Birch Bog on Anthropogenically Transformed Raised Bogs. A Case Study from Pomerania (Poland)

Birch bog is formed on the margins of or within raised bogs, on secondary habitats. The study aim was to understand the vegetation and mycological diversity of birch bog on the background of habitat conditions on raised bogs subject to anthropogenic changes, including 15 areas located on seven bogs. Two of the analyzed areas were located on a peat bog not subject to human impact. Phytosociological and mycosociological relevés were taken and substrate analyses were carried out (pH, humidity, N-NH4, N-NO2, N-NO3 and P-PO4). Based on habitat predictors, two area groups were distinguished, differing primarily in humidity. More humid habitats were present on the margins of bogs, and were characterized by lower acidity and higher N-NH4 and P-PO4 abundance. Despite the fact they were enriched by runoffs from the neighboring arable fields, this was not always reflected in the plant and fungi species richness. Quercus robur appeared on less humid habitats, which may be a symptom of unfavorable changes toward habitat drying. In the majority of cases, changes in the habitat independent of the birch patches located and the human impact type are not yet reflected in the vegetation. However, they may be indicated by the fungal diversity, highest in former peat extraction pits, and lowest in pristine peat.


Introduction
Bogs are among the most valuable ecosystems, not only due to the rare and valuable species found therein, but also due to the fact that they are natural retention basins. They contain 10% of the freshwater volume of the Earth [1]. Moreover, they are of key importance for the long-term sequestration of atmospheric carbon [2]. It is estimated that peat contains circa 26% of all the terrestrial carbon accumulated since the Last Glacial Maximum [3] thus they are among the largest terrestrial carbon reservoirs. Close to 95% of all peat bogs around the world are located in the northern hemisphere in a cold and at the same time humid temperate climate [4]. It is also here where the majority of raised bogs are present, supplied solely by rainwater. Such ecosystems are highly susceptible not only to natural factors, such as unfavourable climatic changes but above all hydrological disturbances resulting from human activity [5]. For the purpose of the study, 15 patches of birch bog (Vaccinio uliginosi-Betuletum pubescentis) representative of the aforementioned bogs were selected (Table 2). Two patches in Mszar near Stara Dobrzyca were treated as reference points for the remaining surfaces due to the fact that they have not been disturbed (no human impact present). The patch sizes were 400 m 2 , with the exception of patch S2, measuring 200 m 2 . In order to determine the plant species diversity and their share in the community structure, phytosociological relevés were taken using the Braun-Blanquet method. The patches also constituted permanent plots for the observation of macromycetes. Systematic mycological observations were carried out every 2-3 weeks on average; between 22 and 26 observations were carried out for each plot. The share of fungi in the community was analyzed within bioecological groups: mycorrhizal fungi and saprotrophic fungi (growing on peat and among mosses, litter-inhabiting).  [47], 3 [48], 4 [46]. The nomenclature of vascular plants is described in [50] and the nomenclature of mosses is according to [51]. The nomenclature of fungi is given in [52]. The herbarium documentation is deposited in the Herbarium of the Department of Botany and Nature Conservation, Szczecin University (SZUB-F).
Assessment of soil and water conditions was performed based on the analyses of cumulative soil samples. Samples were collected in three consecutive years of the study, on three occasions in the growing season: in spring, summer and autumn (they comprised replications in the statistical analysis), from a depth of 0-20 cm. The following parameters were determined in the samples: ammonium nitrogen content (N-NH 4 )-via distillation, nitrite nitrogen content (N-NO 2 )-via distillation, nitrate nitrogen content (N-NO 3 )-via colorimetry with Griess method, available phosphorus content (P-PO 4 )-via colorimetry with phosphomolybdate blue (Egner-Riehm method), pH-potentiometry, humidity-by weight in a moisture balance. The analyses were performed at the Department of Soil Sciences, Grassland and Environmental Chemistry of the West Pomeranian University of Technology.
The results concerning the parameters of soils were developed on the basis of the univariate analysis of variance. Additionally, such factors as the place of occurrence of the raised bog, drying up of the habitat, the influence of anthropopressure (surface runoff from agricultural fields and meadows) and the location of the habitat were tested. The significance of differences was evaluated using the Tuckey's HSD (honestly significant difference) test at the significance level α = 0.05. Based on the standardised physical-chemical data, the investigated soils were grouped by use of hierarchical cluster analysis, Ward's square Euclidean distance method [53]. This method consists in presenting similarities between objects as a function of distance. The variables describing the object (in our case parameters of soils) are more similar to one another when the distance between them is smaller.
Correlations between plants and fungi and habitat predictors and between plant predictors and fungi were determined by calculating the Pearson's correlation coefficient. In statistical analyses, the vegetation cover in tree layer 'a' and shrub layer 'b' was considered jointly, as 'a + b', since both layers played a similar role with regards to the group of mycorrhizal fungi and due to the correct interpretation of results they should be analyzed jointly.
These statistical analyses presented in this paper were achieved using the statistical software package for Windows (Statistica ® v.12 PL, StatSoft, Szczecin, Poland).

Results
The upper layers of peat of all the analyzed areas were characterised by strongly acidic pH and very low, although variable, value of available phosphorus ( Table 3). The birch bog in Stramniczka and Zielone Bagno encompasses large areas, forming on dykes and very dry areas, on a layer of humipeat or in poorly hydrated, overgrown former peat extraction pits. In the Stramniczka bog, the upper layers of peat of the S1 and S2 plot have similar humidity and higher amounts of N-NH 4 and N-NO 2 than N-NO 3 ( Table 3). On the other hand, plots of the Zielone Bagno bog have variable humidity, yet on ZB2 a significantly higher amount of N-NH 4 was found. On Torfowisko Toporzyk, birch bog occurs primarily in the western part of the bog, covered by transition peat deposit, and it remains in contact with alder bog, growing on more eutrophic habitats. In this bog, the upper layer is characterised by high humidity and higher amounts of N-NH 4 and N-NO 2 than N-NO 3 . Birch bog on the Niewiadowo bog was formed primarily in its southern and south-western part, creating extensive patches and from the north, it is adjacent to coniferous bog forest. The upper layer of the bog indicates variability in terms of humidity and nitrogen content (at N1 and N2-N-NO 3 forms are predominant). The birch bog in Mszar near Stara Dobrzyca covers small areas at the bog margins, indicating a high degree of naturalness. The SD1 and SD2 plots differ significantly in terms of humidity but not soil composition. They have a higher share of the N-NH 4 and N-NO 2 forms over N-NO 3 . In the case of the Ziemomyśl, birch bog grows over the majority of its surface, and it is characterised by good hydration of the upper peat layer as well as a high share of the N-NH 4 form. On the other hand, at Roby this community occurs primarily on the margins and on slightly dried surfaces (R1-3), which do not exhibit significant  (Table 3). As a result of habitat predictor analysis based on the estimation of the distance between clusters utilizing analysis of variance (Ward's method), two major groups of the analyzed plots were obtained ( Figure 1). The first group included plots S1, S2, T1, ZB1, ZB2, N1, SD1 and Z1 which, with the exception for T1 plot, were characterized by very high humidity. The second group included plots R1, R2, R3, ZB3, N2, N3 and SD2, which, apart from SD2, were characterized by lower humidity in comparison with the second group.
Both the less humid habitats (on the basis of the division with Ward's method, Figure 1), as well as human impacts (surface runoffs), indicated significant variability of soils in terms of humidity, pH, ammonium nitrogen and orthophosphate (V) phosphorus. Such differences between soils were not determined for the concentration of nitrate(V) and (III) nitrogen (Tables 4 and 5). Soils of the study areas exposed to surface runoffs are enriched with N-NH 4 and P-PO 4 relative to natural and other areas.  As a result of habitat predictor analysis based on the estimation of the distance between clusters utilizing analysis of variance (Ward's method), two major groups of the analyzed plots were obtained ( Figure 1). The first group included plots S1, S2, T1, ZB1, ZB2, N1, SD1 and Z1 which, with the exception for T1 plot, were characterized by very high humidity. The second group included plots R1, R2, R3, ZB3, N2, N3 and SD2, which, apart from SD2, were characterized by lower humidity in comparison with the second group.  Table 2.
Both the less humid habitats (on the basis of the division with Ward's method, Figure 1), as well as human impacts (surface runoffs), indicated significant variability of soils in terms of humidity, pH, ammonium nitrogen and orthophosphate (V) phosphorus. Such differences between soils were not determined for the concentration of nitrate(V) and (III) nitrogen (Tables 4 and 5). Soils of the study areas exposed to surface runoffs are enriched with N-NH4 and P-PO4 relative to natural and other areas.   Table 2.
Taking into consideration the habitat locality, soils of the analyzed study areas differed statistically significantly only in terms of humidity, soil pH and concentration of ammonium nitrogen ( Table 6). The middle parts of the bogs are characterised by higher soil acidity and lower humidity and N-NH 4 and P-PO 4 abundance than surfaces on the margins and in former peat extraction pits. Table 6. Characteristics of peat soil habitats depending on their location on the bog. Former peat extraction pits (S1-2, ZB1-3, N1 and N3), bog margin (T1, SD1-2 and Z1), middle part of the bog (R1-3 and N2). Soils of the analyzed study surfaces originating from mid-forest bogs were characterized by significantly higher pH H 2 O values than those located on mid-field bogs. The remaining physical and chemical parameters of soil were similar (Table 7). Table 7. Characteristics of peat soils of mid-field and mid-forest bogs. Mid-forest bogs (N1-3 and SD1-2), mid-field bogs (S1-2, T1, R1-3, ZB1-3 and Z1).

Parameter
Humidity A total of 78 plant taxa were found in birch bog, and in individual patches from 14 to 34 species were recorded (Figure 2). Class Vaccinio-Picetea, in which this community is included, is represented depending on the patch from 2 to 12 species, and moss species from the class Oxycocco-Sphagnetea from 0 to 7. The vegetation cover in certain layers of the examined patches was occasionally variable (Figure 3). This is visible, e.g., in the layers of the tree and shrub stand on Roby bog (R1-3), as compared with the remaining objects. The tree stand of all the analyzed patches is dominated by Betula pubescens, frequently accompanied by Pinus sylvestris (sometimes rather common-N2 and SD1), and less commonly Sorbus acuparia and Betula pendula. The shrub layer is predominantly formed by Betula pubescens and Frangula alnus undergrowth, sometimes with an admixture of Salix spp. In some patches (R1, S2 and SD1-2), the herbal layer is frequented by species of open moss area from class Oxycocco-Sphagnetea, including Erica tetralix, Eriophorum vaginatum and Oxycoccus palustris. The contribution of Sphagnum is marked in the moss layer, and in some areas (SD1-2, S1-2 and T1) abundance of Sphagnum fallax, Sph. palustre and Sph. squarosum, and among brown mosses, Pleurozium schreberi (ZB2 and N1-3) and Aulacomnium palustre (SD2 and R1-2) was recorded. Sphagnum magellanicum was rare, and was recorded only on the preserved habitat (SD1-2). The presence of Lycopodium annotinum-a species regionally characteristic of birch bog was found on T1, N2-3 and ZB1-3. Encroachment of Phragmites australis-a species with wide ecological scale-was recorded in patches R2, S2 and Z1, and a considerable contribution of Molinia cearulea in R1 and R3, with the concomitant appearance of juvenile Quercus robur, whereas Picea abies was growing singularly only in T1. The contribution of Sphagnum is marked in the moss layer, and in some areas (SD1-2, S1-2 and T1) abundance of Sphagnum fallax, Sph. palustre and Sph. squarosum, and among brown mosses, Pleurozium schreberi (ZB2 and N1-3) and Aulacomnium palustre (SD2 and R1-2) was recorded. Sphagnum magellanicum was rare, and was recorded only on the preserved habitat (SD1-2). The presence of Lycopodium annotinum-a species regionally characteristic of birch bog was found on T1, N2-3 and ZB1-3. Encroachment of Phragmites australis-a species with wide ecological scale-was recorded in patches R2, S2 and Z1, and a considerable contribution of Molinia cearulea in R1 and R3, with the concomitant appearance of juvenile Quercus robur, whereas Picea abies was growing singularly only in T1.     A total of 144 macromycetes species were recorded on the permanent plots of birch bog, and from 33 to 74 species were found on particular plots. As a rule, the highest number of species was found on plots located in former peat extraction pits, and the lowest on bog margins (with the exception of the T1 plot). The share of fungi in the selected ecological groups was variable (Figure 4). Cortinarius flexipes, Laccaria proxima, Lactarius tabidus, Russula betularum and R. claroflava were distinguished among mycorrhizal fungi (46 species) in terms of the frequency of occurrence and abundance. This group was most frequently represented on the plots in former peat extraction pits, and least frequently on bog margins. Lignicolous fungi (47) were predominant among saprotrophic fungi (90), and some of them form permanent, annual or several-year-old fruiting bodies, including Daedaleopsis confragosa, Diatrypella favacea, Fomes fomentarius, Inonotus obliquus and Fomitopsis betulina. The second group of saprotrophic fungi in terms of abundance, including a considerably lower number of species, consisted of peat-growing fungi (15), such as Gymnopus dryophilus, Rhodocollybia maculata, Entoloma cetratum and E. sericatum. Their contribution was variable between individual plots; however, it was lowest on the peat bog margins. Bryophilous fungi (12) were represented by, among others, Bogbodia uda, Hypholoma elongatum and Galerina tibiicystis, and litter-growing fungi (13) by e.g., Mycena galopus and Gymnopus androsaceus. The presence of representatives of both groups on individual plots was variable. A total of 144 macromycetes species were recorded on the permanent plots of birch bog, and from 33 to 74 species were found on particular plots. As a rule, the highest number of species was found on plots located in former peat extraction pits, and the lowest on bog margins (with the exception of the T1 plot). The share of fungi in the selected ecological groups was variable (Figure 4). Cortinarius flexipes, Laccaria proxima, Lactarius tabidus, Russula betularum and R. claroflava were distinguished among mycorrhizal fungi (46 species) in terms of the frequency of occurrence and abundance. This group was most frequently represented on the plots in former peat extraction pits, and least frequently on bog margins. Lignicolous fungi (47) were predominant among saprotrophic fungi (90), and some of them form permanent, annual or several-year-old fruiting bodies, including Daedaleopsis confragosa, Diatrypella favacea, Fomes fomentarius, Inonotus obliquus and Fomitopsis betulina. The second group of saprotrophic fungi in terms of abundance, including a considerably lower number of species, consisted of peat-growing fungi (15), such as Gymnopus dryophilus, Rhodocollybia maculata, Entoloma cetratum and E. sericatum. Their contribution was variable between individual plots; however, it was lowest on the peat bog margins. Bryophilous fungi (12) were represented by, among others, Bogbodia uda, Hypholoma elongatum and Galerina tibiicystis, and litter-growing fungi (13) by e.g., Mycena galopus and Gymnopus androsaceus. The presence of representatives of both groups on individual plots was variable. The results of Pearson's correlation (R) used to determine the type of correlation between plants and fungi and the selected habitat parameters of birch bog indicate the existence of significant relationships between the group of species of class Vaccinio-Picetea, the total number of fungi, M, Sh and Sl fungi groups, and substrate pHKCl, and N-NO3, N-NO2, N-NH4 and P-PO4 content (Table 8). Moreover, the M fungal group exhibited a positive correlation with substrate pHH2O and humidity (R = 0.62, R = 0.64, p < 0.05, respectively). A positive correlation with pHH2O was also found for vegetation The results of Pearson's correlation (R) used to determine the type of correlation between plants and fungi and the selected habitat parameters of birch bog indicate the existence of significant relationships between the group of species of class Vaccinio-Picetea, the total number of fungi, M, Sh and Sl fungi groups, and substrate pH KCl , and N-NO 3, N-NO 2, N-NH 4 and P-PO 4 content (Table 8). Moreover, the M fungal group exhibited a positive correlation with substrate pH H 2 O and humidity (R = 0.62, R = 0.64, p < 0.05, respectively). A positive correlation with pH H 2 O was also found for vegetation cover in 'a + b' and 'd' layers (R = 0.51, R = 0.56, p < 0.05, respectively). Furthermore, a positive significant relationship occurred between humidity and certain vegetation parameters and the total number of fungi species (Table 9). Moreover, a negative correlation was only found between the species group from class Oxycocco-Sphagnetea, and substrate pH KCl (R = −0.53, p < 0.05).
The existing relationships between fungi and plant predictors are presented in Table 8. A significant positive correlation was obtained between class Vaccinio-Picetea species group, and the total number of fungi species and M, Sh and Sl fungi groups (R = 0.87, R = 0.67, R = 0.72, R = 0.78, p < 0.05, respectively). A significant negative correlation was obtained between class Oxycocco-Sphagnetea species group and the total number of fungi species and Sh fungi group (R = −0.60, R = −0.61, p < 0.05, respectively). Furthermore, the Sh group was negatively correlated with the plant cover of the 'd' layer (R = −0.59, p < 0.05, respectively).

Discussion
In Poland, birch bog is a rather rare community, restricted to the north-west part of the country. Its mature phytocenoses occupy the habitat with the total area of only 8.75 km 2 [30]. Typically, this community is species-poor, which has been observed both in Ireland [27], as well as in Poland, where normally about 20 plant species are found in one patch [30]. Among the birch bog patches we analyzed, 2/3 contained less than 20 plant species (Figure 2), and their low numbers were found not only on the undisturbed peat bog (SD2), but also on certain bog areas subject to human pressure (e.g., N2). Consequently, the low species diversity cannot be linked solely to natural habitats.
Ward's analysis of birch bog distinguished two habitat groups, less and more humid. The more humid habitats were characterised by lower acidity and were more ammonium nitrogen and phosphorus rich. They were located primarily on the best-hydrated bog margins, which were accessed by runoffs from neighbouring, more nutrient-rich minerotrophic areas. These runoffs often contained biogens from arable lands adjacent to bogs. The patch with the highest floristic diversity (T1, 34 species) was formed on such a habitat. This may suggest that substrate biogen abundance and high humidity play a significant role in both the species richness and composition of the community. However, this was not always reflected by soil richness, for example on Z1-a patch characterised by the highest biogen abundance and high humidity-only 16 plant species were determined on bog margin. Correlation analysis results suggest that the total number of plant species in a community patch is significantly associated only with humidity (Table 8). On the other hand, nutrients such as N-NH 4 , N-NO 3 , N-NO 2 and P-PO 4 , may have a positive impact on the abundance of class Vaccinio-Picetea species, in which the discussed community is included. The type of phytocenoses neighboring birch bog significantly influences the number of species found therein. In the case of T1, the neighboring alder bog, rushes and minerotrophic forests resulted in the patch being enriched with plants which penetrated from those habitats, including Peucedanum palustre, Picea abies and Galium palustre. On the other hand, the species poverty of the Z1patch, which was the most biogen-rich, may be associated with i.a. the vicinity of a moss area, which comprises oligotrophic species, exhibiting the lower capacity to penetrate more nutrient-rich habitats. The appearance of Sphagnum magellanicum-a species strictly associated with raised bog habitats, in the SD1-2 patches, located on bog margins and not subject to human pressure, also resulted from the direct vicinity of moss area with birch bog. The negative correlation between the number of species from class Oxycocco-Sphagnetea, and pH KCl (Table 8) is typically explained by the considerable share of Sphagnum spp., as they additionally impact habitat acidification, by binding cations (Ca and Mg) present in the environment and the release of hydrogen ions [54]. However, at Roby (lowest pH) this correlation can be linked with slightly higher (with regards to the majority of the remaining study objects) number of herbaceous plant species included in this class, and not with Sphagnum spp., as those mosses grew there only at low amounts. Typically, the share of raised bog species in birch bog is low [30].
Less humid habitats were mostly located within the bogs. The least humid birch bog patches were located in Roby (R1-3) and Niewiadów (N2). They were richer in N-NO 3 than N-NH 4 , which could be a symptom of unfavourable changes toward habitat drying [55][56][57]. This phenomenon may be also suggested by the presence of single juvenile Quercus robur individuals [58] recorded in patches R1 and R3, where moor grass was also found, as well as in R2. Moreover, in the vegetation structure of R2, a high share of shrub 'b' layer was recorded, sometimes higher than trees 'a', this could have stemmed from the lower habitat humidity, which may have promoted numerous appearances of Betula pubescens in the understory which was definitely predominant here.
The spatial structure of birch bog was in some cases otherwise, e.g., by comparing herbs layer 'c' and moss layer 'd' on extracted bogs with undisturbed bog (Mszar near Stara Dobrzyca). In bog patches subject to human change, the 'c' cover was usually higher than the 'd'. A reverse relationships were found in Betuletum pubescentis on the previously exploited peatlands in Ireland [27], as well as in a similar community described as bogged birch forest with peat located on similar disturbed habitats in Siberia [42]. Former peat extraction pits, in which birch bog (S1-2, ZB1-3, N1 and N3) was formed, were characterised by similar humidity and slightly lower acidity in comparison with areas on the bog margins. However, the vegetation did not demonstrate considerable differences with relation to this community patches located on other sites. In these sites, birch bog was typically well-developed, which was also indicated by the presence of Lycopodium annotinum in half of these patches, a characteristic species for this community, as well as common Pleurozium schreberi, which had a considerable share in some patches. Former peat extraction pits constitute secondary habitat for birch bog and it may thrive in them only when the high water level is not maintained in this habitat, otherwise Betula pubescens will gradually wither, and under favorable conditions succession may have the direction of moss community restoration. Expansion of forest vegetation is determined by the groundwater level in the bog and the air availability in the upper peat layer [57,59]. For the normal development of the root system, trees require more than a 10% share of peat pores to be filled with air [59]. The analyzed bogs, independently of the study area location (middle, margin, former peat extraction pits), fulfilled this condition, only on peat bog not subject to human impact, the study area SD1 had pores filled with air in 8% and with water in 92%. Furthermore, trees require a nutrient supply for normal development. Birch requires considerably higher amounts of nutrients than pine [60]. On no disturbed peat bogs has the development of trees been inhibited primarily by the excessive humidity and bog growth, and to a lesser degree by the nutrient deficiency [61,62].
Molinia cearulea is a common element of birch bog and its share in this community may vary. In Austria, apart from Vaccinium uliginosum, it is a common predominant plant species of the herbs layer of this community [21]. In patches R1 and R3, where moor grass was most common, it was observed that peat mosses were characterised by lower cover percentage. Competition between these plants for light could be one of the causes of this phenomenon. The importance of this factor was indicated by Hogg et al. [63], who observed that cutting back M. coerulea restricts its competitiveness for light, having a positive impact on the development of peat mosses. On the other hand, Limmper [64] determined that the high share of herbaceous plants on peat bogs may have a negative impact on the development of these mosses. The shadow effect by vascular plants has been indicated to be a cause for this state by Hayward and Clymo [65] and by Heijmans et al. [66]. In turn, the living layer of peat moss may inhibit the growth of vascular plants by binding large amounts of nitrogen [64,67].
Birch and moor grass may appear on ombrotrophic bogs not only on their natural habitats, that is the margins of raised bog, but also on moss areas. It is a highly unfavourable phenomenon as the increase of their total cover and expansion of other vascular plants stimulates evapotranspiration [67,68]. This may contribute to habitat drying, which is very sensitive to changes in water relations, and in such cases susceptible to invasion of Picea abies, which is an alien species in terms of habitat and geography. However, no such phenomenon has been observed on the analyzed patches located in the middle of the bogs. The presence of singular individuals of spruce was recorded only in T1, located in a more nutrient-rich bog margin.
Birch bog is one of the bog forest communities least known in mycological terms. Fungi constitute its important, integral structural element, as they enter into a series of interactions with different plant species and they influence not only the maintenance but also the habitat transformation [69]. They also play the dominant role among decomposer organisms in acidic peat ecosystems [44]. The majority of them are saprotrophs, participating in the decomposition of organic matter [70,71]. Birch bog, other than for plants, is typically rich in macroscopic fungi species, which has been determined in both the Słowiński National Park (78 species, [72]) as well as in the Goleniów Forest (85 species, [73]). Among the analyzed plots, ZB1-2 and N1 (61-74 species) are the richest in fungi species, and are located in former peat extraction pits. In turn, the SD1-2 plots not subject to human pressure turned out to be the least rich in fungi species (33)(34)(35)(36)(37)(38)(39). The contribution of peat mosses could be one of the factors restricting the number of fungi species on an undisturbed habitat, as it was here highest among all the analyzed plots. The locally compact layer of peat mosses typically inhibited the development of fungi growing on a different substrate, including peat, which is further suggested by the negative correlation, whereas it favored the occurrence of a small group of fungi associated with moss areas, e.g., Galerina paludosa, G. tibiicystis and Sphagnurus paluster. The role of these moss species is confirmed by the obtained negative correlation between the total number of fungi species per plot, and the number of plant species from class Oxycocco-Sphagnetea. An analysis of Pearson's correlation indicates that both the total number of species of fungi found within one surface and the number of mycorrhoidal species were significantly and positively correlated with the number of plant species from the Vaccinio-Picetea class. This relationship stems from the community structure in which the Vaccinio-Picetea class plant species are among its basic elements, and in particular B. pubescens and its accompanying trees interacting with fungi. Numerous fungi species, including mycorrhizal species, are specific for given tree species [74], which is reflected by the biota of macromycetes of the given community. Plots N2 (in the middle of the bog) and SD1 (bog margin) are examples here, where the considerable share of pine resulted in the appearance of fungi associated with them, including Russula emetica, Lactarius rufus and Auriscalpium vulgare.
The fungal species diversity of birch bog does not only depend on the multi-plane relationships between these specific organisms and plants but also on chemical properties of the soil. Results presented in this study indicate the existence of significant relationships between the occurrence of fungi species and selected environmental factors, i.a. humidity and soil pH. In birch bog, all the study areas were characterised by more or less acidic soil. The determined positive correlation between the number of fungi and certain bioecological fungi groups, and pH suggests that excessively acidic soil may have a negative impact on the diversity of macromycetes. Numerous fungi species prefer determined values of pH and react differently to its change [75,76]. A similar situation occurs in the case of phosphorus and nitrogen compound content in soil, as many fungi species e.g., ectomycorrhizal, exhibit different tolerance to the high or low content of nitrogen in soil [77]. Moreover, the availability of different nitrogen forms plays a significant role in the production of fruiting bodies [76] and formation of ectomycorrhiza [78]. Based on the obtained positive correlation between the total number of macromycetes on the given plot and certain bioecological groups of fungi, as well as the content of phosphorus and various nitrogen forms in the soil it can be expected that with their increase macromycetes diversity will increase. Trees growing on humid peat soils largely depend on ectomycorrhizal symbionts which facilitate their absorption of nutrients and participate in capturing P [79,80]. In the case of Betula pubescens saplings [67], it was determined that the presence of ectomycorrhizal symbionts did not guarantee trees growth, which could stem from inhibition of the activity of those symbionts by acidic conditions. The correct content of nitrogen and phosphorus in the soil is of particular importance for the development of birch [11,67], given that birch is incapable of fully utilising nitrogen in the event of the absence of P in soil [81].

Conclusions
Changes occurring in the habitat of Vaccinio uliginosi-Batuletum pubescentis birch bog regardless of (1) location of bog patches, (2) anthropopression type (drainage, peat exploitation, surface runoff), (3) time of anthropogenic impact (peat exploitation -lasted about 40 years, completed approx. 60 years ago, dewatering-started a few years before exploitation of peat, lasts with lower intensity to today) and (4) intensity of human impact, in most cases are not yet reflected in the vegetation, as found on the basis of a comparison with birch growing in a peat bog not disturbed by humans. Plant species, e.g., Quercus robur, started to appear only in singular areas, but in the case of their greater share they may have indicate ongoing transformations of these habitats. Habitat conditions enable the presence of only a strictly limited number of widely distributed plant species, such as Phragmites austrialis. However, the slow rate of change is demonstrated by the high diversity of fungi species, which was highest in the areas of former peat extraction pits, whereas lowest on bog margins, and particularly on the undisturbed bog. On the other hand, the considerable presence of fungi that are strictly associated with bogs constitutes an indicator for the still good preservation state of the studied objects.
When birch bog appears on secondary habitats in the middle of a bog, a dilemma arises as to whether humans should allow for its further development. In the case where birch bog would threaten the existence of a moss area on an ombrogenic peat bog, activities aiming toward its eradication for the advantage of the moss area should be undertaken by using e.g., long-term submerging of the area where birch bog is found. However, if this community is well developed, which would be demonstrated by, i.a. presence of its characteristic species, and its state, based on the accepted indicators, can be evaluated as good [58], then such a community should be protected also on secondary habitats, as it is rare and has priority among Natura 2000 habitats. It is a permanent community in stable hydrological conditions. Despite the fact that the majority of the studied birch bog habitats have been and still are being mostly subject to different types of human impact, their physicochemical properties, including humidity, enable further development of this community. The majority of the analyzed birch bog patches are located on protected areas, thus granting them a higher chance of survival.
Author Contributions: Z.S. and M.S. designed, conducted the research, partly analyzed the data, and wrote the paper; R.M. carried out laboratory analyzes, partly analyzed the data and participated in writing; R.G. participated in fieldwork and reviewed the final draft of the manuscript; M.G. partly analyzed the data and reviewed the final draft of the manuscript. All authors read and approved the final manuscript.
Funding: Studies supported financially in part by the Ministry of Science and Higher Education (Poland), grant N N305 2617 33.

Conflicts of Interest:
The authors declare no conflict of interest.