Decay Of Wood And Litter

Since fungal hyphae have a large surface to volume ratio, fungi are particularly effective agents in decomposing the biomass—though strictly speaking, communities other than fungi comprised of bacteria and actinomycetes also participate in this process. Cellulose and hemicelluloses are the chief polysaccharides in plant cell walls which in woody tissues are encrusted with lignin. Most fungi living as saprophytes on organic matter produce cellulases and hemicellulases. However, the occurrence of a fungus where cellulose or lignin is abundant is not enough to infer that the fungus is responsible for decomposing these polymers, since a non-cellulolytic fungus may live commensally on products formed by a cellulolytic fungus. The removal of certain plant cell wall polysaccharides by one species may improve the accessibility of another species to cellulose (polysaccharide composed of glucose molecules joined in (3-1,4 linkage) or hemicelluloses (non-cellulose polysaccharide, composed of (-linked pentose with side chains). Direct observation of sporulating structures and cultural (plating) techniques demonstrate that the decomposition of biomass involves activities of a mixed microflora comprising fungi, actinomycetes and bacteria.

Studies with pure cultures of fungi show that the enzymes involved in biomass degradation are synthesized in significant amounts only when inducing substrates are present. How the insoluble substrates induce the synthesis of the enzymes requires more study. Fungi belonging to the Basidiomycotina are the most important organisms contributing to the decay of wood. Their leathery or woody fruit bodies (basidiocarps) project out from tree trunks (Figure 12.1), signaling that their mycelium inside the plant tissue has been slowly attacking lignin and polysaccharide constituents. The spores or mycelia of these fungi gain entry into wood through wounds. They are carried by wind, water, insects, birds or by rodents that feed on and breed inside the weakened or dying trees. The early stages of decay of standing or fallen trees is due to several species but soon competition between mycelia sets in and a single individual mycelium can extend to several meters (Boddy and Rayner, 1982). Decay of biomass is an aerobic process requiring oxygen for the oxidation of lignin phenols. Moisture is vital not only for the synthesis of

Figure 12.1 Wood decay. Fruit bodies (basidiocarps) protruding from a decaying fallen tree in a rainforest. Wood-rot fungi often attack the standing tree. (Photo: Jean Paul Ferrero/Ardea.) (See color insert following page 140.)
Table 12.1 Chemical Analysis of Decayed and Sound Wood Samples1
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