Terricolous Lignicolous Decomposers and Fairy Rings

For most terricolous (i.e. fruiting on soil) basidiomycetes, ecological information is largely reliant on spatiotemporal analysis of fruiting, though some studies on mycelia, notably those of Warcup (see below), have provided valuable insights. Vertical stratification of grassland soils is usually less than in woodland due to invertebrate activity and it is not known whether mycorrhizal fungi dominate the deeper soil horizons, as is the case in woodland (Lindahl et al., 2007).

The most obvious manifestations of basidiomycete activity in grasslands are fairy rings, which are more visible in close-cropped and homogeneous vegetation than in other habitats where they also occur (Dowson et al., 1989; Chapter 5). Radial expansion rates of fairy rings range from 8 cm year—1 for Marasmius oreades (Smith, 1980) to over 100 cm year—1 for Lepista sordida (Terashima et al., 2004). Maximal ring diameters of 100-300 m (Shantz and Piemeisel, 1917; Kreisel and Ritter, 1985) have been reported with estimated ages of up to 200-700 years (Shantz and Piemeisel, 1917; Burnett and Evans, 1966; Kreisel and Ritter, 1985). Fairy rings are classified according to whether vegetation is killed at the ring margin (type 1), grows more vigorously (type 2) or is unaffected (type 3) (Shantz and Piemeisel, 1917). More than 50 species of grassland basidiomycetes have been reported to form type 1 or 2 fairy rings, mostly belonging to the genera Marasmius, Lepista, Agaricus, Clitocybe, Lycoperdon and Calvatia (Couch, 1995), with others, such as Hygrocybe and Panaeolus spp., forming type 3 rings occasionally (Figure 1). Several studies have demonstrated the genetic integrity of fairy rings, by investigation of mating type factor distribution (Burnett and Evans, 1966), molecular markers (Abesha et al., 2003) or mycelial pairings to determine somatic compatibility (K. Roderick, unpublished). There has been speculation, but no experimentation as to whether unmated mycelia (homokaryotic primary mycelia) can form rings without fruiting (Parker-Rhodes, 1955).

Fairy rings or arcs are formed by the annular growth of a mycelial system with apparent dieback of mycelium internal to the growth front. It has often been d

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Figure 1 Differential GPS mapping of Agaricus campestris fairy rings on the UW Aberystwyth campus (SN595818), showing altered Lolium/Festuca vegetation (lines) and basidiocarps (dots), during the summer of 2004. Note that many rings did not produce basidiocarps and the heterogeneous distribution of the rings.

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Figure 1 Differential GPS mapping of Agaricus campestris fairy rings on the UW Aberystwyth campus (SN595818), showing altered Lolium/Festuca vegetation (lines) and basidiocarps (dots), during the summer of 2004. Note that many rings did not produce basidiocarps and the heterogeneous distribution of the rings.

noted that such growth is simply an emergent property of localized nutrient depletion/toxin accumulation behind the growth front. Soil organic matter and nitrogen are depleted within the rings of several species (Lawes et al., 1883; Edwards, 1984, 1988; Kaiser, 1998). It is likely that nutrient redistribution via dung would increase nutrient levels within larger rings but there would still be an annular region of depleted nutrient internal to the mycelial front. The study of Dowson et al. (1989) is the only study to our knowledge to have explored the reasons for continued outward expansion of a ring, demonstrating that polarity of growth of Lepista nebularis was maintained after translocation of ring fragments, though why this should be the case remains unclear (Chapter 1). Their observation that mycelia disappeared when their orientation was reversed to face mycelia from their original ring is consistent with observations that when adjacent rings intersect the underlying mycelium degenerates (Parker-Rhodes,

Excavation of soil at the margin of type 1 and 2 rings reveals dense mycelial growth visible to the naked eye, whereas for species forming type 3 rings (e.g. Hygrocybe spp.) even microscopic observation of soil beneath the basidiocarps does not reveal an abundance of clamped mycelia (Warcup, 1959; G.W. Griffith and G.L. Easton, unpublished). The areas of mycelial abundance in type 1 and 2 rings coincide with areas of more luxuriant or killed/'scorched' vegetation,

Figure 2 Type 3 fairy ring of Hygrocybe pratensis (Ystumtuen, Aberystwyth; SN731799). (Inset) An ISSR fingerprint gel showing the genetic identity of basidiocarps from the same ring and difference compared with basidiocarps from an adjacent ring (10 m away). One sample (*, arrow) contained additional bands due to the presence of an endophyte (Paecilomyces marquandii).

Figure 2 Type 3 fairy ring of Hygrocybe pratensis (Ystumtuen, Aberystwyth; SN731799). (Inset) An ISSR fingerprint gel showing the genetic identity of basidiocarps from the same ring and difference compared with basidiocarps from an adjacent ring (10 m away). One sample (*, arrow) contained additional bands due to the presence of an endophyte (Paecilomyces marquandii).

sometimes in concentric rings (Edwards, 1984; Terashima et al., 2004). Appearance of vegetation symptoms is highly seasonal and linked to soil moisture conditions, with rings (both basidiocarps and vegetation effects; Figure 2) often visible only in certain years (Shantz and Piemeisel, 1917), and possibly linked to growth or reproductive phases of the fungal life cycle (Fisher, 1977). This has led to some confusion regarding the classification (type 1 or 2) of species (Halisky and Peterson, 1970). Soil respiration and nutrient content are elevated beneath zones of luxuriant vegetation associated with Agaricus arvensis (Edwards, 1984), suggesting that enhanced decomposition of SOM is responsible for increased plant nutrient availability. In the same rings, symptoms of K deficiency were observed in associated grasses, suggesting that fungal tissues concentrated nutrients (with basidiocarps containing 6% N, 3% K and 1% P by dry weight), at the expense of adjacent plants. Edwards (1984,1988) estimated that basidiocarps contained ca. 25% of all the K present in areas of dense fungal growth. The ability of grassland basidiomycetes to concentrate K and related elements in fruit bodies has subsequently received attention in the context of radiocaesium (137Cs) accumulation following the Chernobyl reactor explosion (Dighton et al., 1991; Anderson et al., 1997).

The high mycelial density in annular areas of rings causes changes in the hydrological properties of the soil (Warcup, 1959; Terashima and Fujiie, 2005). Increased soil hydrophobicity is linked to hyphal secretions, possibly hydropho-bins, which coat soil particles. On managed turfgrasses the resulting 'dry patch' symptoms can be alleviated by use of surfactants and fungicides (York and Canaway, 2000). Under suitable climatic conditions (rings are usually most visible in dry summers), these localized changes in soil hydrology can alter growth of vegetation, potentially masking the beneficial effects of elevated soil nutrients described above. However, in rings of some type 1 species, secretion of toxins (such as cyanide) has been implicated (Blenis et al., 2004), while other species (e.g. M. oreades, Vascellum curtisii and Bovista dermoxantha) have a necrotrophic ability following colonization of healthy root and leaf tissues (Filer, 1965; Terashima et al., 2004). Several type 1 species exhibit host specificity with regard to symptom production, with Terashima and Fujiie (2005) reporting a ring of L. sordida causing type 2 symptoms on Zoysia japonica, but disappearing on reaching an area vegetated by Lolium perenne.

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