Fungal Fairy Rings (FFRs) are observed in diverse environments, ranging from woodlands to grasslands and are widespread globally (Gregory 1982; Dowson et al. 1989; Peter 2006; Fig. 5). In woodlands, the formation of FFRs is influenced by the symbiotic relationships with specific plant hosts or the presence of particular types of litter. Conversely, in grasslands, the environmental requirements for FFR fungi depend heavily on the soil-water balance. Bayliss (1911), Shantz and Piemeisel (1917), Hardwick (1978) and Gramss (2005) highlighted that FFRs typically appear in natural grasslands with well-drained soils. Recent studies in the Laramie Basin by Miller and Gongloff (2021) identified that adequate, but not excessive, precipitation is a critical factor for FFR formation. In regions with low precipitation, there is a notable decline in FFR colonies, while higher frequencies of FFRs were documented in north-facing slopes. This aligns with observations by Allegrezza et al. (2022), who reported higher fungal colony densities in sloped areas compared to flat zones, where stagnant water and soil hypoxia inhibit fungal development.

Figure 5. 

World map distribution of FFR study cases available in the literature (A) and their numbers in different countries (B).

In the Italian Apennines, Allegrezza et al. (2022) found that FFRs are most frequently observed in areas with annual rainfall ranging from 900 to 1300 mm and completely disappear when precipitation exceeds 1700 mm. These findings suggest that geomorphological and hydrological conditions significantly influence the spread of Basidiomycete colonies.

While precipitation patterns largely determine the presence of FFR fungi, altitude and temperature are less restrictive. In the Italian Apennines, high frequencies of FFR colonies were recorded at altitudes between 500 and 2200 m above sea level, with mean annual temperatures ranging from 3 to 13 °C. FFRs are also documented in high-altitude grasslands, such as the Tibetan Plateau, where Floccularia luteovirens (Alb. & Schwein.) Pouzar forms rings at around 3800 m above sea level, with a mean annual temperature of -3.9 °C (Wang et al. 2005).

Despite these extremes, many species have wide altitudinal ranges, such as M. oreades, observed from sea level (Molliard 1910) to 1730 m above sea level (Norstadt et al. 1973). Certain species, like Chlorophyllum molybdites (G. Mey.) Massee is confined to tropical climates (Reid and Eicker 1991), highlighting the variability in ecological niches occupied by FFR-forming fungi.

Soil type is another key factor in FFR formation. Miller and Gongloff (2021) noted that, amongst 16 soil types studied, FFRs occurred in eight types belonging to Aridisols and Mollisols. Taxa such as Agaricus braendlei L.A. Parra & M.M. Gómez, Agaricus lilaceps Zeller, Calvatia Fr., Disciseda candida (Schwein.) Lloyd and Geastrum Pers. prefer Aridisols, while M. oreades and Bovista plumbea thrive in Mollisols, likely due to lower levels of dissolved salts from precipitation.

Nitrogen availability also affects FFR density in grasslands, as observed in studies linking FFR prevalence to cattle manure (Cosby 1960; Hardwick and Heard 1978). Excessive nitrogen, however, is detrimental to fungal growth. High nitrogen levels impair mycelial health (Kües and Liu 2000) and enzymatic activity (Kachlishvili et al. 2006). In agricultural systems with high nitrate release from soil tillage, FFRs are rare, with only one case reported in a barley field previously occupied by grassland (Shantz and Piemeisel 1917). Phytopathological research further confirms that excessive inorganic nitrogen limits the spread of FFR-forming fungi like M. oreades in gardens and turfs (Drew Smith 1957).

In the Italian Apennines, biogeographic surveys reveal that FFRs are more prevalent at higher altitudes, likely due to reduced grazing pressure, as these grasslands are accessible to herbivores for only a few months annually (Allegrezza et al. 2022). Interestingly, some fungi benefit from the presence of herbivores. For instance, A. arvensis thrives in horse-grazed grasslands in the Rogedano Mountain, Marche, Italy, where grazing is restricted to wild horses and a few wild herbivores such as deer (Bonanomi et al. 2012; Bonanomi et al. 2013).

Undisturbed environments with moderate nutrient levels and abundant decomposing organic matter are conducive to FFR formation. Although there is no direct evidence linking specific nutrient levels in grasslands to particular FFR fungi, stable grasslands often support Basidiomycetes, which serve as bioindicators of environmental disturbance (Arnolds 1992; Griffith et al. 2002; Durall et al. 2005).

Specific conditions, such as litter type and the absence of competitors, can also promote FFR development. For example, C. nebularis specialises in degrading broadleaf litter in woodlands, but cannot thrive in grasslands due to the unsuitability of grass litter. Similarly, C. gambosa (Saint George’s mushroom) prefers to grow beneath Rosaceae Juss. plants such as Prunus spinosa L. and Rubus ulmifolius Schott or under old plantations of Sorbus L., Malus Mill. and Pyrus L. Although the reasons for this preference remain unclear, it is speculated that the lack of ectomycorrhizal symbiosis in most Rosaceae plants favours the growth of saprotrophic fungi (Gadgil and Gadgil 1971).

A distinctive feature of fungal fronts is the regular arrangement of their mycelial mats, originating from a single point and spreading centrifugally, akin to fungal growth observed in Petri dish cultures. These fronts may arise from germinating spores or vegetative expansion of mycelium fragments. Studies on M. oreades in Norwegian sandy dunes provided evidence that most fungal fronts (~ 90%) are generated by spore germination rather than mycelium fragmentation, as indicated by their distinct genetic structures (Abesha et al. 2003). In rare cases, fungal fronts form vegetatively, involving spatial advancement and intraspecific coalescence. Formation of FFR colonies with more than one genet can also be observed (Mallett and Harrison 1988), as observed in pathogenic species like A. ostoyae (Legrand et al. 1996; Worrall et al. 2004) and ectomycorrhizal fungi such as T. matsutake, where up to four genets can form the same Shiro (Lian et al. 2006; Peter 2006).

The expansion of fungal mycelium in soil is driven by various factors, including precipitation and temperature (Moore et al. 2008), soil resource distribution (Ritz 1995), microbial competition, parasitism and chemical interference (Dix 2012). Self-regulation mechanisms, such as the release of inhibitory substances, also play a critical role (Keller et al. 2005; Mazzoleni et al. 2015a; Mazzoleni et al. 2015b; Salvatori et al. 2023). Actively growing hyphae colonise outer soil regions in search of organic matter (Mathur 1970; Miyamoto and Igarashi 2004; Baldrian 2008; Dix 2012) (Fig. 1A, F) or plant hosts as in the case of fronts formed by pathogenic fungi (Shaw III 1980; Anselmi and Minerbi 1988; Pukkala et al. 2005), releasing enzymes that degrade organic material and assimilating nutrients from newly colonized areas (Šnajdr et al. 2011; Rodrigues et al. 2015). Fungal fronts form regular patterns only when resources are homogeneously distributed in soil. Otherwise, mycelial growth becomes patchy, reflecting resource availability (Bayliss 1911; Shantz and Piemeisel 1917; Mathur 1970; Dowson et al. 1989; Griffith and Roderick 2008).

As new mycelium explores the soil, the internal regions of fungal fronts are conditioned by senescent mycelial residues, rendering these areas unsuitable for recolonisation by younger hyphae, which remain confined to the fungal front’s margins (Mazzoleni et al. 2015b; Álvarez-García et al. 2020). This phenomenon, observed across diverse fungi under laboratory conditions, underpins a central question raised by Bayliss (1911): why do fungal fronts form rings rather than dish-like colonies?

Historically, the nutrient-based hypothesis posited that nutrient depletion in the internal zones of fungal fronts prevents recolonisation (Wollaston 1807; Way 1847; Lawes et al. 1883). However, this explanation lacked empirical support. Way (1847) and Schreiner and Reed (1907) endorsed De Candolle’s excretory theory (1830–1832), suggesting that recolonisation is inhibited by species-specific excretory by-products. Experimental evidence supporting unidirectional fungal growth was provided by a sod inversion study, where C. nebularis failed to grow in soil it had previously conditioned (Dowson et al. 1989).

The inability of fungal fronts to recolonise inner areas is now understood within the broader framework of biological pattern formation. Ring-like patterns are also observed in plant tussock rings (Bonanomi et al. 2014; Cartenì et al. 2016). Mazzoleni and co-workers (2015a, 2015b) suggested that patterns of circular growth, both for fungi and plants, can be explained by self-DNA autotoxicity, as decomposition of senescent organic matter releases chemical compounds with specific detrimental effect on the species that produced it. The presence of self-produced waste products impairs the ability of vegetative organisms to recolonize the inner portions of the colony despite resources availability. Recently, the self-inhibitory hypothesis mediated by self-DNA was reproduced by means of mathematical modelling. In the model, the authors successfully reproduced the FFRs circular shapes, demonstrating that autotoxicity provides an explanatory mechanism of fungal front dynamics in the soil space (Salvatori et al. 2023).

In sloped terrains, fungal fronts often develop into arc-like patterns, with degeneration observed downslope due to leaching of water-soluble self-DNA (Miller and Gongloff 2021; Allegrezza et al. 2022). This phenomenon results in a transition from closed rings, typical in flat areas, to open arcs, as fungal colonies retreat uphill leaving behind a self-toxicity tail.

Both ecologists and plant pathologists commonly estimate the annual growth rate of fungal fronts (FFRs) by tagging the external edge of the zones where active mycelium is present and monitoring the subsequent metrical advancement over the course of a vegetative season. The rate of mycelial expansion depends on several factors, including fungal species, seasonality and vegetation type (Ramsbottom 1926; Pugh 1980; Toohey 1983). In unfavourable conditions, the widening of fungal front regions (FFRs) can be temporarily halted for several years, suggesting that age calculations, based on previous expansions, may have been overestimated (Ramsbottom 1926). Nevertheless, this remains one of the most reliable observational methods to assess colony growth rate and age.

More recently, Miller and Gongloff (2023) estimated the size and age of 304 FFRs in the Laramie Basin by comparing aerial photos taken from 1947 to 2022. Their approach, which incorporated field estimates of the FFR-forming fungi’s specificity, provided the most comprehensive insight into the colonisation dynamics of FFRs in grassland ecosystems.

The growth rates of FFRs vary considerably across different fungal taxa (Fig. 7). Species with the highest growth rates are typically those within the Clitocybe (Fr.) Staude genus, with average rates of approximately 75 cm per year (Toohey 1983; Dowson et al. 1989). For example, Lepista (Fr.) W.G. Sm. species exhibit particularly high growth rates, with Lepista sordida (Schumach.) Singer showing an annual growth of 125 cm in turf environments (Terashima et al. 2004), though the average growth rate for this genus is closer to 60 cm per year. Similarly, fungi in the Agaricus L. genus have average growth rates of around 55 cm per year (Edwards 1984; Bonanomi et al. 2012). The Marasmius Fr. genus shows a broader growth range, from 7 to 39 cm per year (Cosby 1960; Burnett and Evans 1966; Ingold 1974; Hardwick and Heard 1978; Dickinson 1979). Slower-growing taxa include ectomycorrhizal species, such as Russula Pers. and Tricholoma (Fr.) Staude, with average growth rates of 22 cm per year and 17 cm per year, respectively (Ohara and Hamada 1967; Lian et al. 2006; Kataoka et al. 2012; Narimatsu et al. 2015).

Figure 7. 

Annual expansion rate of fungal fronts from different FFR-forming fungi (60 observations) divided by genera and ecological groups. Dashed line connects mean points. Sph and Ecm abbreviation refers to FFRs formed by saprotroph and ectomycorrhizal fungi, respectively.

The methodology for estimating growth rates, in combination with the diameter of the FFRs, has enabled the estimation of fungal front ages. In French grasslands, large FFRs of I. geotropa, with a diameter of 800 m, were estimated to be around 700 years old (Drew Smith 1957; Gregory 1982). Similarly, a FFR with a diameter of 137 m and an average growth rate of 60 cm per year would be approximately 100 years old (Allegrezza et al. 2022). Seasonal extension methods, combined with rhizomorph log colonisation rates, have also been used to assess the growth rates of pathogenic Basidiomycetes, such as Armillaria gallica Marxm. & Romagn. With a growth rate of around 20 cm per year and a minimum expansion of 15 hectares, these fungi were estimated to be 1,500 years old, with an estimated biomass of 10,000 kg (Smith et al. 1992). Similar methods have been applied to other fungal species, including A. ostoyae. In Oregon, this species formed an expansive front of 890 hectares, estimated to be 2,400 years old, while a similar front in Switzerland, covering 500 hectares, was approximately 1,000 years old (Ferguson et al. 2003; Bendel et al. 2006). These fungal fronts are amongst the largest living organisms on Earth and their longevity is remarkable, particularly considering that they belong to microbiological life forms. The long lifespan of Basidiomycetes may result from a poorly-understood mechanism that preserves genetic integrity and prevents the accumulation of disadvantageous mutations during cell division (Hiltunen et al. 2019; Hiltunen et al. 2021).

While tracking the radial expansion of FFRs is relatively straightforward, representing mycelial development along the soil horizon is more challenging and has been documented only in a few studies (Shantz and Piemeisel 1917; Norstadt et al. 1973; Edwards 1984). Soil trenching has provided valuable insights into how mycelial mats of FFRs are distributed along the soil horizon during the vegetative season and at varying soil depths. The depth of mycelial mats varies amongst FFRs formed by different species (Suppl. material 1). For instance, Lycoperdon curtisii Berk., L. sordida and Holocotylon dermoxanthum (Vittad.) R.L. Zhao & J.X. Li form dense mycelial mats that extend up to 4 cm deep (Terashima et al. 2004). In contrast, species such as A. campestris, T. matsutake and A. arvensis have mycelial mats that extend to greater depths up to 22 cm, 15 cm and 10 cm, respectively (Edwards 1984; Bonanomi et al. 2012; Kataoka et al. 2012).

As with other filamentous fungi, the fungal fronts of FFRs fungi form at the outermost periphery of the mycelial mats, where hyphae extend their apices to colonise organic matter. Once the target is reached, intense sub-apical branching fills all available space in the substrate. In M. oreades, young vegetative hyphae at the outer edge of the mycelial mats secrete elevated levels of extracellular laccases, which catalyse the oxidation of organic matter (Mathur 1970; Yaropolov et al. 1994; Gramss et al. 2005; Griffith and Roderick 2008). This process breaks down compounds such as lignin and cellulose, which, in turn, lead to soil denitrification and the release of CO₂, resulting in soil acidification due to the reduction of pH (Fig. 8) (Gramss et al. 2005; Bonanomi et al. 2012; Zotti et al. 2021).

Figure 8. 

Chemical analysis of soil affected by fungal fronts of FFRs. Data from a total of 180 published FFRs and additional data from 13 FFRs of Calocybe gambosa, Agaricus arvensis, and Agaricus crocodilinus in Italian grassland/woodland. Data were collected as chemical parameters in the fungal front and outer areas with a high density of mycelial mats and soil external to FRs, respectively. Collected data were converted to mg kg^-1 (ammonia, nitrates, phosphorous, potassium, magnesium, calcium) or % (total organic carbon, total nitrogen, pH, water content), Data were log-transformed to reduce dimensionality. Values of n represent the number of FFRs where soil chemical variables were studied.

Concurrently, in the bulk of the soil affected by FFRs, water content decreases (Fig. 8), a side effect of hydrophobin secretion (Sietsma et al. 1995; York and Canaway 2000; Fidanza 2007b). Hydrophobins in FFR fungi likely serve multiple functions (Wösten 2001), including regulation of water within the substrate (Lugones et al. 1998), converting hydrophobic substrates into hydrophilic ones (Wösten and de Vocht 2000), providing protection from drying and air exposure (Wösten 2001), acting as mechanical barriers against pathogen attacks and aiding in correct sporophore development (De Groot et al. 1996; De Groot et al. 1999). These hydrophobins also regulate water content, making the secreted chemicals more effective (Lebeau and Hawn 1961; Blenis et al. 2004; Caspar and Spiteller 2015), which benefits FFR fungi in their biological activity. Similarly, some ectomycorrhizal mats also produce hydrophobic mycelial aggregates, suggesting that both symbiotic and saprobic fungi share similar strategies (Griffiths et al. 1994; Koo et al. 2003; Koo et al. 2009).

Significant changes occur in the levels of ammonia and nitrates in the soil affected by FFRs (Shantz and Piemeisel 1917; Fisher 1977; Edwards 1988; Fidanza 2007b; Espeland et al. 2013; Yang et al. 2019). The mycelial mats absorb decomposed material from the outer edge of the colony, releasing excess intracellular nitrogen in the form of ammonium (De Groot et al. 1998; Moore 2003). Once released, ammonium is converted into nitrates by the activity of associated bacteria (Shantz and Piemeisel 1917; Fisher 1977; Edwards 1984; Zotti, Bonanomi et al. 2021). These nitrates undergo denitrification and are released as gaseous nitrogen (Knowles 1982). However, basidiomycete mycelium cannot utilise nitrogen in this gaseous form (Deacon 2013) and it is, instead, absorbed by plants. In the case of ectomycorrhizal fungi, high ammonium concentrations have been observed in T. matsutake (Kim et al. 2013) and mycelial mats of Hysterangium spp. Vittad. (Griffiths et al. 1994; Kluber et al. 2010; Trappe et al. 2012), but not in nitrates, leading to the hypothesis that nitrogen compounds released by decomposition are directly passed to the plant symbionts without undergoing bacterial transformation.

Parallel to the release of ammonium and nitrates, several FFR fungi have shown consistent enrichment of phosphorus in the soil (Fisher 1977; Edwards 1984; Fidanza 2007b; Yang et al. 2018a, 2018b). Phosphorus is essential for many biological processes and its high enrichment in FFR soil may be due to its decomplexation from organic matter (Jennings 1989). It may also play a role as a co-factor in the selective decomposition of recalcitrant compounds (Tripathi and Yadav 1991) and could be translocated to the colony edge for nutritional purposes, similar to rhizomorph-forming fungi (Wells and Boddy 1995; Wells et al. 1998). Recent studies have shown that the high levels of phosphorus in FFR soils are due to its available forms, as measured by Olsen P methodologies, indicating that it is released through organic matter decomposition (Gramss et al. 2005; Bonanomi et al. 2014; Yang et al. 2019; Zotti et al. 2021). However, slightly decreased phosphorus levels have occasionally been observed within FFRs, suggesting that a portion of phosphorus is sequestrated in the active mycelium (Fisher 1977; Edwards 1984; Gramss et al. 2005). Phosphorus is used in sporophore formation, a high-energy-demanding process (Kües and Liu 2000) and is, therefore, reabsorbed from the soil during fruit-body emergence (Fisher 1977).

Mycelial activities also lead to the accumulation of higher levels of potassium, magnesium and calcium, resulting from the solubilisation of these elements during organic matter decomposition (Gramss et al. 2005). While the specific dynamics of these elements in FFR fungi are not well understood, their contribution to fungal nutrition is acknowledged (Deacon 2013).

Lastly, iron levels increase with the passage of FFR mycelial mats. This has been observed in C. gambosa (Zotti et al. 2021) and M. oreades (Gramss et al. 2005), where iron concentrations doubled. It is likely that iron sequestration occurs due to the high demand for the element in carbohydrate consumption and homeostasis. Additionally, iron may be gathered from the external soil environment through the production of siderophores (Renshaw et al. 2002).

The impact of FFRs on grassland ecosystems and vegetation can be complex and varied. In some cases, the mycelium can damage the grass cover, while in others, it can stimulate lush plant growth. Both outcomes can be observed in many FFRs and, when these fungi spread at the edges of forested and grass-dominated environments, their influence on vegetation is evident, even in ectomycorrhizal species (Toohey 1983). Given the dual nature of these effects, this section first provides an overall description of the influence of FFR fungi on vegetation, then divides the discussion into two subsections. This structure aims to clarify the mechanisms behind both the detrimental and stimulating effects of FFR fungi on plant growth, while also exploring the ecological implications of advancing mycelial mats on plant communities.

The understanding of the effect of FFRs fungi on soil microbiota has evolved alongside advancements in microbiological techniques, reflecting the growing interest in this topic within modern research (Table 1). Early studies focused on the microbial characteristics associated with the formation of the greener belt of vegetation. Meanwhile, the soil microbial community in woodland ecosystems has been investigated since the isolation of mycorrhizal-helper bacteria in the Shiro of T. matsutake, representing useful information for the stable cultivation of valuable edible fungi and for reforestation (Powell 1993; Yun and Hall 2004).

Initially, the plate dilution method was the primary technique used to study the changes in soil microbiota induced by FFR fungi (Ohara and Hamada 1967; Norstadt et al. 1973). This method was later replaced by more advanced community-based techniques, including FAME profiling (Espeland et al. 2013), PCR-DGGE (Kataoka et al. 2012) and Next-Generation Sequencing (Marí et al. 2020; Duan and Bau 2021; Zotti et al. 2021).

Amongst culturable bacteria, the development of FFR mycelial mats is associated with a general simplification of the bacterial community in T. matsutake (Ohara and Hamada 1967; Kataoka et al. 2012). In contrast, in M. oreades, no significant changes are observed, although a reduction in enzymatic activities has been noted (Norstadt et al. 1973). Studies have demonstrated that FFR soils can harbour a large number of prokaryotic colonies (Gramss et al. 2005) and associated enzymes (Bonanomi et al. 2012). Using FAME profiling combined with DNA sequencing, it was found that, in soils colonised by the mycelial mats of A. lilaceps, Pseudomonas fluorescens Migula, Stenotrophomonas maltophilia (Hugh) Palleroni & Bradbury and Agrobacterium radiobacter (Beijerinck & van Delden) Conn were the most prominent species. However, there is limited information on the changes in fungal communities associated with culturable species studies, with the only exception being an increase in Mortierella Coem. in the soil of T. matsutake (Ohara and Hamada 1967), which highlights the limited affinity of many fungi for culture-based methodologies.

Field surveys combined with PCR technologies applied to woodland FFRs revealed that, during the passage of the fungal front, root tips are dominated by T. matsutake, suggesting competitive exclusion amongst fungal symbionts (Lian et al. 2006). Following this, the application of 454 pyrosequencing in soil colonised by T. matsutake revealed a strong simplification in the fungal community, except for the equitability amongst different taxa, which appeared unaffected (Kim et al. 2013). In contrast to the fungal community, bacterial communities appeared more responsive to the progression of the fungus, although no remarkable changes in diversity metrics were observed (Kim et al. 2014).

Contrary to these pioneering works, other studies from the same ecoregion reported differing results, indicating changes in both eukaryotic and prokaryotic microbial communities following the development of T. matsutake. No apparent changes were observed in the eukaryotic community, but specific changes were detected in the bacterial community in soils dominated by T. matsutake. Using various metrics, such as Bray-Curtis similarity for fungal communities and UniFrac distance for bacterial communities, the study found that geographic location was a better predictor of fungal community composition than the passage of the FFR fungus. In contrast, bacterial community composition showed a stronger association with the developmental stage of T. matsutake. Furthermore, some bacterial genera, such as Burkholderia, Bacillus Cohn and Paenibacillus Ash et al., exhibited a common advantageous response to the fungal front (Oh et al. 2016). These genera were subsequently evaluated as mycorrhizal helper bacteria under controlled conditions, revealing that most bacterial taxa, including Burkholderia, had negative effects on T. matsutake growth in Petri dishes, while species from the Paenibacillus and Staphylococcus Rosenbach genera promoted fungal growth (Oh and Lim 2018).

In Tibetan grasslands, the effects of A. gennadii and A. campestris on the bacterial community were evaluated across alpine and temperate climates within stimulated vegetation (Yang et al. 2018a, 2018b). The results indicated varying effects of FFR development on bacterial diversity, with one area showing increased diversity indices and another showing a decrease, though no specific bacterial associations were identified.

More recent studies have focused on the community structure associated with the development of A. arvensis. These studies revealed few significant changes in the number of OTUs, Shannon diversity index and Pilou’s evenness for the bacterial community (Zotti et al. 2020). However, marked changes were observed in the fungal community, particularly in alpha diversity decreasing in fungal dominated soil. In the same study, a significant increase in Burkholderia amongst bacteria and Trichoderma amongst fungi is thought to result from a phenomenon of mycoparasitism. Evidence from other studies supports this idea, suggesting that Burkholderia can convert harmful toxins produced by the fungus into less toxic forms (Choi et al. 2016) and Trichoderma species are less sensitive to toxins produced by other fungal mycelium (Blenis et al. 2004). Both taxa also exhibit inhibitory effects on the growth of the associated fungal species (Oh and Lim 2018; Oh et al. 2018) and they are known for their role in biological control of fungal populations in agricultural fields (Kubicek et al. 2001; Parke and Gurian-Sherman 2001; Benítez et al. 2004). These findings suggest that the persistence of these taxa in FFRs may represent an evolutionary advantage developed to counteract the defensive strategies of dominant fungal mycelium. However, direct evidence of mycoparasitism is still required to fully understand the role of Burkholderia in FFRs.

Further studies emphasise the impact of FFR fungi on grassland ecosystems, particularly in terms of increasing microbial species richness. A study on FFR-forming fungi in Spanish grasslands revealed that, within the rings, relative abundances of Pleosporales Luttr. ex M.E. Barr and Eurotiales G.W. Martin ex Benny & Kimbr. decreased, while Clavaria Vaill. ex L., Psathyrella (Fr.) Quél., Tricholoma (Fr.) Staude, Amanita Pers. and Lycoperdon Pers. genera increased (Marí et al. 2020). The authors suggested that fungal diversity increases within the rings, although the interpretation of results requires caution, as ectomycorrhizal species such as Tricholoma and Amanita could contribute to the genetic signal detected, possibly from spores in the soil, as observed for Cantharellus Adans. ex Fr. in the inner zone of A. arvensis FFRs (Zotti et al. 2020).

In FFRs of Leucocalocybe mongolica (S. Imai) Z.M. He & Zhu L. Yang in the Mongolian Tibetan Plateau, higher species richness was observed in the zone of stimulated vegetation compared to the surrounding grassland. This increase in richness was more pronounced in the bacterial community than the fungal community (Duan and Bau 2021). Similar increases in bacterial species richness in areas with active mycelial mats have been reported in other metagenomic-based studies on FFRs (Zotti et al. 2021). In FFRs of C. gambosa, an association between the bacteria belonging to genera such as Pseudomonas Migula, Sphingobacterium Yabuuchi, Pedobacter Steyn, Parapedobacter Kim, Advenella Coenye and Rhodanobacter Nalin were observed. The authors, integrating metagenomic data with soil physiochemical data, hypothesised that the high levels of ammonia and nitrate around the mycelial mats could be the result of consortia with soil bacteria, rather than being solely produced by fungal saprobic activity. This supports the idea that dominant fungal fronts should be considered holobionts, formed not just by a single organism, but by a complex consortium of microbial taxa (Zotti et al. 2021).

The authors have declared that no competing interests exist.

No ethical statement was reported.

No Fungal strains were used in this study.

No funding was reported.

M.Z. and G.B. conceived the present idea; M.Z. wrote the manuscript with the support of S.M. and G.B. M.Z. developed the data analysis. S.M. revised the theoretical formalism. Both M.Z. and S.M. authors contributed to the definitive version of the manuscript. S.M. revised the manuscript and supervised the whole project. All the authors approved the manuscript.

Maurizio Zotti https://orcid.org/0000-0001-5540-9477

Giuliano Bonanomi https://orcid.org/0000-0002-1831-4361

Stefano Mazzoleni https://orcid.org/0000-0002-1132-2625

All of the data that support the findings of this study are available in the main text or Supplementary Information.