The Cellulose Decomposition Phase

After the early decomposition phase, a community of secondary colonisers replaces the primary colonisers. In our studies of pine litter colonisation, and also in an American study of mixed deciduous litter (O'Brien et al., 2005), Mycena species dominated the Basidiomycota DNA pool. The ecophysiology of Mycena galopus has been well investigated by Frankland and co-workers (reviewed by Frankland, 1998). In our studies, identities of the Mycena species remain unknown, due to lack of database references. More detailed studies of the molecular phylogeny of litter basidiomycetes in general, and of the genus Mycena in particular, are required. Throughout the cellulose decomposition phase, the Mycena species and other basidiomycetes were accompanied by ascomycetes. In litter of Pinus sylvestris, P. taeda, Picea abies and mixed deciduous species, as-comycetes accounted for 50-70% of the fungal taxa. The distribution of clones more or less reflected the distribution of taxa, suggesting that ascomycetes constituted a major fraction of the mycelial biomass in the litter. Almost all of the ascomycetes recorded as DNA in the litter were either Leotiomycetes (primarily within the Hyaloscyphaceae, Helotiales) or Dothideomycetes. The fact that as-comycetes constituted a major fraction of the litter-colonising fungi does not, however, necessarily imply that they were responsible for a major part of the decomposition. Osono and Takeda (2002) tested the litter-degrading capacity of 60 different species over 21 months. They found that, apart from Xylariaceae, ascomycetes caused little mass loss of the litter, and their litter-decomposing capacity was exceeded by that of basidiomycetes by an order of magnitude. In the study of Osono and Takeda (2002), however, Leotiomycetes were represented by one species only. In the pine forest study of Lindahl et al. (2007), each litter sample (taken with a 28 mm diameter corer) contained on average 1.6 bas-idiomycete taxa, ranging up to 5. In 15% of the samples, no basidiomycetes were detected. These samples did not diverge from the rest in decomposition, suggesting either that some ascomycetes may cause significant decomposition, and/ or that there is a high degree of temporal variation in basidiomycete colonisation.

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