Decay of the components of wood. Despite its highly integrated matrix of cellulose, hemicellulose, and lignin, which gives wood superior strength properties and a marked resistance to chemical and microbial attack, a variety of organisms and processes are capable of degrading wood. The decay process is a continuum, often involving a number of organisms over many years. Wood degrading agents are both biotic and abiotic, and include heat, strong acids or bases, organic chemicals, mechanical wear, and sunlight (uv degradation).
Abiotic degradation
Heat degrades both cellulose and hemicellulose, reducing strength and causing the wood to darken. At temperatures above 451°F (219°C), combustion occurs. Strong acids eventually degrade the carbohydrate portion of wood, reducing its strength. Strong bases attack the lignin, leaving the wood appearance bleached and white. Other chemicals, such as concentrated organics or salt solutions, can also disrupt the lignocellulosic matrix, reducing material properties of the wood. Sunlight, primarily through the action of ultraviolet light, also degrades wood through the creation of free radicals which then degrade the wood polymers. Mechanical wear of wood can occur in a variety of environments.
Biotic degradation
Biotic damage can occur from a variety of agents, including bacteria, fungi, insects, marine borers, and birds and animals. Birds and animals generally cause mechanical damage in isolated instances. All biotic agents have four basic requirements: adequate temperature (32–104°F or 0–40°C) with most optima between 77–90°F (25–32°C), oxygen (or other suitable terminal election acceptor), water, and a food source. Water is a critical element for biotic decay agents: it serves as reactant in degradative reactions, a medium for diffusion of enzymes into wood and degradative products back to the organism, and a wood swelling agent.
Bacteria are not major degraders of wood products, but they can damage pit membranes, thereby increasing permeability, and some are capable of cell wall degradation. See also Bacteria.
Fungi are among the most important wood-degrading organisms because they play an important role in terrestrial carbon cycling. Wood-degrading fungi can be classified as molds, stainers, soft rotters, brown rotters, and white rotters on the basis of the attack patterns. Molds, stainers, and soft rotters are members of the ascomycetes and the deuteromycetes (Fungi Imperfecti). See also Ascomycota; Deuteromycotina; Fungi.
A number of insects have evolved to attack wood, including termites (Isoptera), beetles (Coleoptera), and bees and ants (Hymenoptera). Termites are the most important wood-degrading insects in most environments, and their activity causes severe economic losses. See also Coleoptera; Hymenoptera; Isoptera.
In saline environments, marine borers can cause significant wood losses. Three groups of marine borers—shipworms, pholads, and gribbles (Limnoria)—cause most wood damage in these areas. See also Boring bivalves; Shipworm.
Wood protection
Protecting wood from degradation can take a number of forms. By far the simplest method is to employ designs which limit wood exposure to moisture. In some cases, however, water exclusion is not possible and alternative methods must be employed. The simplest of these methods is the use of heartwood from naturally durable species. Decay- or insect-resistant species include redwood (Sequoia sempervirens), western red cedar (Thuja plicata), and ekki (Lophira alata), while marine-borer-resistant heartwoods include greenheart (Ocotea rodiaei) and ekki. Most marine-borer-resistant woods contain high levels of silica which discourages marine borer attack, while species resistant to terrestrial decay agents often contain toxic phenolics. Wood can also be protected from degradation by spraying, dipping, soaking, or pressure treatment with preservatives. See also Wood properties.
