Vegetation types and habitats with participation of plant species – local endemics to the marble part of Northern Pirin Mts., Bulgaria

This study was carried out in the carbonate (marble) part of North Pirin Mts. Its upper parts are declared protected area of National Park category according to the Bulgarian Protected Areas Act. It is also a NATURA 2000 protected zone under the Habitats Directive, and a natural UNESCO World Heritage Site. The main reason for the conservation of this territory is the exceptional concentration of rare, relict and endemic plant species, its specific flora, vegetation and fauna. Subject of the study are the plant communities and habitats, with participation of 8 local endemic plant taxa, such as Cyanus achtarovii, Oxytropis kozhuharovii, O. urumovii, Odontarrhena orbelica, Alyssum pirinicum, Veronica kellereri, Brassica nivalis subsp. jordanoffii and Arabis ferdinandi-coburgii. The plant communities were studied according to the classical Braun-Blanquet’s approach by sampling the phytocoenoses, where the local endemics participate. The major syntaxa to which these communities belong, were determined. The plant communities with the participation of local endemics were also referred to lower units according to EUNIS habitats classification. The study found that most of the studied endemic species occur in various vegetation types and exhibit relatively wide ecological flexibility, despite inhabiting restricted area. The main reason for this is the comparative uniformity of the high mountain marble relief. This study also highlights the high conservation value of the vegetation cover and habitat diversity of the carbonate alpine and subalpine vegetation belts of Northern Pirin Mts., as a habitat for numerous endemic and relict plant species.


Introduction
Pirin Mts. in Bulgaria are known for their exceptional concentration of numerous rare, relict and endemic plant species, their specific flora and vegetation.More than 70 endemics have been documented in the mountain, over 30 of them being local or narrow regional endemics [1][2][3].
Although Pirin Mts. are remarkably diverse in endemic plants and glacial relicts [1], studies on their vegetation cover are relatively few and have not been performed over a long period of time.The available studies on the flora and vegetation cover of Pirin Mts. are of a more general character [4][5][6][7][8][9].One of the main studies [10] giving review on the general plant communities of Pirin Mts., is relatively outdated and the classification schemes published therein need to be revised and updated.A more recent detailed work on the vegetation of rock fissures, screes, and snow-beds in Pirin Mts.[11] contains information on some of these endemic plants as participants in various described syntaxa.Subjects of phytocoenological studies have also been the communities of Centhranthus longiflorus subsp.kellererii (Stoj., Stef.& T. Georgiev) I. Richardson [12] and forests of Pinus heldreichii Christ.[13].Among the local endemics subject of the present study, a special autecological study [14] was made only on the two species of the genus Oxytropis.There are only short notes of the ecology and habitats of Cyanus achtarovii, Oxytropis urumovii, Alyssum pirinicum, Veronica kellereri, Brassica nivalis subsp.jordanoffii and Arabis ferdinandi-coburgii published in the main book of endemic plants in Bulgaria [1] and in the descriptions of the habitats in the Red Data Book of Bulgaria, vol. 3 [15].One paper [16] indicates some of the endemic species, studied by us, as related to certain habitats of conservation importance in this part of Pirin Mts.
The focus of the present study are the plant communities and habitats, distributed on marble bedrock where occur 8 local endemic plant species -Cyanus achtarovii (Urum.)Holub, Oxytropis kozuharovii D. Pavlova The aim of the present study is to determine synecological features of these local endemics for Pirin Mts. and their affinities to some specific plant communities and habitats.This will help us to determine the diversity and extents of the ecological niches and also capabilities of glacial relicts and neoendemics in this very important Arcto-Alpine refugium on the marble rocks.

Study area
The mountain is conditionally divided in three parts -Northern, Southern and Middle.Our study area (Figures 1, 2) is located in the Northern carbonate (marble) and highest in altitude part of Pirin Mts., covering the three characteristic marble areasthe main ridge named over the highest peak of the mountain Vihren (2914.3m), the ridge of Sinanitsa peak (2516 m) and a rocky site in Okadenski Cirque.Sampling plots are situated in altitudinal range from 1723 m to 2914 m.According to geological and geomorphologic studies [17][18][19][20][21], Pirin Mts. is a horst-anticlinorium, separated from all sides by powerful faults.The fold structure is represented by several subordinate, granitoid-embedded anticlines and the narrow synclines formed between them.The mountain morphostructure is built from the rocks of the metamorphic complex: shists, amphibolites, marbles, marbleized limestones and granito-gneisses, divided into two or more formations.The granite rocks form three intrusive bodies, all exposed at the surface.These bodies overlap with the three main parts of Pirin Mts.At a depth of 200 to 800 m, the covered granites are exposed in the river valleys under the metamorphic mantle, forming the main aquifer horizon of ground and karst waters.The marbles from Northern Pirin Mts. are weakly susceptible to physical weathering and denudation.The sharp cirque walls and waterless bottoms are formed.Main factor for the specifics of relief in this marble part of the mountain is the structure of the marble rocks, which combine carbonate mineral content and crystalline properties.They favour both the occurrence of karstification and frost weathering of the rocks.Because of the high elevation of Northern Pirin the large areas of former glaciation enhanced the karst processes by weathering of the morphological surface.At present, surface karst is most developed on elevated sections of cirque bottoms, which were ice free before the beginning of the Holocene and are away from rockfall creep accumulation.According to [21] in the high mountain karst in the most representative area, Vihren part, several vast and deep cirques-uvalas were formed, which comprise a large diversity of landforms.However, all marble area with pronounced karst geomorphology belongs to one general type of relief named as "glaciokarst".
Pirin marble rocks are usually distinctly white in color, which indicates the silicate poverty of the protolith; and results in a very high albedo, which is a reason for some differences also in climatic conditions comparing to the granites.As a product of metamorphism, marble has obtained structural features that bring some similarities to other crystalline rocks, including those with silicate composition (such as gneiss, granite, etc.).These concern the tendency to develop crack sat right angles, and fragment IOP Publishing doi:10.1088/1755-1315/1305/1/0120014 on larger angular blocks (rockfalls) under the action of physical weathering instead of pebble size particles, as it is with limestone.In such particular structural frameworks, marble could be more prone to physical weathering (especially frost action) than limestone.Especially in the high-mountain marble areas frost defragmentation is strongly pronounced and is often predominant on slopes and ridges [19], [21].
In the higher parts of Pirin Mts. the climate is typical mountainous.Average January temperatures range from -1 to -2 °C in the foothills and up to -5 °C.The average annual temperature at Vihren hut is +3.5°C.The amount of precipitation varies from 600-800 mm in the lower parts to 1400-1500 mm in the higher ones [22,23].The snow cover at highest altitude in this part of the mountain remains for 7-8 months.A specific characteristic of glaciokarstic relief in Pirin Mts. is the presence of whole year lasting snow/ice features (glaciers and snow patches), two of them are considered to be centuries old, and remain despite temperature warming [21].

Sample methodology
A total number of 72 unpublished phytocoenological relevés were used in the present research.They were carried out according to the Braun-Blanquet's methodology [24] with a size area between 14-50 m 2 , mostly of 25 m 2 .Vegetation sampling was conducted during 2022-2023 vegetation season.Permission for the field study conducted on public land was not provided because it is not mandatory since the study is conducted by visual observation and assessment of the cover-abundance values of the vegetation as it is according to the Braun-Blanquet's methodology and since the study is not related to collection of genetic material of the studied endemic species.Our study involved the following protected species: Cyanus achtarovii, Odontarrhena orbelica, Alyssum pirinicum, Brassica nivalis subsp.jordanoffii and Arabis ferdinandi-coburgii.

Vegetation analysis
The relevés were classified using cluster analysis in the SYN-TAX computer program [25].An averagelinkage method UPGMA was used as a sorting strategy.The floristic similarity between relevés in the (dis)similarity matrix was calculated by using the Horn index [26].This index was preferred as an information theory index of community similarity (community overlap) which was recommended for such type of vegetation data [27].
All the relevés were then imported into JUICE 7.0 program [28] and arranged according to their order in the resulting cluster dendrogram (Figure 3).Seven clusters of relevés were defined on the basis of coenological considerations using the separators according to their grouping in the dendrogram.One relevé was excluded from further analysis because of its non-typical character.Diagnostic species of each cluster were determined on the presence/absence basis by calculating the fidelity of each species to each cluster, using the phi coefficient of association [29,30].In order to remove the dependence of the fidelity measures on the vegetation unit size, each cluster was compared with the rest of the relevés in the data set, which were taken as a single undivided group.The size of target groups was fixed to 10% of the total data set while holding the percentage occurrences of species within and outside the target groups the same as in the original data set [31].The threshold phi value for the diagnostic species was set to 0.20 and Fisher's exact test was calculated to give a zero value to these species whose fidelity was not statistically significant (p < 0.05).Diagnostic species of the "group" 7 including only one relevé, were proposed based on floristic, biogeographic and ecological features.
The results of the classification were summarized in Table 1 (combined synoptic table), where diagnostic species were ranked by decreasing fidelity.Species were considered constant if their frequency exceeded 30% for the given community.Dominant species were defined as those attaining a cover higher than 50% and the number of relevés (percentages in brackets from their total number) where these species are dominant.
An indirect ordination analysis was conducted to visualize the relationships between the defined vegetation groups.An initial running of a Detrended Correspondence Analysis (DCA) resulted in a length of gradient higher than 3, allowing the application of unimodal methods [32].The DCA was performed on the basis of the original percentage species cover values which were squared root transformed.Rare species were downweighted.Environmental variables (elevation, inclination, total cover of vegetation) were used as passive variables in the DCA and projected onto the ordination diagram.The ordination analysis was performed by using CANOCO 5.0 software [32].

Taxonomic, syntaxonomic and habitat affiliation
The names of the vascular plants are presented according to [33], except the name Odontarrhena orbelica (≡ Alyssum orbelicum Ančev & Uzunov) based on the published revision of section Odontharrhena [34].
The major syntaxa at the alliance level to which these communities belong are indicated.They are determined according to the last summarized publication for syntaxa up to alliance level in Europe of [35].
The plant communities with the participation of local endemics were also referred to lower units according to the classification of habitats in Europe of EUNIS [36].

Vegetation communities with participation of local endemic plant species
The cluster analysis resulted in the distinguishing of 7 groups (Figure 3, Table 1).A smooth gradient of environmental factors was observed in the alpine marble part of Pirin Mts.It is evidenced by the low diversity at the habitat level, mainly due to the karst relief where water is not retained.It is characteristic that all communities develop on the basis of the marble rocks, with the soil cover being very poor or absent, and more mesophyllous conditions are found only around the snow beds, as the water there also quickly disappears into sinkholes.The ordination analysis supports the differences between the classified vegetation groups (Figure 4, left).The interpretation of the ordination diagram is assisted by the passively projected environmental variables.Along the first DCA-axis the communities are ordered according to the increasing elevation.The gradient on the second axis is the floristic richness which increases due to a decrease in the extremity of the environmental conditions.The scattergram for species (Figure 4, right), reveals an increase in pH along the first DCA-axis and, an increase in humidity along the second axis.The group of more mesophyllous species such as Salix reticulata, Plantago atrata, Bartsia alpina, etc. is positioned near the bottom of the diagram.
The identified vegetation groups are presented below with their main ecological and floristic characteristics, as well as their suitability as habitats for the studied local endemic species.(Table 1).Names of the taxa are indicated by eight-character labels containing the first four letters from both the generic and specific epithets of the species name except the following: GaliDemi -Galium demissum subsp.demissum; GaliStoj -Galium demissum subsp.stojanovii.Each taxon is indicated by the symbol corresponding to the group for which it is diagnostic or dominant, except for several constant taxa participating in more than one group -Achillea ageratifolia, Brassica nivalis subsp.jordanoffii, Cyanus achtarovii, Festuca penzesii, Odontarrhena orbelica, Carex kitaibeliana, Arabis ferdinandi-coburgii, Poa ursina.

Group 1 EUNIS: E4.437 Helleno-Balkanic stripped grasslands Number of relevés: 26
Diagnostic species: Aster alpinus, Carum appuanum subsp.bulgaricum, Leontodon crispus, Linum alpinum subsp.pirinicum, Minuartia setacea, Paronychia kapela, Saxifraga ferdinandi-coburgi, Silene saxifraga var.pirinica, Thymus thracicus Constant species: Anthyllis vulneraria subsp.vitellina, Carex kitaibeliana, Festuca penzesii, Helianthemum cinereum, Saxifraga luteoviridis, Sesleria korabensis Dominant species: Achillea ageratifolia, Poa ursina This group is represented by typical alpine tussock grasslands on poor soils and often opened marble base rocks.The vegetation cover varies between 20% and 90%, but mostly between 40 and 80%.The exposition is mostly southern or with a southern component.The slope of the terrain is between 5 and 60°, but mostly about 30-40°.The floristic and ecological structure of its communities are very similar to the association Achilleo-Seslerietum klasterskyi Simon 1958.Sesleria klasterskyi is a synonym of Sesleria korabensis [33] which is a species with a high constancy.Many of the species, including characteristic species of the group correspond to those described for the association [10].According to [10], they belong to the class of alpine and subalpine grassland communities of Elyno-Seslerietea, as well as to the alliance Anthyllido-Seslerion klasterskyi described as new to science from Pirin Mts.This group is the most important as a habitat for some of the local endemics, especially for Brassica nivalis subsp.jordanoffii (Figure 5).These grasslands are important habitat also for Alyssum pirinicum, Odontarrhena orbelica (the populations there are the most numerous) and less important for Arabis ferdinandi-coburgii and Cyanus achtarovii.This group is very similar to the previous one and forms a large cluster with it.The only difference is that it is more distinctly chasmophytic and with dominance in most of the plots included in cluster of the dwarf shrubglacial relict Dryas octopetala.The vegetation cover also varies, but it is mostly between 20 and 40%, respectively less than in Group 1.All the communities are open, located on highaltitudinal (2480 m average) insolated steep slopes (more than 40°, even often 50-60°) with predominantly southern exposition (or with a southern component).The substratum is composed by cracked rocks intermingled with gravel, yet solid rock outcrops in few plots.According to its ecological and floristic characteristics, this group is the closest to the association Lentopodio-Potentillietum stojanovii Simon 1958, which the author, however, placed in the chasmophytic class Asplenietea trichomanes.This scheme was also accepted by [11].However, [10] emphasizes that the transition between the chasmophytic and grassland communities is so smooth and they are difficult to be distinguished from each other.We therefore could consider this classification tentative and conclude that the groups 1 and 2 are very similar, but the first one is rather related to the open alpine grasslands, and the second one is rather representative to the alpine calcareous chasmophytic communities.This group is the second most important as a habitat for the local endemics.Only in this habitat Oxytropis urumovii is registered and it is also diagnostic species to the group.These rocky grasslands are however  6) is described upon small number of relevés and represents transitional chasmophyticalpine calcareous grassland communities.The differences with the previous Group 2 are that the substratum consists of relatively large marble outcrops holding thin soil layer with somewhat stronger water holding capacity.This allows retention of the tufts of Balkan endemic species such as Festuca pirinica, but also Sesleria korabensis and Alyssum pirinicum.The richness of Saxifraga species is another evidence for improved soil moisture conditions than in the previous groups.These communities are close to the association Festuco (pirinensis)-Seslerietum klasterskyi Simon 1957, which was classified by the author in the grassland class Elyno-Seslerietea.It is a representative habitat for Alyssum pirinicum, which is also a diagnostic species.Odontarrhena orbelica, Arabis ferdinandi-coburgii and Veronica kellereri are also represented in this group, but their populations are not numerous.

Group 4 EUNIS: F2.12111 Alpide Salix retusa-reticulata snowbed communities Number of relevés: 6
Diagnostic species: Armeria alpina, Bartsia alpina, Campanula grossekii, Carex parviflora, Doronicum columnae, Galium anisophyllon, Linum capitatum, Pedicularis orthantha, Pinus mugo, Plantago atrata, Salix reticulata, Silene pusilla, Taraxacum apenninum, Veronica kellereri Constant species: Carex kitaibeliana, Dianthus microlepis, Poa ursina, Sesleria korabensis Dominant species: Salix reticulata Group 4 (Figure 7) represents vegetation of snow-beds on stabilized calcareous screes in the alpine belt of Pirin Mts.These communities, dominated by Salix reticulata (low creeping semi-shrub) are classified by [11] in the association Bartsio-Salicetum reticulatae Mucina et al. 1990 (class Thlaspietea rotundifolii).It is the most typical snow-bed community on the calcareous substratum in Pirin Mts.They 1305 (2024) 012001 IOP Publishing doi:10.1088/1755-1315/1305/1/0120019 are observed either in small north-facing terraces under beetling rocks, either in shallow depressions in between large boulders in bottoms of glacial cirques, where snow remains for a longer period of time even during summer and all the populations are exposed to long-lasting snowy conditions.Veronica kellereri (Figure 8) is a diagnostic species with an ecological optimum in this group.No other local endemics have been documented in this specific habitat.In such open woodlands, affected by spontaneous fires and avalanches, some typical woodland or somewhat nitrophilous herbaceous species enter their floristic composition.Their syntaxonomic affiliation has not been studied until now, and its relations to other similar communities in the high limestone mountains of the Balkans are not known.Once again Alyssum pirinicum is the most widespread local endemic in these communities.However, Brassica nivalis subsp.jordanoffii and Cyanus achtarovii also participate but their populations were not numerous and representative for the habitat.

Dominant species: no
The communities of Group 6 are located at the most extreme areas for vegetationthe highest alpine belt of Vihren Peak, where the stony terrain with mobile screes and rock outcrops dominate the landscape.Vegetation cover is affected by extreme conditions such as strong winds, major temperature amplitudes, lack of soils, etc., therefore the communities are poor and with low coverage -10-15%.In the work of [11], a new endemic for Pirin Mts.alliance (Veronico kellereri-Papaverion degenii), represented by the association Papaveri degenii-Armerietum alpinae Mucina et al. 1990, was described.The most adaptive endemic species found in these grasslands is again Alyssum pirinicum.Odontarrhena orbelica and Veronica kellereri have small populations here because of the unsuitable rocky substrate which is permanently crumbling by strong erosion processes and is often barely or very sparsely vegetated.The last Group 7 (Figure 9) is represented by one relevé made in the Okaden Cirque.Although the marbles are dominant substratum in the locality "Yellow Rocks", there is a boundary area between marble formation and metamorphic volcanic rocks (biotite-gneisses, gneisses-schists, and amphibolite) formation, therefore the established community is in fact acidophilic which is also proven by the typical plant species such as Juncus trifidus, Deschampsia flexuosa, Festuva valida, etc.It is very likely that this community belongs to the association Festucetum pictae Domin 1931 which is distinguished by growing on relatively well-developed and well moistened during the vegetation season soils.This community is the habitat of only one endemic species -Oxytropis kozhuharovii.  . ---.

Synecological features of the endemic plant species.
The main results show that there are no great differences in the synecological preferences of the analyzed local endemics with exception of Oxytropis kozuharovii, which is commented separately.Their main characteristics are presented as follow: Alyssum pirinicum -It is the most ecologically flexible species with numerous populations, occurring in various grassland and chasmophyte communities (Groups 1, 2, 3, 5 and 6) in the whole marble part of Pirin Mt, except sites where soil moisture is hold for longer period places with or snow beds.It does not demonstrate preference in terms of the presence of soil cover, cover of other herbaceous plants, exposition, etc., but it is clearly a pronounced xerophyte that does not tolerate even temporary soil moisture.
Arabis ferdinandi-coburgii -The species shows to be dependent on thin soil cover and unable to compete with taller plants.It is rarely distributed in the study area and was found only in Groups 1 and 3, with small populations in the first one.Its synecological optimum is in Group 3, where other low herbaceous plants predominate, such as Festuca pirinica, several species of Saxifraga spp., Thymys perinnicus, Euphrasia minima, Rhodax alpestris, Poa ursina, Dianthus microlepis, etc.
Brassica nivalis subsp.jordanoffii -Populations of this species are characterized by a relatively good ecological flexibility, prevailing in the open grassland communities on the shallow, but developed soil cover.It is best represented in Group 1, consisted of communities with similar ecology, less represented in the Group 2, which has more pronounced chasmophytic features, and was found in a single relevé in Group 5, where it is in difficult position to compete with tall and tufted grasses and other tall herbaceous plants.
Cyanus achtarovii -Among all the 8 studied species, it is clearly the species that is most dependent on the presence of some (even not thick) soil cover, and it is the reason to be found in Groups 1 and 5, which represent open to almost-closed alpine and subalpine grasslands.It apparently withstands the competition of tufted grasses and even single shrubs and low trees.Probably, these ecological features came from the probable ancestral and closest, but more widespread species Centaurea triumphetti, which inhabits some lightly and open shrubby areas and forest fringes.
Oxytropis kozuharovii -Although the species was described as inhabiting mainly open grasslands on carbonates [14], [37], the community with its population described in the relevé, is definitely acidophilus, developed upon gneiss bedrocks.This species has only one locality in the world, which is disposed in Okadenski Cirque.In this marginal glacier valley of the marble part of Pirin Mt, there is a transition between two big Precambrian geological formations: Assenovgrad group (marble Sitovo group represented by gneiss-schists, biotite, gneisses, schists, marbles and amphibolite [19], which is generally with metamorphic, volcanic origin.This diversity of rocks with different structure, pH and other features, also affects and changes quickly in small patches some important to the plant's environmental conditions.[14] noted that at one and the same site with 3-15 m distances between the samples the values of pH varied markedly.According to the same authors, O. kozhuharovii is rather associated with the contact zone with the granites), while O. urumovii occurs strictly on marbles.In general, the synecological features of this species emphasized its place in the floristic composition of the acidophilic subalpine vegetation.Probably, this represents some type of geologically determined "hyper-isolation", because the other two similar species (O.urumovii and O. campestris) do not have populations in proximity.According to [38] it is possible that this species has evolved as an allotetraploid derivative of O. urumovii and O. halleri (including the closest taxon O. prenjasynonym of O. halleri var.prenja from Bosnia and Herzegovina).To be noted that further and more detailed ecological, physiological and molecular studies could give more precise answers about the centres of origin and the ancestral relationships between these Oxytropis species.
Oxytropis urumovii -This is the species with the fewest populations and with the most limited ecological flexibility, compared to all other seven species.Our study concluded that it occurs in compact, loose patches or as a scattered individual in comparatively wide territory, which is in line with [14].This species was found only in Group 2 (where it is also a diagnostic species) and occurs only in the alpine chasmophytic (open grassland) communities, together with other glacial relicts (including neoendemics) such as Galium demissum subsp.stojanovii, Leontopodium nivale subsp.nivale, Dryas octopetala, Potentilla apennina subsp.stojanovii, etc.
Odontarrhena orbelica -This species is ecologically similar to Alyssum pirinicum.However, it apparently demonstrates somewhat more limited ecological flexibility comparing with the previous one.Although it was designated by the authors who described it [39] as a chasmophyte, it prefers the presence of even a thin soil cover, so it mainly occurs in Groups 1 and 3, although it is also found in one plot in each of the Groups 2 and 6.Therefore, it is rather typical for open grasslands than to clearly rocky substrates.
Veronica kellereri -The species is a typical calciphilous chasmophyte.Its populations are found in Groups 2, 3, 4 and 6.However, it could also be determined as more mesophyllous element comparing with the rest of the studied species and typical for the snow bed vegetation as well as for the moist screes and pebble deposits.It was determined [11] as a diagnostic species and name-giving taxon for the alliance Veronico-Papaverion degenii Mucina et al. 1990.In addition, our study shows that the species has broader ecological niche comparing with previously described communities.

Conclusion
The present study identified some synecological characteristics of the local endemic plants distributed in the marble part of Northern Pirin Mts.As a general conclusion, they have common ecological preferences and are normally found in more than one community/ habitat, which is due to their adaptation capacity to occupy specific ecological niches and furthermoreto a somewhat similarity in habitat features, that are formed under these specific high-altitudinal conditions.The geological and geomorphological features of this part of the mountain, combined with climatic conditions, have resulted in specific and isolated environment for plants development and speciation.It is also the reason for the high endemism at species and subspecies level.However, the uniform geomorphological features over a large area are the reason for two of the key environmental factors for vegetation development, namely water presence and soil fertility, being practically at a significant minimum.As these factors do not vary significantly in spatial and temporal scale, they cannot support this diversity of habitats, as in the places where water and soil cover are retained.It is also comparable with the neighbouring granite parts of the mountain, where greater water availability and soil type diversity are the reasons for providing greater habitat diversity including various vegetation types.In contrast, the gradual and small differences in water availability and soil richness in the marble part of Pirin Mts.result in uniform conditions and narrow variations of the habitats' traits.These variations are primarily due to altitude, absence or scarcity of soil coverage, and longer retention of air and soil moisture in deeper cirques or rock crevices near snow beds.It is indicative that the dominant grasses and sedges such as Sesleria korabenis, Festuca penzesii, Poa ursina, Carex kitaibelliana, have a high constancy and are widespread practically throughout the studied territory.For this reason, endemic plant species inhabit more or less the whole marble massif and they appear, albeit to varying degrees, in most of the plant communities.Still, there are endemic species with relatively wider ecological flexibility such as Alyssum pirinicum and Brassica nivalis subsp.jordanoffii.Another group includes species with more limited requirements to the environmental conditions like the two Oxytropis spp., more or less Veronica kellereri and Arabis ferdinandi-coburgi.In fact, Oxytropis kozhuharovii is a different case.So far, there is only one known population, which can be tentatively assumed as located in the marble part of Pirin Mts.However, in the Okaden Circus, there is one wide transitional zone, where two large geological formations are bordered.Despite the predominance of marbles there and calciphillous flora and vegetation, the population of the species is found to inhabit metamorphic volcanic rocks: gneisses and gneiss-schists.It participates in the floristic composition of a typical acidophilic community from Juncetea trifidi class.
The present study has also nature conservation importance.It emphasized once again the uniqueness of the highest marble massif of the Pirin Mts., as a center of origin of endemism under very specific environmental conditions.It emphasized the high conservation value of the vegetation cover and habitat diversity of the alpine and subalpine vegetation belts of the Northern Pirin Mt, as a habitat for numerous endemic and relict plant species and globally important protected area for nature conservation purposes.

Figure 1 .Figure 2 .
Figure 1.Map of the study area in the carbonate (marble) part of Northern Pirin Mts.

Figure 3 .
Figure 3. Cluster dendrogram of the established 7 main groups of communities in the carbonate part of Pirin Mts.

Figure 4 .
Figure 4. Ordination diagrams of relevés (left) and diagnostic, constant and dominant taxa (right) on the basis of the first two DCA axes.Environmental variables are passively projected onto the ordination space.The numbers of groups correspond to their numbers in the cluster dendrogram (Figure 3) and the synoptic table(Table 1).Names of the taxa are indicated by eight-character labels containing the first four letters from both the generic and specific epithets of the species name except the following: GaliDemi -Galium demissum subsp.demissum; GaliStoj -Galium demissum subsp.stojanovii.Each taxon is indicated by the symbol corresponding to the group for which it is diagnostic or dominant, except for several constant taxa participating in more than one group -Achillea ageratifolia, Brassica nivalis subsp.jordanoffii, Cyanus achtarovii, Festuca penzesii, Odontarrhena orbelica, Carex kitaibeliana, Arabis ferdinandi-coburgii, Poa ursina.

Table 1 .
Combined synoptic table for the 7 clusters distinguished in the carbonate part of Pirin Mts.Frequencies of species are presented as percentages and fidelity values calculated using the phi coefficient and multiplied by 100, are indicated in superscript.The percentages and fidelity values of the diagnostic taxa are bordered.