 and 2018, with most plots sampled in July or the beginning of August, and the majority of the plots have a standard plot size of 16 m 2 . We recorded all vascular plants and bryophytes using the nine-grade Braun-Blanquet scale (Westhoff and van der Maarel 1978) for cover and abundance estimation (r = few individuals covering < 1% of the area; + = more individuals covering < 1%; 1 = cover 1-5%; 2m = many tiny individuals or ramets covering < 5%; 2a = cover 5-15%; 2b = cover 15-25%; 3 = cover 25-50%; 4 = cover 50-75%; 5 = cover 75-100%). The total percentage cover for all bryophytes and all vascular plants was also recorded.

Water pH, conductivity and adjusted pH
We measured water pH and conductivity from the centre of the patch of fen being studied using portable instruments (mostly HACH HQ40d or CM 101 and PH 119, Snail Instruments). A shallow hole was dug before each measurement was taken to allow a pool of water to form. For testing the differences between associations, we further combined these two variables into a single variable called adjusted pH (Plesková et al. 2016) that expresses the joint physiological effect of pH and calcium richness on dominant moss species. For this calculation, we first estimated calcium concentration from water conductivity, using the imputation model of Hájek et al. (2021a). Secondly, we calculated adjusted pH by adding the decadal logarithm of the millimolar Ca 2+ concentration to the actual pH value (Plesková et al. 2016).

Classification of vegetation
As a first step, we ran unsupervised hierarchical classifications, using two different approaches. One was based on partitioning the major gradients (modified TWINSPAN, Roleček et al. 2009; with total inertia as a measure of cluster heterogeneity), and one was based on agglomerative clustering (the Beta-Flexible Clustering Method with the beta value -0.25 and the Bray-Curtis distance). The pseudospecies cut-off levels of 0, 5 and 25% were used in both cluster analyses in order to take into account the estimated percentage covers of individual species ). The number of interpreted clusters (four and five, respectively) corresponded to the number where the OPTIMCLASS 1 algorithm (Tichý et al. 2010), with Fisher exact test threshold for diagnostic species being set to P < 10 -4 , started to flatten or decrease. For each group we present the most diagnostic species (with the highest phi-coefficient; simultaneously with Fisher Exact test significance of p < 0.05).
As a second step, we tested whether Southern Carpathian rich fens (Romania) belong to the same association as Bulgarian high-mountain rich fens, and whether some low-elevation fens of Bulgaria belong to the same association as Romanian S. warnstorfii rich fens. The goal was to clarify the national-level syntaxonomical synopses. For this purpose, we constructed three species groups (named Pinguicula balcanica group, Sphagnum warnstorfii group and Geum coccineum group; cf. Table 1) using the COCKTAIL method (Bruelheide and Chytrý 2000) and utilised them in simple formal definitions for the three major vegetation types appearing in the unsupervised hierarchical classifications (Table 1). According to formal definitions we classified 49 vegetation-plot records, and 21 remaining records were classified by the semi-supervised k-means classification with three pseudospecies cut-off levels to take account of species covers (0, 5, 25%), 10 starts and two vegetation-plot records forming a centroid. We allowed one additional cluster to appear (i.e, the final number of clusters was four), because four groups has resulted from the initial beta-flexible clustering.
In the synoptic table, we consider a species as diagnostic if it has a statistically significant association with a cluster (P <0.05; Fisher exact test). We also present the species occurring in at least 20% of vegetation-plot records.

Differences among vegetation types
Differences among associations in edaphic and climatic variables were visualised by box-and-whisker plots showing medians, interquartile ranges, extremes and outliers, and tested by one-way ANOVA with Tukey's pairwise test with Copenhaver-Holland correction. Water conductivity was log-transformed prior to testing to achieve normal distribution. Normality of the data was tested using the Anderson-Darling normality test. All analyses were conducted using the Past 4 software (Hammer et al. 2001).

Unsupervised classifications
Based on the OPTIMCLASS 1 algorithm, modified TWIN-SPAN resulted in five clusters, while beta-flexible clustering resulted in four clusters. However, their interpretation is the same (Figures 2, 3). The group of Bulgarian relevés characterised by Balkan species (especially by Primula frondosa subsp. exigua and Pinguicula balcanica), the small group of Romanian relevés characterised by Ligularia sibirica and Epipactis palustris, and the group of Romanian and Bulgarian S. warnstorfii fens characterised by Calliergon giganteum and Valeriana simplicifolia appeared in both classifications, largely with the same diagnostic species. The group of Bulgarian S. contortum fens with SE-European species (the Geo coccinei-Sphagnetum contorti association) also appeared in Table 1. Species groups used in the formal definitions for the three associations before the run of semi-supervised k-means classification. The Sphagno contorti-Primuletum exiguae association (10 relevés from Bulgaria) had been defined by the presence of the Pinguicula balcanica group (at least two species had to be present), the Sphagno warnstorfii-Eriophoretum latifolii association (17 relevés, out of which two are from Bulgaria) is based on the presence of the Spagnum warnstorfii group (at least two species had to be present) and the Geo coccinei-Sphagnetum contorti association (27 relevés from Bulgaria) is based on the presence of the Geum coccineum group (at least two species had to be present) and the absence of the Pinguicula balcanica group.

Semi-supervised k-means
When we set three formally defined vegetation types (Bulgarian high-mountain rich fens, low-elevation S. warnstorfii rich fens, and Bulgarian S. contortum rich fens) as predefined groups and ran semi-supervised k-means, the small Romanian cluster with Ligularia sibirica also appeared, but this group included only three relevés with Ligularia sibirica and S. warnstorfii. No Romanian relevé was assigned to the cluster of Balkan high-mountain rich fens. A single Romanian relevé was assigned to the cluster of Bulgarian S. contortum rich fens, but it lacks SE-European species and is transitional to poor fens, making its assignment to the Geo coccinei-Sphagnetum contorti association inappropriate.

Syntaxonomical conclusions
We interpret the cluster of Bulgarian high-mountain rich fens as a new plant association, with a distribution range  restricted to the Balkans, and we describe it formally in this paper with the name Sphagno contorti-Primuletum exiguae.
In approximately half of the relevés, Sphagnum warnstorfii dominates, with certain changes in species composition suggesting advanced succession; we suggest treating these as the sphagnetosum warnstorfii subassociation. We further interpret the cluster of low-elevation S. warnstorfii rich fens as the Sphagno warnstorfii-Eriophoretum latifolii association and report it as a new association for Bulgaria. Finally, we discovered that the Geo coccinei-Sphagnetum contorti association (cluster of Bulgarian S. contortum rich fens) does not occur in Romania and is restricted to the Balkans. A small cluster of Romanian rich fens characterised by L. sibirica and Epipactis palustris were not definitively interpreted syntaxonomically. However, as these relevés were dominated by peat moss species and high-mountain species were absent, we merged it with the Sphagno warnstorfii-Eriophoretum latifolii association, where it may be considered as a separate subassociation.
The synoptic table shows the three delimited associations resulted from the beta-flexible classification at the level of four clusters, with the two clusters we interpreted as the same association Sphagno warnstorfii-Eriophoretum latifolii merged (Table 2). The full records for the associations new to Bulgaria are presented in Table 3.
Nomenclatural type:  Nomenclatural note: When the name of a syntaxon is formed from the names of two taxa of which only one belongs to the highest of the dominant strata determining the vertical structure, then the name of that taxon appears in the second place (the Code of Phytosociological Nomenclature; Theurillat et al. 2021). In rich fens with Sphagnum contortum and S. warnstorfii, the moss stratum is the dominant one in terms of cover and biomass, but the herb layer is the highest one that determines vertical structure. Therefore P. frondosa subsp. exigua must appear on the second place in the syntaxon name even if S. contortum usually dominates.

Environmental differences among the three associations
The high-mountain association Sphagno contorti-Primuletum exiguae occurred at significantly higher elevations, while the other two associations did not differ in elevation. The Sphagno warnstorfii-Eriophoretum latifolii association showed the highest water pH, with statistically significant differences compared with the other two associations, while the Geo coccinei-Sphagnetum contorti association exhibited the highest water conductivity ( Figure  4). The Sphagno contorti-Primuletum exiguae showed the lowest pH. When pH and conductivity were joined into a single variable, adjusted pH, the difference between the Sphagno warnstorfii-Eriophoretum latifolii and the Geo coccinei-Sphagnetum contorti was no longer significant, suggesting ecologically equivalent conditions for the occurrence of calcium-tolerant peat moss species.

Discussion
At the margin of their southeastern range in the Balkan Peninsula, rich fens may be robustly classified into three associations, one high-mountain association occurring above the treeline in the Balkans, and two occurring at lower elevations. The high-mountain association is characterised by Balkan species that otherwise occur in the Balkan high-mountain fens of the Narthecion scardici alliance (Peterka et al. 2017; referred to as Caricion fuscae in Roussakova 2000 andHájková et al. 2006) from which the Sphagno contorti-Primuletum exiguae may develop in the course of autogenic succession or succession after a drop in the water table. Such a succession from brownmoss dominated fen communities towards rich fens with calcium-tolerant peat mosses is well known (Rybníček 1974;Kooijman 2012;Vicherová et al. 2017;Singh et al. 2021), and the combination of Balkan fen species with calcium-tolerant peat mosses in Bulgaria was to be expected. Yet, it had not been reported in previous studies from the Balkans (Roussakova 2000;Hájek et al. 2008a;Tzonev et al. 2009) and in our study it was represented by only 13 records, while the Narthecion scardici fens that lack diagnostic species of rich fens, especially calcium-tolerant peat mosses, are much more common. Obviously not all Narthecion scardici fens develop into rich fens with calcium-tolerant peat mosses. The reason is that calcium and pH content is quite low in most Narthecion scardici fens  and succession tends to move towards acidicole hummock-forming peat mosses (Sphagnum capillifolium, S. russowii) with dwarf shrubs such as Bruckenthalia spiculifolia (Hájek et al. 2005;. Enhanced pH and calcium concentrations may be the reason why Sphagno contorti-Primuletum exiguae, especially its subassociation with S. warnstorfii, may develop from the Narthecion scardici fens, but the values measured in the Bulgarian vegetation plots (Figure 4) are quite low, lower than optimum values for calciumtolerant peat mosses (S. warnstorfii, S. teres, S. contortum) in other regions (Mikulášková et al. 2015;Plesková et al. 2016). Mikulášková et al. (2015Mikulášková et al. ( , 2017 studied Bulgarian populations of S. warnstorfii genetically, along with other populations worldwide, and found slight yet apparent pH-and magnesium-related genetic variation within S. warnstorfii, with Bulgarian populations at the acidic and magnesium-poor end of the cline. Another calciumtolerant peat moss species, S. contortum, is more frequent in Bulgarian rich fens including the high-mountain ones. Vascular plants occurring in the Sphagno contorti-Primuletum exiguae (e.g., Eriophorum latifolium) also seem to be adapted to lower levels of calcium and pH as compared to other regions (Hájková et al. 2008). An occurrence of calcicole species in quite acidic and calcium-poor conditions has also been reported from other cold and nutrient-poor areas such as Scandinavia  and also from Central Europe in the recent past, before the period of current eutrophication and warming (Rybníček 1974;Hájek et al. 2015). The species combination that characterises Sphagno contorti-Primuletum exiguae may hence mirror specific refugial conditions, such as cold climate and low nutrient availability. In warmer and nutrient-richer conditions, acidicole peat mosses are expected to outcompete calcium-tolerant moss species (Kooijman 2012;Kolari et al. 2021) (Vicherová et al. 2017), they are quite rare in the submediterranean-subcontinental climate of Bulgaria and they were not delimited in the previous study of Hájek et al. (2008a). When analysed together with Romanian rich fens, the Sphagno warnstorfii-Eriophoretum latifolii clearly occurs in Bulgaria, but only in a few specific areas of the Rhodope and Stara Planina Mts, at elevations of 1,530-1,550 m a. s. l. Although we call them low-elevation fens to distinguish them from high-mountain (subalpine to alpine) fens, such elevations are higher than those at which the association occurs in the Czech Republic in Central Europe (Chytrý 2011, interquartile range 500-700 m a. s. l. ). The elevational shift in climate conditions between Central and Southeastern Europe is mirrored in the distribution of other groundwater-dependent habitats such as wet grasslands (Hájek et al. 2008b). The association Sphagno warnstorfii-Eriophoretum latifolii is a very rare vegetation type in Bulgaria, occurring at the very margin of its distribution. The reason for its rarity in Bulgaria may be that it requires a high precipitation: temperature ratio, especially during the summer (Vicherová et al. 2017) and generally it requires a cold and wet climate. In the Carpathians, most occurrences of this association are in areas where the annual precipitation is at least 800 mm, mean annual temperatures are below 6°C and there are only zero to one hot days with maximum temperature above 30°C (Hájek et al. 2021a).
The Geo coccinei-Sphagnetum contorti association, from which the Sphagno warnstorfii-Eriophoretum latifolii may develop if the abovementioned climate conditions are met, is much more widespread in Bulgaria because it only depends on particular groundwater chemistry and does not require such a specific climate (Hájek et al. 2008a). It may therefore occupy the lowest elevations and warmest areas of the three rich fen vegetation types known from SE Europe, but as such it is quite poor in specialised and relict fen plants that are generally rare in SE Europe (Horsáková et al. 2018) and may contain many wet-grassland and reed-bed species (Table 2). Despite this, a couple of disjunctly occurring and hypothetically relict species such as Hamatocaulis vernicosus, Eriophorum gracile or Carex lasiocarpa have been found (Hájek et al. 2009), making these fens important biodiversity hotspot and refugia for boreal species in South-Eastern Europe. Our analysis has demonstrated that this association is strongly associated with the Balkans, not reaching the Southern and Eastern Carpathians. Although this association shows higher water conductivity than the previous one, water pH is lower. When pH and conductivity are combined to capture their joint physiological effect on peat mosses (Vicherová et al. 2015;Plesková et al. 2016), there is no difference between the two low-elevation associations.

Rich fens with Ligularia sibirica
This delimited cluster was quite small and comprised predominantly vegetation plots with S. warnstorfii. We interpreted it as a specific vegetation type within the Sphagno warnstorfii-Eriophoretum latifolii, but further research on the continental scale is needed. The relevés of this cluster come from the area of the Eastern Carpathians where phosphorus-enriched, nitrogen-limited fens of the Saxifrago-Tomentypnion occur (the Harghita and Covasna regions; Peterka et al. 2017;Hájek et al. 2021b). Ligularia sibirica links this cluster with the Saxifrago-Tomentypnion fens. It seems the cluster represents rich fens that have developed from these nitrogen-limited fens (the Drepanoclado adunci-Ligularietum sibiricae Hájek et al. 2021 association). In the whole-Carpathian analysis of calcium-rich fens (Hájek et al. 2021b), however, this vegetation type was not delimited by the analyses, and individual records were classified as Sphagno warnstorfii-Eriophoretum latifolii or, in a single case, as the Menyantho trifoliatae-Sphagnetum teretis association.
We cannot exclude the possibility that rich fens that have developed from N-limited extremely-rich fens of the Saxifrago-Tomentypnion, but mostly without Ligularia sibirica, may occur in other European areas such as Latvia, Estonia, Finnland, Russia or Swiss Jura Mts (compare distribution of Saxifrago-Tomentypnion in Peterka et al. 2017), but it seems premature to describe a new association based on so few vegetation-plot records. We have therefore classified the plots forming this cluster within the Sphagno warnstorfii-Eriophoretum latifolii association.
To conclude, we have presented evidence for distinguishing three well-supported associations of rich fens in Bulgaria, the Geo coccinnei-Sphagnetum contorti, the Sphagno warnstorfii-Eriophoretum latifolii and the Sphagno contorti-Primuletum exiguae ass. nov., with the latter two being reported for Bulgaria for the first time. All these rich-fen associations are rare in SE Europe, occurring here at the margin of their range and acting as irreplaceable refugia of fen biota in this part of the world.