Introduction

European non-forest wetlands, such as fens, floodplain and wet meadows on mineral and peaty soils, are considered threatened habitats. The main threats of their stand structure and community diversity are related to changes in water regimes, abandonment of agricultural utilization or traditional farming and conversion of meadows into intensively used agricultural land (Brinson and Malvárez 2002; Verhoeven 2014; Straubinger et al. 2023). These landscape treatments, exacerbated by ongoing climate change, also have a negative impact on many vascular plants specialised in these habitats. Wetlands evenly harbour abundant group of rare species, including the Siberian Iris (Iris sibirica) that is threatened in several Central European countries (Schnittler and Günther 1999; Király 2007; Grulich 2012). The species is also listed in the Slovak Plant Red List in the near threatened category (Eliáš et al. 2015).

Iris sibirica L. (family Iridaceae) is a rhizomatous herbaceous perennial species. It has a rhizome and hollow, frequently branched and somewhat vertically compressed stems, 50–120 cm tall. Several basal leaves (25–80 cm × 4–10 mm) are accompanied by only a few small leaves on the stem. The plants have 1–3 (rarely 5) mid-blue to violet-blue (rarely white) flowers, with 5–7 cm in diameter. The fruit is a capsule of 2–4 cm long with an ellipsoidal to sub-cylindrical shape with flat seeds (POWO 2023). This boreal Euro-Siberian species is primarily distributed in the temperate biome. In Europe, I. sibirica grows except for its northern regions and most of the Mediterranean area. It occurs from western France and Central Europe to the Baltic States in the north, the Balkan Peninsula in the south and European Russia in the east. The Asian distribution range extends from western and south-western Siberia to Mongolia, with some isolated occurrences recorded in the Caucasus Mts and Anatolia region (Meusel and Jäger 1992). The species is relatively common with a scattered distribution pattern in Central Europe. It is documented in the Germany (Meusel and Jäger 1992), Czech Republic (Hrouda and Grulich 2010), Austria (Fischer et al. 2008), Hungary (Bartha et al. 2015) and Poland (Zając and Zając 2001). In Slovakia, it has a similarly scattered distribution throughout the country (Dostál and Červenka 1992). I. sibirica is a typical wetland species with a strong affiliation to wet meadows of the Molinio-Arrhenatheretea class, mainly to the order Molinietalia caeruleae (Botta-Dukát et al. 2005), and the alliances Calthion palustris, Molinion caeruleae and Deschampsion cespitosae (e.g. Matuszkiewicz 2008; Chytrý 2010; Borhidi et al. 2012, Hegedüšová Vantarová and Škodová 2014). The association Iridetum sibiricae was described as a plant community with the dominance of this species and it has been reported especially from Central European countries (Salamon-Albert et al. 2010).

The distribution pattern, vegetation and ecological affinities of the target species in Slovakia have been published almost exclusively in local floristic and phytosociological studies, but there is no complex synthesis at the national level. Therefore, the aims of this study were i) to revise and complete the chorological data of Iris sibirica in Slovakia, and ii) to describe the vegetation with the species occurrence and to analyze its ecological pattern.

Materials and methods

Distributional data

The distribution data of Iris sibirica in Slovakia were collected from the following sources: public Central European herbaria (BP, BRA, BRNM, BRNU, EGR, HLO, HNTS, KO, LTM, MMI, MOP, MPS, NI, OL, OLM, PMK, PR, PRC, SAV, SLO, SMBB, SNV, TM, VSM, ZAM, ZV and W), JACQ – Virtual herbaria database (https://www.jacq.org), public photo galleries (FotoNet – wild nature photography, https://www.fotonet.sk, https://www.nahuby.sk), Slovak floristic and vegetation databases (Institute of Botany, Plant Science and Biodiversity Center, Slovak Academy of Sciences in Bratislava, DAPHNE – Institute of Applied Ecology in Bratislava, Slovak Vegetation Database, https://ibot.sav.sk/cdf/) and field research of the authors. Herbarium codes followed Thiers (2022+) and small local museum collections were unified according to Vozárová and Sutorý (2001). These sources were only used if the record was georeferenced and the locality had not been published or supported by a herbarium specimen.

The map of Iris sibirica distribution was prepared in the program ArcGis version 9.2 using the grid template of the Central European Flora Mapping System (Niklfeld 1971). All localities were geographically sorted according to the traditionally used phytogeographical division of Slovakia (Futák 1984).

Vegetation and ecological data

Vegetation affinity of Iris sibirica was analysed based on the phytosociological relevés provided by the Slovak Vegetation Database (code EU-SK-001 in the Global Index of Vegetation-Plot Databeses; Šibík 2012) and completed by our unpublished relevés. Finally, we used 82 relevés (80 treeless, whose sources are listed below in the Table 1, and two relevés of shrub and forest vegetation which are listed in Appendix 2) stored in the Turboveg database (Hennekens and Schaminée 2001). They were subsequently processed in the Juice program (Tichý 2002). Species taxonomy was unified using the concept of broadly defined plant taxa with few aggregate taxa, namely Agrostis stolonifera agg. (A. gigantea, A. stolonifera), Eleocharis palustris agg. (E. palustris, E. uniglumis), Galium palustre agg. (G. elongatum, G. palustre), Luzula campestris agg. (L. campestris, L. multiflora), Molinia caerulea agg. (M. arundinacea, M. caerulea), Poa pratensis agg. (P. angustifolia, P. pratensis) and Solidago canadensis agg. (S. canadensis, S. gigantea). Bryophytes were excluded from the dataset because they were only identified in some relevés. In order to homogenise the vegetation dataset, two relevés of shrub and forest vegetation were excluded before the statistical analyses. Numerical classification was performed with species merged into a single layer using the modified TWINSPAN algorithm (Roleček et al. 2009). We used five pseudospecies cut levels (0%, 5%, 25%, 50% and 75%) and total inertia as a measure of cluster heterogeneity. The differential species of each cluster were determined using frequency and fidelity thresholds (Φ – phi coefficient; Chytrý et al. 2002). They were set at frequency ≥ 20%, phi coefficient ≥ 0.30 and difference in frequency between clusters ≥ 20%. If a particular species was constant (frequency reached ≥ 50%) in two or more clusters, it was not accepted as a differential. The Fisher’s exact test (p < 0.05) was used to eliminate species with a non-significant occurrence in a given cluster (Tichý and Chytrý 2006). Detrended correspondence analysis (DCA) and Ellenberg indicator values (EIVs) for vascular plants, calculated for temperature, moisture, light, soil reaction, nutrients and continentality (Ellenberg et al. 1992), were used to explain species-environmental relationships in the vegetation dataset. Species cover values were logarithmically transformed prior to analysis and EIVs were used as supplementary variables in the DCA ordination. The analysis was carried out in CANOCO for Windows 5.0 software (Ter Braak and Šmilauer 2012). Mean annual temperature and total annual precipitation of vegetation plots were retrieved from WorldClim version 2 [http://www.worldclim.org] on a grid background with a spatial resolution of 30 arc-seconds.

Table 1 Shortened synoptic table of treeless vegetation with the presence of Iris sibirica in Slovakia. Taxa are presented with percentage frequency and phi-values (*100; upper index). Only taxa with the frequency ≥ 20% in the dataset are shown (164 taxa are hidden)
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Nomenclature

Nomenclature of vascular plants and higher vegetation syntaxa follows the checklists of Marhlod and Hindák (1998) and Mucina et al. (2016), respectively. Association names of plant communities are mentioned according to Slovak vegetation overviews (Valachovič 2001; Hegedüšová Vantarová and Škodová 2014; Valachovič et al. 2021).

Results

Distribution of Iris sibirica in Slovakia

Iris sibirica occurs almost throughout Slovakia. Although it has been found in 73 grid-cells (~ 19% of the total) in Slovakia using the Central European Flora Mapping System (Fig. 1, Appendix 1), the distribution is concentrated in the south-western Slovakia (Záhorská nížina lowland) and in the southern regions of central Slovakia (volcanic mountains – Štiavnické vrchy Mts, Poľana Mts, Javorie Mts and Ipeľská kotlina basin). Similar numbers of localities are reported from the Pannonian and Carpathian bioregions (51% vs 49%). In the Carpathians, a much higher ratio of recent localities (≥ 1990 – 69%) was found compared to the Pannonian bioregion (≥ 1990 – 45%; see Appendix 1).

Fig. 1
figure 1

Distribution of plant species Iris sibirica L. in Slovakia

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Vegetation affinity and ecological characteristics

Altogether 283 vascular plant taxa were identified in 80 phytosociological relevés with the occurrence of Iris sibirica. The most common plant species in the dataset were Ranunculus acris (occurrence in 79% of the relevés), Sanguisorba officinalis (70%) and Deschampsia cespitosa (60%), followed by Poa pratensis agg. (59%), Lathyrus pratensis (56%), Filipendula ulmaria (54%), Carex panicea (53%), Serratula tinctoria (51%) and Galium boreale (50%). TWINSPAN analysis indicated five relatively well distinguished clusters of vegetation relevés. First cluster was differentiated by a set of wetland and eutrophic tall plants such as Caltha palustris, Carex buekii, Equisetum palustre, Filipendula ulmaria and Lathyrus pratensis (Table 1). It corresponded to the Calthion palustris alliance [previously recognised as suballiance Filipendulenion ulmariae (Lohmeyer in Oberd et al. 1967) Balátová-Tuláčková 1978 (Fig. 2). This type of vegetation is distributed mainly in the Slovenský kras Mts, rarely in the other parts of Slovakia (Fig. 3). Plant species of lowland floodplain meadows (e.g. Alopecurus pratensis, Carex praecox agg., Lychnis flos-cuculi, Lysimachia nummularia, Poa pratensis agg., Ranunculus acris, R. repens, Sanguisorba officinalis and Taraxacum sect. Ruderalia) were typical for the stands of the second cluster. This vegetation, with a geographical affinity to lowlands and basins, was classified in the Deschampsion cespitosae alliance (previously distinguished alliances Alopecurion pratensis and Cnidion venosi; Figs. 2 and 3). Vegetation of the third cluster was mainly formed by plants of wet meadows and fens (such as Carex nigra, C. panicea, Dactylorhiza majalis, Deschampsia cespitosa, Juncus conglomeratus, Lysimachia vulgaris, Myosotis scorpioides agg., Potentilla erecta, Ranunculus acris, Sanguisorba officinalis), which were accompanied by mesophilous species (e.g. Carex pallescens, Ranunculus auricomus agg., Viola canina). These stands were also enriched by plants of nutrient-poor soils adapted to annual water level fluctuations, such as Carex umbrosa, Galium boreale, Molinia caerulea agg., Serratula tinctoria and Succisa pratenis. This species composition is typical for the alliance Molinion caeruleae (Fig. 2), which was reported especially from the volcanic mountain ranges of central Slovakia (Fig. 3). The fourth cluster is characterised not only by plants of wet meadows and fens, but also by physiognomically and competitively strong tall plant species such as Calamagrostis epigejos, Lythrum salicaria, Phragmites australis, and by the shrub species Salix cinerea and S. purpurea. This cluster grouped vegetation of different successional stages between wet meadows (Calthion palustris and Molinion caeruleae) and fens (Scheuchzerio palustris-Caricetea fuscae; Table 1 and Fig. 2). They were distributed in the Borská nížina lowland, occasionally in the Štiavnické vrchy Mts and Turčianska kotlina basin (Fig. 3). Vegetation of the fifth cluster occurred only in the Borská nížina lowland (Fig. 3). It was composed of plants with an affinity to wet meadows on nutrient-poor soils (e.g. Carex panicea, Galium boreale, Molinia caerulea agg., Ranunculus acris, Sanguisorba officinalis, Serratula tinctoria or Succisa pratenis). The herb-layer mosaic was created by species of fens (e.g. Carex davalliana, C. nigra, Epipactis palustris or Galium uliginosum) and abundant group of mesophilous species (e.g. Anthoxanthum odoratum, Centaurea jacea, Filipendula vulgaris, Galium verum, Leonotodon hispidus or Plantago lanceolata; Table 1, List of sources). The cluster showed transitions between the hygrophilous vegetation of Molinion caeruleae, fen vegetation of Caricion davallianae and/or Caricion fuscae alliances and mesic habitats (Fig. 2). Finally, I. sibirica was rarely recorded in the shrub and forest vegetation of the alliances Salicion traindrae and Salicion albae, respectively (Appendix 2).

Fig. 2
figure 2

The main vegetation types with the occurrence of Iris sibirica in Slovakia: Calthion palustris (cluster 1): Photo 1, Silica, R. Šuvada, 2007; Deschampsion cespitasae (cl. 2): 2, Nižná Jablonka, M. Dudáš, 2022, 3, Breznička, R. Hrivnák, 2005, 4, Malé Kosihy, J. Košťál, 2019; Molinion caeruleae (cl. 3): 5, Banský Studenec, R. Hrivnák, 2023, 6, Budiná, R. Hrivnák, 2022, 7, Detva, D. Galvánek, 2014; Successional stages between wet meadows (Calthion palustris and Molinion caeruleae) and fens (Scheuchzerio palustris-Caricetea fuscae; cl. 4): 8, Krupina, M. Mokráň, 2013, 9, Socovce, D. Tomášiková, 2010; Transition between the hygrophilous vegetation of Molinion caeruleae, fen vegetation of Caricion davallianae and/or Caricion fuscae alliances and mesic habitats (cl. 5): 10, Veľké Leváre, V. Šefferová-Stanová, 2015. Photos are numbered in row order

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Fig. 3
figure 3

Distribution of individual vegetation clusters in Slovakia (Calthion palustris, cluster 1 = blue circles, Deschampsion cespitasae, cl. 2 = purple squares, Molinion caeruleae, cl. 3 = yellow diamonds, successional stages between wet meadows (Calthion palustris and Molinion caeruleae) and fens (Scheuchzerio palustris-Caricetea fuscae, cl. 4 = green triangles and transitions between the hygrophilous vegetation of Molinion caeruleae, fen vegetation of Caricion davallianae and/or Caricion fuscae alliances and mesic habitats, cl. 5 = red stars)

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DCA scatterplot diagram showed good differentiation of the first, second and fourth clasters. On the other hand, the remaining plots partially overlapped (Fig. 4A). The first DCA axis was interpreted as a combined EIV gradient of light, continentality as well as temperature and nutrients, while the second DCA axis represented the EIV moisture gradient (Fig. 4B). Along the first DCA axis, species of nutrient-rich and thermophilous sites were replaced by plants of relatively cold and open habitats on nutrient-poor soils (Fig. 4C). These patterns were also confirmed by mean values of EIVs (Table 2). Species richness highly varied along the second DCA axis, i.e. the number of vascular plants (alpha diversity) decreased from typical wet to semi-wet habitats in the order 2, 3 and 5 → 4 → 1 (Fig. 4D). Clusters 1 and 3 had the highest mean elevation and the lowest mean annual temperature, while the values of total annual precipitation in cluster 1 were lower than in clusters 3, 4 and 5. The lowest elevation and the highest temperature values were found in cluster 5, while the lowest precipitation values were identified in cluster 2 (Table 2).

Fig. 4
figure 4

DCA ordination diagrams of (A) samples (Calthion palustris, cluster 1 = blue circles, Deschampsion cespitasae, cl. 2 = purple squares, Molinion caeruleae, cl. 3 = yellow diamonds, Successional stages between wet meadows (Calthion palustris and Molinion caeruleae) and fens (Scheuchzerio palustris-Caricetea fuscae, cl. 4 = green triangles and Transition between the hygrophilous vegetation of Molinion caeruleae, fen vegetation of Caricion davallianae and/or Caricion fuscae alliances and mesic habitats, cl. 5 = red stars), (B) Ellenberg indicator values (EIVs) as supplementary variables, (C) species with the occurrence at least 20% of all relevés, and (D) species richness-EIVs relationship. The first two DCA ordination axes are shown (horizontal axis = axis 1 and vertical axis = axis 2). First four letters of genus and species name are shown, e.g. JuncCong = Juncus conglomeratus (see Appendix 3 for full names and abbreviations of all species)

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Table 2 Mean EIV values and climatic characteristics of vegetation plots in each cluster
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Discussion

A revision of herbarium specimens, available literature and electronic database sources showed that Iris sibirica is a relatively common plant species with a scattered distribution in Slovakia. Most of the localities were found in south-western Slovakia and southern parts of central Slovakia. However, several localities situated in the lowland areas of the Pannonian bioregion are only historical, i.e. recent records of the species indicate its rarity there, with apparent occurrences in sites embedded in the national network of protected areas. This could be explained by the fact that the Slovak lowlands (e.g. Záhorská and Podunajská nížina lowlands) are intensively used for agriculture and are relatively densely populated with a long history of human settlement (Miklos 2002). This landscape utilization often leads to direct habitat destruction accompanied by changes in site ecology and vegetation, which in turn negatively affect populations of the I. sibirica. In addition, the increasing homogenisation of the landscape reinforced by a reduction in the mosaic structure and secondary succession of grasslands, has a negative impact on the availability of suitable sites for the species, particularly in mountainous regions. In the last decades, these changes have reduced plant diversity and negatively controlled the species compositional pattern of grasslands across Europe (Habel et al. 2013), including Slovakia (Špulerová 2008; Halada et al. 2017). However, populations developed on sandy soils (e.g. in the Záhorská nížina lowland) are under less human pressure than those in the Podunajská nížina lowland due to unsuitable conditions for intensive agricultural activities. The situation seems to be different in the mountainous areas of central Slovakia. Here, the traditional management of wet meadows still persists, which in turn facilitates the presence of species in many localities. Several of them have been newly discovered (Hrivnák et al. 2005, 2016). These facts are fully reflected in the recent conservation status of species in Slovakia (Eliáš et al. 2015). In spite of the relatively widespread distribution of the species in Slovakia, wetland plant communities with its occurrence are under strong negative human impact. Therefore, the classification of Iris sibirica as a “near-threatened” species is correct.

Our vegetation analyses showed that I. sibirica thrives in a wide range of wetland habitats. These include wet meadows of the alliances Calthion palustris and Molinion caeruleae, floodplain meadows of the alliance Deschampsion cespitosae and transitional plant communities towards fens (alliances Caricion davallianae and Caricion fuscae). A coenological affinity to wet and flooded meadows has been frequently reported from adjacent countries (Matuszkiewicz 2008; Chytrý 2010; Borhidi et al. 2012). The association Iridetum sibiricae has been previously mentioned from several European regions (e.g. Salamon-Albert et al. 2010), but contemporary vegetation revisions did not distinguish this community at the association level due to floristic heterogeneity and absence of diagnostic species (Chytrý 2010, Hegedüšová Vantarová and Škodová 2014). Occasionally, the species was also found in shrub and forest swamps, which represent later successional stages of the wet meadows. Similar occurrence of I. sibirica in forest understorey has been documented in Poland, where it grows along rivers in floodplain forests, in mesic sites overgrown by oak-hornbeam and oak forests, but also in different types of open pine forests (Kostrakiewicz 2001). In Central Europe, the present vegetation niche can also include spring and fen communities (e.g. in the Czech Republic, Hrouda and Grulich 2010), or swamp willows dominated by Salix cinerea (e.g. in Hungary, Salamon-Albert et al. 2010).

Most of the phytosociological relevés in our study belong to herbaceous vegetation previously classified as Filipendulenion ulmariae (= Calthion palustris) or to transitional stages of wet meadows (Molinion caeruleae) and fens. They were commonly composed of competitively dominant plant species (Calamagrostis epigejos, Deschampsia cespitosa, Molinia caerulea, Phragmites australis) and shrub species of the Salix genera (S. cinerea, S. purpurea), which importantly shaped the patterns of community diversity. These resource-acquisitive species enriched by other tall herbs of wet meadows (e.g. Filipendula ulmaria, Lythrum salicaria) indicate early successional stages following the abandonment of traditional management practices (primarily mowing). These stands can be considered as optimal habitats for I. sibirica (see also Kostrakiewicz-Gierałt 2013). European Ellenberg-type indicator values proposed by Tichý et al. (2023) for vascular plants evenly suggest an affinity to non-forest and wet vegetation. In more detail, I. sibirica was categorized as a light and moisture demanding species, with an intermediate value for temperature and a preference to nutrient-poor habitats. It has already been found that the moisture regime seems to be an important factor for the vitality of I. sibirica. When population structure was considered as an indicator of species vitality, populations on drier sites were less abundant but in better condition than those on wetter sites (Kostrakiewicz 2008).

Our study provides a first revision of the chorological records and vegetation preferences of Iris sibirica in Slovakia with respect to the Central European context. Although the species was found in many localities throughout Slovakia, we identified two main centres of distribution situated in south-western and central Slovakia. The species prefers flooded meadows developed in lowlands (Deschampsion cespitosae) and wet meadows (Calthion palustris, Molinion caeruleae). An abundant group of relevés also showed transitional features towards fen habitats (Caricion davallianae and Caricion fuscae) or corresponded to successional stages of wet meadows.