The natural history of the degradation of wood

Abstract

To the succession of bacteria and fungi colonizing it, wood consists of a series of conveniently oriented holes surrounded by food. The degradation of wood leading to the utilization of that food supply takes place in a number of clearly defined stages, each of which results from the activity of a particular group of organisms. The success of the entire process is governed by the moisture content of the wood. This has to be high enough to promote a flow path for the products of enzyme action yet low enough to prevent water logging, which might produce an anaerobic habitat hostile to the normal wood-decay organisms. Wood in soil contact takes up an equilibrium moisture content with its surroundings, so that a post or pole will be wet below ground level and comparatively dry above it. Capillary action takes water along the conducting elements of the wood, but once above ground level evaporation will ensure flow through the wood from the high water potential in the soil to a low water potential in the atmosphere. Soil nutrients are carried by the water and subsequent attack by wood-rotting organisms will occur mainly at about ground level where supplies of atmospheric oxygen, soil water and nutrients are plentiful. The initial colonizers are bacteria, some of which destroy the pit membranes, thereby increasing aeration of the wood by gaseous diffusion. They facilitate access to the supplies of nutrient in the wood by the subsequent scavengers, the primary moulds and sapstain fungi. Later still the true wood-rotting fungi, the soft, white and brown rots, make their appearance; each of these has its own particular pattern of cell-wall breakdown. The latter two groups ultimately dominate the soft rots and open up the structure of the wood cell walls. The partial decay of the walls will finally permit the cellulolytic secondary moulds to gain access to and therefore compete for the carbohydrate contents of the wall. The situation out of ground contact (for example, window joinery), is similar, moisture being equally important and the occurrence of a similar sequence of colonizing organisms, suggesting that the structure and composition of the wood itself has a part to play in selecting the ‘pecking order’.