WOOD of Native Trees Compared Regarding Resistance to Decay

Wood kept constantly dry or continuously submerged in water does not decay, regardless of sapwood or species. A large proportion of the wood in use is kept so dry at all times that it lasts indefinitely. Moisture and temperature are the principal factors that affect the rate of decay; they vary greatly with the local conditions surrounding the wood in service. When exposed to conditions that favor decay, wood decays more rapidly in warm, humid areas of the United States than in cool or dry areas. High altitudes are as a rule less favorable to decay than low because the average temperatures are lower and the growing seasons for fungi are shorter.

The natural decay resistance of all common native species of wood is in the heartwood. When untreated, the sapwood of practically all species has low decay resistance and generally has short life under decay-producing conditions. The decay resistance or durability of heartwood in service is greatly influenced by differences in the character of the wood, the attacking fungus, and the conditions of exposure.  Therefore great differences in length of life may be obtained from pieces of wood that are cut from the same species or even the same tree and used under apparently similar conditions.

General comparisons of the relative decay resistance of different species must at best be estimates. They can not be exact and they may be very misleading if considered as mathematically accurate and applicable to all cases. They may be very useful, however, if considered as approximate averages only, from which specific cases may vary considerably, and as having application only where the wood is used under conditions that favor decay. The following classification of common native species is subject to the limitations just mentioned.

On the basis of service records, supplemented by general experience, the heartwood of the following species may be classed as durable even when used under conditions that favor decay: The catalpas, practically all of the cedars, chestnut, southern cypress, the junipers, black locust, red mulberry, Osage orange, redwood, black walnut, and Pacific yew. On the other hand the heartwood of aspen, basswood, cottonwood, the true firs (not Douglas fir), and the willows when used under conditions that favor decay may be classed as low in decay resistance. The heartwood of Douglas fir, red gum, western larch, chestnut oak, southern yellow pine, and tamarack may be classed as intermediate. The heartwood of dense Douglas fir, honey locust, white oak, and dense southern yellow pine may also be classed as intermediate but nearly as durable as some of the species named in the high durability group. The heartwood of the ashes, beech, the birches, the hemlocks, sugar maple, the red oaks, and the spruces may be considered on the horder line between the intermediate and non- durable groups and can not with assurance be placed wholly in either group.