Suillus flavidus, a Peatland-Associated Mycorrhizal Fungus in Poland: Ecology, Distribution, Conservation Threats, and Sustainability Considerations

Abstract

Suillus flavidus is an ectomycorrhizal fungus associated with moist, nutrient-poor habitats, particularly peat bogs and transitional mires, where it forms symbiotic relationships with two-needle pines, especially Pinus sylvestris. This study presents an updated assessment of its distribution in Poland, identifies key ecological factors influencing its occurrence—such as habitat type and phenology—and evaluates its conservation status in the context of sustainability goals. Analysis of available data shows that over two-thirds of the known sites in Poland are located in peatland ecosystems, with more than half occurring within protected areas. Although S. flavidus is distinctly hygrophilous, it appears to prefer moderately wet habitats, particularly swamp forests. Despite an increase in recorded localities over the past five decades, S. flavidus remains an endangered species due to the ongoing degradation of peatland habitats, which are increasingly threatened by land-use change, drainage, and climate-related shifts in hydrology. The species’ long-term survival depends on sustainable landscape management, the preservation of suitable host tree populations, and the inclusion of fungi in conservation and restoration strategies. Thus, S. flavidus should be considered both an indicator species and a relevant element in meeting sustainability goals.

1. Introduction

The ectomycorrhizal fungi of the genus Suillus Gray are widely distributed and ecologically important species typically associated with conifers of the temperate and boreal zones of the Northern Hemisphere [1,2,3,4]. Currently, 112 species of Suillus are known worldwide [5], 13 of which occur in Poland [6,7], with most being characterised by a high degree of host and habitat specificity (e.g., [5,8,9,10]). An example of such a species is Suillus flavidus (Fr.) J. Presl, which forms a symbiosis with two-needle pines, particularly Pinus sylvestris L., in lowland and upland regions, and P. mugo Turra, in the mountains. The fungus grows mainly in peatlands, although it is also found in moist pine and mixed forests with pine. It also occurs in coastal forests and moist heaths with Erica tetralix L., developing on a thin layer of peat in interdune depressions on the Baltic Sea coast (e.g., [11,12,13]).

Suillus flavidus is known in North America, Africa, Asia, and Europe [13]. In Europe, it is mainly found in northern countries, e.g., Estonia, Finland, Norway, Sweden, and the UK [14,15,16,17], and those in its central part, e.g., Austria, Czech Republic, Germany, Poland, Switzerland, and Slovakia [6,7,8,18,19,20,21]; it is also found sporadically in southern regions, e.g., Bulgaria [22] and Romania [23]. Despite this wide distribution, in many European countries S. flavidus is included in the Red List of Fungi as a very rare or rare fungus, and is protected and considered an endangered species: it is classified as Endangered (EN) in Switzerland [24], Critically Endangered (CR) in Austria [20], Vulnerable (VU) in Slovakia [25] and the Czech Republic [26], and Threat of Unknown Extent (G) in Germany [27]. In Poland, S. flavidus is a critically endangered taxon (category E) and has been protected for many years [28,29,30]. It is included in the IUCN Global Fungal Red List Initiative [31].

Suillus flavidus is one of the few ectomycorrhizal fungi found in wet habitats, such as bogs, mires, and swampy and boggy forests, which currently are amongst the most threatened ecosystems globally [32]. The rapid advance of climate change and growing human pressure [33,34] have been responsible for the disappearance of various ecosystem types, resulting in the loss of habitats to many temperature-sensitive species and their shrinking geographic ranges [35,36]. Many species are thought to have become extinct due to climate change [37], decreasing biodiversity on both regional and global scales [35].

Climate change is increasingly affecting forest ecosystems, in some cases with the effects being influenced by the relationships between ectomycorrhizal fungi and their associated plant partners [5,38]. For example, some species of the genus Suillus have been shown to play a significant role in enhancing host plant resistance to drought-induced oxidative stress [39,40]. However, in the longer term, global climate warming is expected to alter the distribution of both tree species [41,42] and associated fungi [43,44,45]. Pioneer tree species such as Picea abies (L.) H. Karst. and Pinus sylvestris, which are associated with fungi of the genus Suillus, are predicted to be heavily affected by climate change, as their range will decrease and shift northwards. It is believed that Scots pine, the main symbiotic partner of S. flavidus, will lose nearly 30% of its current acreage in Europe, even under an optimistic scenario [41].

Contemporary challenges in nature conservation increasingly require the integration of research on rare species with a sustainability-oriented approach that emphasises biodiversity protection, climate change mitigation, and the preservation of ecosystem functions. Mycorrhizal fungi, such as S. flavidus, constitute an essential component of these ecosystems, playing a crucial role in forest health and biogeochemical cycling [46,47]. Their presence can indicate the ecological integrity of wetland habitats, which are among the most threatened yet highly valuable ecosystems in terms of carbon sequestration and climate regulation [48]. Unfortunately, the intensification of land-use, peatland drainage, and ongoing climate change contribute to the degradation and disappearance of natural habitats for this species, placing it among the threatened fungi in Poland [28]. From the perspective of the Sustainable Development Goals, research into the distribution, ecology, and extinction risk of such narrowly specialised fungi is not only a conservation priority but also part of a long-term adaptive strategy to environmental change [49].

A study on the fungal diversity of raised and transitional bogs in north-western Poland [50] showed that the distribution and occurrence of many species, including S. flavidus, remains insufficiently documented. Although some data on the occurrence of S. flavidus in Poland have been published [11,51,52,53], the availability of some articles is limited, especially those dating from the first half of the 20th century [54,55]. Furthermore, not all data, including unpublished findings from inter alia private herbarium collections, have been included in publicly available databases, such as the Global Biodiversity Information Facility (GBIF) [17]. Therefore, one of the aims of this paper is to synthesise and critically analyse the available information on the distribution of S. flavidus in Poland. It reviews all available data from the last 70 years, published and unpublished, to identify ecological factors that may determine the occurrence of this species, with a particular focus on habitat and phenology. An assessment was also made of threats that could contribute to a reduction in the geographic range or extinction of S. flavidus in certain regions.

2. Materials and Methods

2.1. Data Origin and Taxonomic Verification

A database of S. flavidus records from Poland was compiled based on all available data. The sources include the following: (i) mycological literature compiled in the Checklist of Polish larger Basidiomycetes [6] and an online database of literature published since 2000 [7]; (ii) verified herbarium specimens from Polish herbaria (KRAM, LBL, SZUB, WA, and ZBŚRiL PAN; acronyms for public herbaria adhere to the Index Herbariorum [56]) and private collections (BGF-Błażej Gierczyk); (iii) information from the Database of Rare and Endangered Fungi (GREJ) [57]; and (iv) unpublished data from State Forests and Regional Directorates for Environmental Protection, as well as the author’s own data and unpublished records of mycology specialists. In addition, data were also searched for in publicly accessible databases, including GBIF [17], Web of Science, PubMed, and Google Scholar. The following monographs and keys were used to identify the basidiomata and to verify herbarium specimens: Breitenbach and Kränzlin [18], Knudsen and Taylor [16], and Kibby [58]. Characteristics of microscopic structures (spores, basidia, cheilo- and pleurocystidia) were observed and measured using an Olympus BX53 light microscope (LM) with an Olympus DP26 digital camera (Evident Scientific, Waltham, MA, USA). The identification of S. flavidus was based on macro- and micromorphological characteristics, including a lemon-yellow to greyish-yellow, sometimes brownish-yellow cap, 20–60 (80) mm in diameter, initially conical to convex, later becoming flat with a distinct, flat umbo; surface slimy (viscid), smooth, with fine, appressed fibrils; cuticle peelable; tubular hymenophore, with tubes and pores pale yellow to olivaceous yellow, the pores large and angular; cylindrical stipe, 30–80 × 3–10 mm, sometimes bent and somewhat thickened toward the base, concolorous with the cap, with pale whitish basal mycelium; gelatinous ring, yellowish to brownish-yellow, soon disappearing and leaving a narrow band; ellipsoid basidiospores, smooth, almost hyaline, with guttules, measuring 7.5–11 × 3–4.5 µm; clavate basidia, with four sterigmata, lacking a basal clamp, measuring 23–30 × 6–7.5 µm; cheilo- and pleurocystidia cylindric to clavate, with hyaline to brownish incrustation, measuring 40–70 × 4–9 µm [16,18,58]. The basidiomata of S. flavidus are highly distinctive, and their identification is relatively straightforward due to the species’ association with moist habitats. S. flavidus specimens collected by the author are deposited in the Herbarium of the University of Szczecin, Poland (SZUB).

2.2. Dystribution of Suillus flavidus and Ecologicala Data

The distribution of S. flavidus in Poland is presented on the basis of all available published and unpublished data according to the ATPOL (Atlas of Poland) grid square system [59]. To illustrate changes in the distribution of this species, the data was divided into three different periods: localities up to and including 1970 (according to Fraiture and Otto [13]), from 1971 to 2002, and after 2002; the latter two periods were divided based on the date of publication of the Checklist of Polish larger Basidiomycetes [6]. If the date of the basidioma collection was not known, the publication date of the locality was taken into account. The points on the map do not relate to single records but only show ATPOL squares, which may contain one to several S. flavidus localities. A list of localities used to create the cartographic map, and the analysis of phenological and habitat data are given in Supplementary Data S1.

All available published and unpublished records were searched to acquire phenological data for S. falvidus and any information on the habitats and plant communities in which it occurred. Duplicate records relating to the same locality or plant community were assessed as a single-record entry. When a species was found at the same locality in different years, only the year of the first record was considered. In the phenological analysis, all months in which S. flavidus basidiomata were found at a locality were taken into account.

The fungal nomenclatures were based on the Index Fungorum database (http://www.indexfungorum.org/, accessed on 10 April 2025), and the plant community nomenclatures according to Matuszkiewicz [60]. The nomenclature of vascular plants follows Mirek et al. [61]. The conservation status of S. flavidus in Poland was assessed according to the recommendations of Dahlberg and Mueller [62] and the current IUCN Red List Categories and Criteria (version 16) [63].

3. Results

3.1. Verification of Herbarium Vouchers

A total of 23 vouchers of Suillus spp. from five Polish herbaria and one private collection were analysed. Among them, 21 specimens of S. flavidus and two other specimens of the genus Suillus were verified. One voucher from the LBL herbarium (LBL M-031109) was identified as S. bovinus (L.) Roussel, while one voucher from the WA herbarium (WA71549) was identified as Suillus sp. (probably S. grevillei (Klotzsch) Singer).

3.2. Current Distribution, Habitat, and Phenology

The earliest records of S. flavidus in Poland date back to the turn of the 19th and 20th century [54,64], where it was identified several times in the south-eastern part of the country, mainly by Eichler [54,55], in the vicinity of Międzyrzecz Podlaski. Until 1970, it was known to occur in 19 localities [51,65], and in only 15 localities between 1971 and 2002. The highest number of S. flavidus localities (n = 52, 60.5% of all known records) was recorded after 2002. Currently, the species is known to occur in 86 localities, scattered mainly in the north-western and south-eastern parts of the country (Figure 1 and Figure 2). Most of these localities were found in areas under various forms of protection: 33 in nature reserves, 10 in national parks, 13 in Natura 2000 areas, and three in landscape parks. Twenty-seven sites were located in areas affected by various forms of anthropogenic pressure, such as intensive forestry, infrastructure development, and pollution and eutrophication associated with intensive agriculture.

The majority of S. flavidus localities had a wetland character (Figure 3). More than two-thirds of the records, i.e., 64 out of 86, are from different types of peatlands: 31 records from raised bogs, 12 from transitional mires, 2 from alkaline fens and 6 from bogs developed in inter-dune depressions; for 13 records, the type of bog was not given. S. flavidus was also recorded on the shores of lakes and in wet depressions (seven records in total), as well as in anthropogenically transformed habitats—iron ore mining heaps (one record). It also occurred in dry habitats, i.e., on coastal grey dunes (n = 9). S. flavidus was found in a variety of forest and non-forest communities (Figure 4) in which its symbiotic partners also occur. Among forest communities, it was most frequently found in pine forests: Vaccinio uliginosi-Pinetum (n = 29) and Empetro nigri-Pinetum (n = 9), while among non-forest communities it was most frequently recorded in Sphagnetum magellanici (n = 9) and Ledo-Sphagnetum magellanici (n = 7). Information on the plant community was absent from 30 localities, and the habitat from 5 localities.

In Poland, basidiomata of S. flavidus were observed from June to November (Figure 5). Most records were obtained in autumn, i.e., in September (n = 52) and October (n = 21). In the other months, the basidiomata were found much less frequently.

4. Discussion

Suillus flavidus is widely distributed in Europe, with a more frequent occurrence in its northern regions [21,66,67,68,69]. Most records of S. flavidus in Poland date from the last twenty years, with tree or more locations reported annually more frequently after 2005 (Figure 2). This significant increase in the number of records is mainly related to higher intensity of mycological studies of peatland ecosystems [50,52,70] and the increase in the number of observations by amateur mycologists and specialists [30,57]. Compared to other European countries, especially those in Scandinavia, the number of S. flavidus records in Poland (n = 86) may seem relatively small: Norway has 468 records [66], Finland 236 [68], Denmark 212 [69], Germany 137 [21], and the Czech Republic has 96 [71]. However, only 44 localities have been noted in Switzerland [67], i.e., 50% of the records known from Poland, and only 10 in Austria, including only 6 known after 1990 [20]. This variation in the number of S. flavidus sites between countries may be largely due to differences in the distribution and area of peatlands in Europe and their conservation status [72,73,74]. This is also indicated by data from Poland, where the species was found mainly in areas of peatland concentration (e.g., [75,76,77,78]), particularly in raised and transitional mires (Figure 3). On rare occasions, S. flavidus was recorded in quite different habitats, such as pine-forested grey dunes and interdune depressions on the Baltic coast, e.g., in Poland and Germany [11,12,79]. The species has also been found occasionally in sites influenced by human activities, such as on pine plantations and the margins of fishponds [13]; it has also been noted in pine monocultures occurring on heaps following iron ore mining [80], where a thin layer of peat develops in the depressions of the area, similar to the interdune depressions.

The distribution of S. flavidus in Poland and other European countries largely corresponds to the natural range of P. sylvestris [13,81]. Hence, it is likely that the occurrence of S. flavidus in North America [17] has resulted from the introduction of P. sylvestris to the area [82]. A similar phenomenon is encountered with the North American S. lakei (Murrill) A.H. Sm. & Thiers, which was introduced into Europe, South America, and Australia with its mycorrhizal partner Pseudotsuga menziesii (Mirb.) Franco [10,83]. The absence, or presence, of an appropriate tree partner is a key factor influencing the distribution of many ectomycorrhizal [84,85,86] and wood-inhabiting fungi [45]. In turn, the distribution of trees is significantly influenced by climate [35,42,87]. In Europe, climate change is predicted to result in the loss of localities and the northward migration of many native tree species [41]. One tree believed to be the most vulnerable to climate change is the Scots pine [[87], and references therein], which, based on the most pessimistic climate change scenarios, could see its range reduced by half in the next 50 years [41]. As a result, the population of S. flavidus would also decrease in the same area, especially in central and southern Europe; such a scenario is highly likely, as a decline in the number of its localities has already been observed in some countries, including France and Germany [13]. Ectomycorrhizal fungi are predicted to be disproportionately affected more than other organisms by climate change, due to their narrow climatic tolerance [38].

Despite its wide range in Europe, S. flavidus is a rare species in many of its regions [17,20,22,23]. A good example is north-western Poland, where despite the abundance of suitable habitats and intensive field studies, the number of newly discovered S. flavidus localities was relatively low: out of 134 objects, the species was only noted in 15 of them over 10 years of investigations [50]. In turn, the small number of localities in the central and north-eastern parts of the country may result from both insufficient knowledge of the mycobiota in these areas [6,7] and the actual rarity of the species in these regions, which may be related, for example, to the absence of suitable habitats—primarily moist pine forests on acidic soils [60]. It is possible that S. flavidus tends to occur rarely and patchily in its habitats, similar to other fungal indicator species for peatlands, e.g., Arrhenia gerardiana (Peck) Elborne and Cortinarius chrysolitus Kauffman [88]. However, the most likely explanation is that the occurrence of fungi is primarily determined by the local weather conditions [89]. Peatlands are mainly supplied by precipitation waters [76,77,90], and their deficit results in the upper layers of the substrate (peat) drying out, thus preventing fungal colonisation [50]. In Poland, as in many other European countries (e.g., Denmark and Finland), S. flavidus produces basidiomata mainly in September and October, and much less frequently in June and July and later in November (Figure 5) (e.g., [67,68,69]). Data from Switzerland show that the species is able to produce basidiomata even in December [67], which was probably related to weather anomalies. The fact that the basidiomata are more commonly observed in autumn can be explained by the relatively high annual precipitation. Numerous studies have shown that the production of basidiomata is strongly related to weather conditions, especially autumn rainfall (e.g., [91,92]). Extreme weather events, such as prolonged droughts or heavy rainfall, can significantly affect fungal populations, particularly those already threatened with extinction. For instance, extended periods of drought reduce soil moisture and disrupt the development of mycelium and basidiomata [93,94,95], which limits sexual reproduction and leads to a shortage of spores in the environment. Drought-related stress can not only impact symbiotic relationships between plants and fungi but may also, through indirect effects, influence overall ecosystem functioning [96].

Although there has been a significant increase in the number of S. flavidus localities in Poland [7], and the rest of Europe [66,67,68,69,71], over the last seven decades, the species should still be considered endangered. Firstly, the species prefers moist habitats (peatlands), whose area has decreased dramatically as a result of climate change and human activity [32]. The peatland in Poland has undergone considerable transformation, as none of the original raised bogs remain intact, and at least half of them have disappeared as a result of inter alia peat exploitation or transformation into agricultural land [76,77]. The analysis showed that the majority of S. flavidus localities are located in peatlands, particularly raised and transitional mires (Figure 5), covered by various forms of protection, with half (43 localities) in nature reserves and national parks. Protected peatlands are less exposed to harmful external factors than other areas. In the context of progressive peatland habitat degradation, for example, as a result of drainage, peatland species are declining, while species with a broader ecological range are increasingly encroaching upon these habitats [50]. However, the most significant threat to all peatland ecosystems is climate change, and its associated reduction in precipitation [77,90].

However, the collected data and numerous field observations show that although S. flavidus is clearly a hygrophilous species, it prefers less hydrated habitats, primarily swampy forests (Figure 4). This is in accordance with Stenström [97], who demonstrated that S. flavidus is highly sensitive to flooding, which negatively affects its ability to colonise pine roots. Stressful conditions such as flooding, or high water levels, and its associated low oxygen availability limit the development of ectomycorrhizae [98]. Periodic flooding of vegetation patches inhibits the development of fungi, even species that prefer more moist substrates [50]. Furthermore, studies suggest that other species of the genus Suillus are relatively weak competitors for host root tips in the face of competition from other ECM fungi or are easily excluded by other ectomycorrhizal fungi during fungal community succession [5].

Considering the data presented above, the recommendations of Dahlberg and Mueller [62], and the current IUCN criteria [63], S. flavidus meets the criteria (A2c + 3c + 4c) for the Vulnerable (VU) category in Poland, as its population is estimated to have declined by more than 30% during the last 50 years due to the loss and degradation of its suitable habitats (mainly peat bogs). Furthermore, the decrease is predicted to continue over the next 50 years.

The conservation of S. flavidus is closely tied to broader sustainability goals, particularly the protection of biodiversity and climate action. As a rare, mycorrhizal species restricted to peatland pine forests, S. flavidus depends on stable, undisturbed habitats that are increasingly threatened by drainage, land-use change, and climate-related alterations in hydrology [48,99,100]. Peatlands serve as both biodiversity reservoirs and major carbon sinks. Their degradation not only endangers specialist fungi like S. flavidus but also compromises carbon storage, exacerbating climate change. Therefore, conserving this species supports ecosystem integrity and contributes to sustainable land and climate management [49]. Integrating fungal diversity into conservation frameworks is essential for effective sustainability strategies. S. flavidus can act as a bioindicator for peatland health and the success of ecological restoration. Protecting its habitats through rewetting, limiting habitat fragmentation, and incorporating fungal monitoring into environmental policies aligns with the principles of sustainability and long-term ecosystem resilience [46,101].

5. Conclusions

Although S. flavidus was previously considered rare in Poland, this review reveals a broader distribution across various regions, likely underestimated due to limited sampling. Further field surveys, especially in underexplored areas (e.g., NE Poland), may provide a more complete picture of its range.

The new records presented here contribute to understanding the species’ wider biogeographical patterns and global distribution. This highlights the value of regional data in addressing gaps in fungal biodiversity knowledge and supports informed conservation and land-use planning.

Given that S. flavidus is an indicator of peatland habitats—now rapidly declining due to human activity and climate change—it should be considered for inclusion in the IUCN Red List of Threatened Species. Without conservation efforts, many known localities may disappear. Improved knowledge of its distribution and habitat requirements can support actions to safeguard both the species and its ecosystem.

Moreover, protecting S. flavidus and its peatland habitats aligns with broader sustainability objectives, particularly those related to biodiversity conservation and climate regulation. Integrating fungal data into land-use strategies and restoration planning can enhance the long-term resilience of these vulnerable ecosystems.