Decay in Felled Logs and Large Branches

Fungal community development in bulky wood on the forest floor has been studied in a number of cases, especially in Fagus spp., which we describe here as a model system for wood decay in angiosperms. Decay community development in other deciduous tree species seems to follow similar pathways (Gricius et al., 1999; Hood et al., 2004; Lindhe et al., 2004), though some differences are evident reflecting differences in bark and wood morphology and wood chemistry. In many Betula spp. the bark is usually intact until final decay stages (J. HeilmannClausen, personal observation), making access for secondary colonizers more difficult. Similarly, species which possess true heartwood, e.g. Quercus spp., present widely different decay environments, where decay proceeds following different pathways in sapwood and heartwood.

There have been detailed studies on fungal community structure of felled beech logs, at the mycelial level, during the first 4.5-5 years of decay (Coates and Rayner, 1985a, 1985b, 1985c; Chapela and Boddy, 1988b; Chapela et al., 1988; Boddy et al., 1989; Willig and Schlechte, 1995), and of fruit bodies to very late stages of decay (Ueyama, 1966; Siepmann, 1973; Lange, 1992; Heilmann-Clausen, 2001). The different studies revealed similar patterns of community structure and development though species composition sometimes differed. Primary colonizers derived from those latently present in functional sapwood were prominent during early stages of decay, including a number of pyrenomycetes (Chapela and Boddy, 1988a, 1988b; Heilmann-Clausen, 2001; Hendry et al., 2002), but also bas-idiomycetes, especially F. fomentarius (Baum et al., 2003), and possibly I. nodulosus and Exidia plana (Heilmann-Clausen, 2001). After a few weeks secondary Basidiomycota, including Bjerkandera adusta, T. versicolor and S. hirsutum, arrived by spores at cut surfaces, and their decay columns became clearly resolved from 24 weeks (Chapela and Boddy, 1988b) (Figure 3). On some sites, though not others, the Ascomycota, X. hypoxylon, became increasingly evident between

Figure 3 Diagram illustrating the way in which a hypothetical community might develop in a single beech log, placed upright with the base in contact with soil, over time. No two communities will ever be identical because differences in fungi arriving, and in environmental conditions, at every time point will result in different patterns of development. Nonetheless, species general temporal patterns are discernable (from Boddy, 2001; 6-18 months adapted from Coates and Rayner, 1985c).

Figure 3 Diagram illustrating the way in which a hypothetical community might develop in a single beech log, placed upright with the base in contact with soil, over time. No two communities will ever be identical because differences in fungi arriving, and in environmental conditions, at every time point will result in different patterns of development. Nonetheless, species general temporal patterns are discernable (from Boddy, 2001; 6-18 months adapted from Coates and Rayner, 1985c).

12 and 52 weeks. In large logs the same species dominated by 3-9 years after felling (Willig and Schlechte, 1995; Heilmann-Clausen, 2001) probably following establishment in foci on the bark surface or in small crevices exposing sapwood. In sun-exposed beech logs the bark often loosens within 1 year and communities of stress-tolerant fungi quickly develop, including secondary invaders of which Schizophyllum commune and Trametes hirsuta are the most prominent (Willig and Schlechte, 1995; Heilmann-Clausen, 2001).

In small logs, cord-forming saprotrophs, e.g. Hypholoma fasciculare, Megacollybia platyphylla, Phallus impudicus and Phanerochaete velutina, and the rhizomorphic Armillaria gallica were colonizing by 6 months, the latter usually subcortically, and then in peripheral regions (Coates and Rayner, 1985a, 1985b, 1985c; Boddy, 2001). Eventually, the combative cord-formers occupied substantial decay columns after replacing earlier colonizers (Figure 3). After 1-1.5 years many of the early colonizers had declined, though S. hirsutum persisted in a few logs, and T. versicolor and X. hypoxylon (Ascomycota) were still present in at least 45% of logs by 4.5 years (Chapela et al., 1988; Boddy, 2001). Cord-forming Basidiomycota dominated many logs by 4.5 years though other Basidiomycota, Coprinus spp., L. betulina (invading by temporary mycoparasitism of T. versicolor; Rayner et al., 1987), Psathyrella piluliformis, and the Ascomycota, Lopadostoma turgidum and various soil Mucorales and Deuteromycota were also evident by then. The presence of the latter groups generally reflected disturbance by invading soil invertebrates.

In large logs, colonization by cord-forming Basidiomycota and other combative secondary decay fungi seems to progress slower than in smaller logs (Willig and Schlechte, 1995; Heilmann-Clausen, 2001), probably because access is more difficult due to the protective bark layer and longer distance from cut ends. This allows more time for latent decay fungi to develop large mycelia, and especially E. spinosa (Ascomycota) and F. fomentarius are able to sustain sporu-lating mycelia for decades (Lange, 1992; Heilmann-Clausen, 2001). In the Ascomycota E. spinosa, and X. hypoxylon in smaller logs, this seems to reflect a defensive strategy in which these species maintain occupied wood drier than its surroundings (Boddy et al., 1989; Heilmann-Clausen, 2001).

Little is known of community development during late stages of wood decay, but sporocarp based studies indicate that agarics, especially Mycena and Pluteus spp., become increasingly dominant (Lange, 1992; Heilmann-Clausen, 2001), together with cord-formers, which often persist throughout the decay process. In addition, ectomycorrhizal species and soil/litter saprotrophs are often present, as evidenced by sporocarps (Lange, 1992; Heilmann-Clausen, 2001) and ectomycor-rhizal roots (Harvey et al., 1976; Tedersoo et al., 2003).

The nutrient content in well-decayed wood is higher per unit volume than in undecayed wood, due to loss of carbon as respiratory CO2 and import of mineral nutrients via mycelia of non-resource-restricted fungi (Swift and Boddy, 1984; Boddy and Watkinson, 1995; (Odor and Standovar, 2003; Laiho and Prescott, 2004; Chapters 1 and 3). Most nutrients are, however, bound in living fungal hyphae, bacteria and fauna, while the wood tissue per se is depleted of readily available nutrients. Some of the fungi inhabiting wood at late stages of decay may accordingly depend much more on alternative sources of nutrition. There is a dearth of information in this area, but it is well known that many wood-inhabiting fungi can utilize invertebrates, bacteria and algae (Barron, 2003; Chapters 8 and 9), and some, including not only saprotrophs (Niemela et al., 1995) but also ectomycorrhizal species (Lindahl et al., 1999; Leake et al., 2001), are able to retrieve nutrients from mycelia of other fungi, due to combative or mycoparasitic interactions. Competition/combat for space is then no longer the main driving force for community change. Rather, drivers are likely to be nutrient stress, stress relating to shifting microenvironmental regimes and disturbance by larger animals destroying dead wood and mycelia, and grazing by invertebrates. Grazing results in dramatic morphological and presumably also physiological changes in mycelia of Basidiomycota, and comminution (Chapter 9). At very late stages the composition of the remaining wood components becomes increasingly similar to soil, and R-selected soil fungi and bacteria often dominate (Swift and Boddy, 1984), though ectomycorrhizal fungi may also become established (Tedersoo et al., 2003).

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