There were clear trends in the effect of water potential on fractal dimensions of Stropharia caerulea: DBS decreased (1.7-1.35 approximately) with increasing water potential (i.e. getting wetter) from -0.02MPa to -0.002MPa (Figure 8.9) [69]. For Phanerochaete velutina, there was no significant effect of soil water potential on Dbs, although DBM was sometimes higher on soils drier than -0.006 MPa. These experiments were performed under constant conditions; however, soil water potential is rarely constant in the natural environment. Rather, it increases following rain and then gradually decreases. Such occurrences were mimicked in an experiment in which cord systems established in 24 cm x 24 cm trays were subject to wetting and drying events immediately after addition of a wood resource behind the mycelial margin, or 20 days after addition of the new resource [68]. Wet shifts were achieved by gradually adding deionised water over several days to raise the water potential from -0.019 MPa to -0.009 MPa, equivalent to field capacity; dry shifts were achieved

4 Defined as the free energy of water in a system relative to that of a reference pool of pure water (megaPascals). Effectively, it indicates the ease/difficulty that a microorganism would have in extracting water from an environment. The lower the water potential (i.e. the more negative), the harder it is to obtain water.

by removing lids for 11 days, allowing soil to dry to -0.056 MPa. Wet shifts resulted in a significant increase in DBM in the part of the system containing the new resource, whereas there was no such change following dry shifts.

Figure 8.9 Effect of soil water potential on morphology of Stropharia caerulea extending from 4 cm3 beech wood resources, across nonsterile soil compacted in 13 cm diameter trays at (a) -0.02 MPa (b) -0.06 MPa and (c) -0.006 MPa.

Soil pH considerably affects some species: Phanerochaete velutina had a much lower fractal dimension at pH 7 than at pH 5-6 (1.3 and 1.55-1.67 respectively) [60], though the reason for this is not known. In contrast, Coprinuspicaceus often failed to grow out of wood resources at pH 4.4, but when it did it grew significantly slower than at higher pH. However, similar amounts of extra-resource biomass were produced at all pHs, and this was achieved at lower pHs by much greater space filling.

8.5.6 Changes in Fractal Dimension during Interspecific Microbial Interactions

As mycelia grow through soil they inevitably encounter other mycelia, resulting in antagonistic interactions [70]. The outcomes can be: (a) deadlock, where neither fungus gains any territorial advantage; (b) replacement, where one fungus replaces the other and gains the replaced fungus's territory; (c) partial replacement, where some but not all of the opponents territory is gained; and (d) mutual replacement, where fungus A gains some of the territory held by fungus B, but the mycelium of fungus B simultaneously makes inroads into the territory held by fungus A. DBM and DBS have only been quantified in two papers on interactions between mycelia in soil [33, 61]. Stropharia caerulea mycelia were allowed to interact with mycelia of four other saprotrophic basidiomycetes, individually, in trays of compressed soil [61]. Dbm or Dbs varied depending on interaction combination (Table 8.4). Importantly, there were often localized changes in mycelial morphology (Figure 8.2) and fractal dimension that reflected defensive responses and sometimes resulted in penetration into the opponent's territory. In another study Armillaria luteus rhizomorph systems encountered those of other Armillaria species and of conspecifics in sand microcosms [33]. Only occasionally did this result in differences in DBM compared with systems growing alone. Changes in foraging pattern also occur during interactions between saprotrophic and ectomycorrhizal mycelium [71] (Figure 8.2).

Table 8.4 Changes in fractal geometry of mycelial systems during interactions between Stropharia caerulea and other cord-forming, saprotrophic basidiomycetes in trays of compressed soil. Data from [61].

Other species

Outcome of interaction

Change in DBM and DB

Stropharia caerulea

Other species

Agrocybe Initially deadlock, but gibberosa Agrocybe gibberosa completely encircling Stropharia caerulea. Subsequently, fans of Stropharia caerulea mycelium extended over Agrocybe gibberosa

Hypholoma Deadlock, but mutual fasciculare replacement in one system

Production of dense localised fans (Dbm = 1.89 ± 0.04)

Differences not usually significant

Phanerochaete Stropharia caerulea was velutina rapidly replaced

Phallus Temporary defensive impudicus ridges produced by

Stropharia caerulea, but these were breached in a few places and the fungus was replaced when Phallus impudicus reached the wood blocks

No change in Dbm or

Dbm and Dbs were significantly reduced

Differences not usually significant

High Dbm (1.83 ± 0.03), non-invasive, lateral fans were produced elsewhere (Figure 8.2) No change in DBM consistently lowered

DBM was significantly reduced

No change in DBM or

8.5.7 Effects of Invertebrate Grazing and Destructive Disturbance

Fungal mycelia concentrate mineral nutrients as they decompose organic resources; therefore, they form a highly nutritious food for soil inverterbrates [72]. Much research has focused on the grazing of soil invertebrates on fungal mycelia, but the spatial implications due to fungal growth responses have received little attention. Microarthropod grazing results in dramatic changes in mycelial morphology (Figure 8.10), including changes in fractal structure [73]. When the collembolan

Folsomia candia grazed on Hypholoma fasciculare, at different grazing intensities (20, 40 or 60 collembola added to mycelia with a radius of 1.5 cm), there was no significant difference in DBM, but DBS differed significantly: from 30 days DBS was significantly higher in controls and decreased monotonously with increased grazing intensity. Not only do effects vary depending on grazing intensity, but also on collembolan species and fungal species [74].

Figure 8.10 Mycelium of Phanerochate velutina extending from 4 cm3 beech wood resource, across nonsterile soil compacted into 24 cm x 24 cm trays: (a) ungrazed; and grazed by (b) Folsomia candida or (c) Proistoma minuta (collembola) (images courtesy of George M. Tordoff). Collembola were added when the mycelial systems had extended 4 cm from the wood resource.

Figure 8.10 Mycelium of Phanerochate velutina extending from 4 cm3 beech wood resource, across nonsterile soil compacted into 24 cm x 24 cm trays: (a) ungrazed; and grazed by (b) Folsomia candida or (c) Proistoma minuta (collembola) (images courtesy of George M. Tordoff). Collembola were added when the mycelial systems had extended 4 cm from the wood resource.

Grazing is one form of destructive disturbance. More extensive disturbance could result from the activity of large animals. In an experiment to simulate major destructive disturbance, cord systems of Phanerochaete velutina, developing from wood blocks on soil, were completely severed and removed from zero, three or four faces of the wood cubes [75]. Where mycelia were completely severed, regrowth had the same Dbm (1.6) as undisturbed systems. However, when mycelium were severed from three sides, regrowth initially had the same DBM (1.6) as unsevered parts, but following subsequent removal there was only limited regrowth from the severed sides, and a much lower DBM (1.4). The systems had developed a distinct polarity (i.e. growth predominantly in one direction).

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