Woody detritus

Woody debris, be it small twigs, branches or whole stems, contribute significantly to the detrital biomass in forests, freshwaters, and coastal marine areas. These inputs derive from both natural and anthropogenic activities including senescing trees, natural disturbance (e.g., windfall and slope failure), silviculture, and forest management. In temperate forests, the input of woody debris amounts to roughly 1000 kgha-1 yr_1. Generally, wood decomposition is slow, in the case of logs and trunks roughly equaling the lifetime of the tree, while small twigs in the litter layer may decompose at almost the same rate as leaf litter. Thus, particularly coarse woody debris adds long-lasting unique habitat structure and resources to both terrestrial and aquatic habitats, by being an important source of energy and nutrients for microorganisms and detritivores, trapping sediments, offering protection against harsh environmental conditions, and serving as refugium from predation to both consumers and inhabitants of decomposing wood. Coarse woody debris increases the diversity of biological communities in aquatic and terrestrial habitats. Certain species of fish depend on the wood in streams to survive and to spawn. Logs, trunks, and rootmats still attached to fallen trees are a key structural component in streams that deflects flows in ways that scour stream pools and induce meanders, but also anchors streambanks against high flows and floods by fixing the sediment, thus, reducing erosion and sedimentation downstream. Woody debris also serves as food source (albeit of low quality, owing to low nutrient and high lignocellulose contents) to xylophagous (Greek: Xylon: wood) detritivores. Even more important, the surface of woody debris is densely colonized by a microbial biofilm that, in turn, is grazed upon by various animals. In contrast to most other types of detritus, dead wood in forests is not necessarily in contact with the forest floor -standing dead trees are also subject to decomposition, but the position of the trunk (along with the tree species, the composition of the forest community, soil characteristics, and environmental conditions) determines the processes of decomposition. Even more than for angiosperm litter of photosynthetic tissue, the degradation, or at least modification, of lignin is a prerequisite for wood decomposition. Thus, it is solely fungi (albeit of various taxonomic affiliations) that drive wood decay, being limited by the extremely wide C:N ratio of roughly 300-1000 (cf. Table 2). While soft- and brown-rotting fungi hydroly-tically degrade cellulosic and hemicellulosic moieties of the cell walls, leaving brownish lignin remains, white-rot fungi oxidatively degrade lignins, but concomitantly also hydrolyze cellulose and hemicellulose - the main goal of lignin degradation is not the nutritive utilization of aromatic carbohydrates, but the removal of an embedding matrix to gain access to fiber polysaccharides. Fungal attack of freshly dead wood is facilitated by insects that actively intrude the wood by burrowing holes and galleries. Xylophagous insects are by more than 90% Coleoptera, at later stages of wood decomposition being accompanied by Collembola and noninsects, such as millipedes, isopods, and lumbricids. In the tropics, it is mainly termites that destroy rotting wood. The role of (coarse) woody debris is well-studied in forests and fresh-waters. It provides habitat (and food) and increases environmental heterogeneity; it traps and accumulates wind- or current-driven detritus; it affects microclimatic conditions in the litter layer and the upper soil horizons (terrestrial). By contrast, there is a dearth of information on its importance in marine coastal ecosystems. In high-intertidal tidepools with little algal cover, woody debris increases habitat complexity and heterogeneity and is readily colonized by motile invertebrate in search for shelter from harsh environmental conditions and predation. In low-intertidal pools and the open coast, the complex habitat structure of algal communities increases the relative significance of woody debris and its microbial biofilm as supplementary food source at the expense of its role as habitat structure.

While leaf and needle litter in grasslands and woodlands is more or less evenly distributed, albeit with patchily scattered hot spots of high substrate quality and decomposing activity, the distribution of woody debris in a given habitat is rather heterogeneous, resulting in patches of refugia and supplementary food sources by means of biofilms. Similarly, coastal detritus, be it macroalgae or angiosperms, accumulate in patches, either at the upper margin of the intertidal zone (wrack line or drift line), in areas of little water movement beneath stems, shoots or thalli of macrophytes, or aggregated in drift mats floating subtidally. In either case, the patchy nature of these types of detritus provides habitat heterogeneity, although the effects on the community of animals making use of these patches may differ substantially, being either favorable (see above) or detrimental in terms of reducing the oxygen availability and producing hypoxic or even anoxic microhabitats underneath and within these patches.

Animal detritus

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