Mycelia Foraging Between Relatively Homogeneously Distributed Resources

Fungi have evolved a variety of foraging and behavioural responses to encounters with new resources. Fungi that utilize individual, relatively homogeneous resources, e.g. a leaf litter layer, effectively colonize as if individual components are simply parts of a larger resource. Mycelia form large patches with no particular pattern, e.g. Collybia spp. and Marasmius spp., or form fairy rings, e.g. Clitocybe nebularis (Dowson et al., 1989). Nothing is known of the network architecture of mycelial patches, but fairy rings of C. nebularis extend through the leaf litter layer as an ever increasing annulus of mycelium ^30-40 cm wide (Dowson et al., 1989; Figure 1a-d). The band is differentiated into three distinct zones: (1) the leading edge comprises mycelial cords (linear organs of predominantly parallel hyphae) spreading across the leaf litter layer and up to 6 cm into soil beneath; (2) a central region of dense mycelium which ramifies throughout, and presumably causes, intensely bleached leaf litter but does not extend into the mineral soil; (3) mycelium at the trailing edge which becomes progressively fragmented before completely disappearing. (Fruit bodies are produced from the middle of zone 2.) This outwardly extending annulus does not form as a result of lack of nutrients in central areas, as these are replenished every autumn, nor are toxic metabolites likely to be the cause, since when part of the annulus was transplanted into this region it grew well (Dowson et al., 1989). Rather, these mycelia exhibit highly polarized growth, such that when a turf containing all zones of the annulus was relocated elsewhere, growth continued in the original direction of travel with limited lateral growth (Dowson et al., 1989). Young mycelia of C. nebularis form patches, but what triggers annulus formation is unknown. Presumably ring formation is related to size and might be expected to start when a patch is over 80 cm diameter (i.e. double the width of the mycelial band).

Figure 1 (a-d) Mycelium of a Clitocybe nebularis fairy ring which had developed under a paving slab in a garden. (a) Location of fruit bodies in relation to mycelium. Note aggregation into cords, but still with diffuse mycelium, towards the leading edge (left). (b) Mycelium aggregating into fine cords at leading edge. (c) Thicker cords amidst dense fine mycelium. (d) Very dense, fine mycelium in central zone of annulus. (e) Mycelial network of Megacollybia platyphylla in a mixed deciduous woodland, revealed by removal of surface litter. Digital images (a)-(d) courtesy of David Moore.

Figure 1 (a-d) Mycelium of a Clitocybe nebularis fairy ring which had developed under a paving slab in a garden. (a) Location of fruit bodies in relation to mycelium. Note aggregation into cords, but still with diffuse mycelium, towards the leading edge (left). (b) Mycelium aggregating into fine cords at leading edge. (c) Thicker cords amidst dense fine mycelium. (d) Very dense, fine mycelium in central zone of annulus. (e) Mycelial network of Megacollybia platyphylla in a mixed deciduous woodland, revealed by removal of surface litter. Digital images (a)-(d) courtesy of David Moore.

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