All wood is biodegradable. Many timber species have, however, sufficient natural durability to permit their use, particularly in protected situations, without any special precautions. Many others, of course, are readily attacked by insects and where there is sufficient moisture and air, by wood-decaying fungi. It is these timber species which benefit most from treatment with wood preservatives. Even the so called durables may also benefit so since all sapwood is perishable and durable heartwood in the sea or in ground contact eventually fails from attack by animals and/or micro-organisms. In recent years it has become apparent that for high hazard end uses it is necessary to consider the timber species/preservative type/treatment method as a single unit. It is of course possible, after accumulation of experience and test data, to group certain timber species into classes which can be processed together but until this has been established it is necessary to consider each and every timber species as a distinct biological entity which will respond differently to other timber species to preservative treatment. It has also been found that the most widespread treatment process, vacuum/pressure treatment, is inappropriate for many of the timber species available from the world's forests, particularly hardwoods. In ground contact, the commercially available preservatives do not provide the needed protection against fungal decay and in low hazard uses other preservatives and processes may be used more cost effectively for at least equally satisfactory results. In many temperate countries depletion of durable hardwoods led to wide use of perishable conifers in high hazard sites; wood preservation using tar oils or water-borne fixed preservatives applied by pressure improved these conifers so that in many uses they outlasted the best of the durable hardwoods. Thus, and rightly so, emphasis has been placed in most temperate countries on this approach, non-leachable preservatives applied by pressure to permeable coniferous wood. It is thus reasonable to assume that wood preservation has been very successful. Or has it? It depends on the approach made, and whether the objectives of those using wood preservation have been met. The selection of treatment methods depends heavily upon the objectives and the way successful wood preservation is gauged.

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