Box 76 Colonial insects and wood decomposition

A number of termites, beetles and ants attack undecayed wood. Of these, termites account for the very rapid (and to us, potentially devastating) disappearance of wood in warm temperate and tropical forests. There are more than 2500 species of termites worldwide and the genus Coptotermes has 28 pest species, of which the Formosan subterranean termite or FST C. formosanus is the most widely distributed and economically important. Although probably endemic to southern China it was apparently transported to Japan prior to the 1600s and is now found in many other countries, having spread widely in the USA since the 1960s. As with other termites the FST has three primary castes: the reproductives, soldiers and workers. New colonies are each founded by two 'alates', one male and one female, from the huge swarms which fly on calm and humid evenings in early summer. The colony establishes on moist wood and takes 3-5 years to become large enough to cause severe damage. This termite attacks living plants and a number of structural materials as well as wood. Laboratory studies show that termites can consume up to 10% of their body weight in a day which, with colonies containing several million individuals, makes them significant and very difficult to control.

contribution of most burrowing insects is in producing galleries through the wood and in bringing fungal spores with them for direct inoculation inside the wood. Both of these help fungi to colonize more rapidly.

The larvae of the British stag beetle Lucanus cervus can reach a length of 12 cm and produce large and significant galleries in the decaying roots of deciduous trees which they help to break down. This is the largest beetle in Britain; adults can most often be seen from May to August, occasionally flying on warm evenings. The beetle, which causes no damage to garden plants, has become less frequent in recent years and it is now a UK Biodiversity Action Plan priority species.

Some of the wood-boring insect grubs found in fallen rotting logs, and for that matter tree trunks and limbs, of the forests of Australia and Papua New Guinea are surprisingly large and rapidly accelerate the decomposition process. Moreover, they are very attractive food items and the striped marsupial possum Dactylopsida trivirgata locates them by tapping the wood for the betraying hollow sound caused by the tunnels and then breaks in, further breaking up the log and skewering the grub with the sharp claw of the elongated fourth finger. This is by no means a modern strategy, indeed it may well have been employed by the now extinct Heterohyus nanus, which had two very long spindly fingers on each hand. These were probably used to extract insects from deep within rotting wood. Fossil remains of this animal which belonged to the apatemyids, a small group of insect-eating mammals known from the Palaeocene to the Oligocene, were found in the Messel pit near Frankfurt, Germany. The kaka Nestor meridionalis, a parrot which lives in large tracts of southern beeches in New Zealand, also uses its strong curved bill to dig into decaying trees for grubs and other insects.

Although insects are important, microbes are by far the most important biological agents responsible for wood decomposition. Of these, fungal rot, particularly by basiodomycetes, is responsible for the disappearance of most wood at moisture levels above 20%. Bacteria can break down woody cell walls but tend to be localized in their effect. They dominate in waterlogged wood over fungi, which is the main reason why submerged wood decomposes so slowly. As with insects, fungal decomposers go through a succession of types. The first invaders of freshly dead wood are normally the moulds and staining fungi which live on the easily digested cell contents (the stain fungi, mostly Ascomycetes, have pigmented hyphae which stain the wood, lowering its commercial value which is why foresters try to process cut wood as soon as possible). These early invaders fail to affect the integrity of the wood itself because they cannot digest structural carbohydrates or lignin, and in fact may slow subsequent decomposition by using up the free sugars and starches (the non-structural carbohydrates).

Three types of structural rot are often distinguished. A relatively small number of wood decay fungi break down just the cellulose leaving the brown-coloured lignin intact and so are called brown rots. These occur mainly on coniferous wood and cause the commonest type of wood decay in boreal forests. Other fungi break down both the lignin and cellulose components of wood leaving a weak white residue and so are known as white rots. Finally the soft rot fungi produce spongy pockets of decay in the outer layers of wet wood and are major players only in wet environments. Both brown and white wood rots attack wood using extracellular enzymes secreted outside the hyphae. Celluloses are very long unbranched strands of linked glucose units and, although insoluble, their repeating unit structure makes them readily degrad-able by cellulase enzymes. Hemicelluloses are more complex, being composed of branched polymers that contain several different sugars, and so are less easily decomposed. Lignin is particularly difficult to break down and requires around 12 enzymes, including ligninases and ring-splitting deoxygenases. The lignin is not itself used as a food source but its removal allows the attached cellulose to be digested.

The decomposition of a log usually involves several species of fungi working in succession, perhaps related to different complements of enzymes held by different species. However, the pattern is not normally clear and a large number of fungi may inhabit the same log at the same time. When this happens, the territories occupied by different species or different colonies of the same species are demarcated by black zone lines. These are double walls, built by the two touching colonies, composed of gnarled dark-coloured hyphae and functioning to reduce competition. Woodworkers enjoy this 'spalted' wood for its attractiveness.

A number of wood-decaying basidiomycetes that rely on wood as their only source of food are able to forage or search for new pieces of fallen wood on the forest floor. The hyphae of the fungus grow out through or over the soil in all directions from the wood it is currently living in. When the hyphae happen upon a new piece of wood all new growth of the fungus is concentrated on reinforcing the link between old and new wood with more hyphae to form a mycelial cord. This root-like structure transports water and nutrients to the new wood aiding the invasion process. A superb example of this military-like invasion is shown by the honey fungus Armillaria whose well-developed mycelial cords, called rhizomorphs, are so effective that they allow the fungus to invade living tissue (see Sections 5.4.2 and Fig. 5.10).

In most forests, wood will be colonized by fungi within a year and completely colonized within 5-10 years. However, decay rates of wood vary tremendously depending upon the climate, decaying organisms available and the size and type of wood. Small-diameter logs of Monterey pine left over from forestry operations in New Zealand were found by Ganjegunte et al. (2004) to be largely intact after 4 years, to have lost their bark by 9 years with wood still superficially sound, and to have lost 59% of their initial mass by 13 years. In tropical forests wood above 3 cm diameter takes at least 15 years to decompose; logs may disappear more slowly than might be thought (Anderson and Swift, 1983). In Fiby urskog, Sweden, Norway spruce Picea abies logs observed for 50 years were set to decay completely in less than 90 years. Conversely Franklin et al. (1981) found that 80-cm diameter Douglas fir logs in the Pacific Northwest lost around 40% of their original wood only after 150-200 years, and 90% after 480-580 years. Rates of decay are variable even within a single genus; the lifetime of eucalyptus logs ranges from 7 years in mountain ash Eucalyptus regnans in high rainfall mountainous areas to 375 years in the river red gum E. camaldulensis along water courses (Mackensen et al., 2003). In general terms, pioneer trees invest less energy in protecting their wood from rot (they go for speed rather than defence and logs on the ground have a half-life of a few decades at most) compared with longer-lived late-successional trees which may persist as dead wood for a century or much longer. But again, environmental conditions play an important role in determining decay rates; balsam poplar Populus balsamifera logs which would decay away within 40-60 years on land have been found to last over 250 years when waterlogged in a beaver pond.

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