The natural durability of wood, or its resistance to decay and insect attack,
is related to species and anatomical characteristics. In general, the sapwood
of all species has little resistance to deterioration and fails rapidly in
adverse environments. When heartwood is considered, natural durability depends on species. Since the time of the Phoenicians, carpenters have
known that the heartwood of some species exhibits greater durability in
ground or marine environments. As discussed earlier, heartwood forms as
the living sapwood cells gradually become inactive. In some species,
sugars and other extraneous materials present in the cells are converted to
highly toxic extractives that are deposited in the wood cell wall. In addition,
some heartwoods contain internal crystalline deposits that inhibit
attack by marine borers and insects. There are many species of wood in
the world that provide durable heartwood, but few are found in North
America. Baldcypress (old growth), cedars, and redwood are three North
American commercial species that are recognized as naturally durable;
however, durability varies within a tree and among species (Table 3-4).
Because of this variability, it is unreliable to depend on natural durability
for protection in structural applications, although many electric utilities
continue to use untreated cedar poles installed in the 1930’s. To ensure
uniform performance, wood used in bridge applications is treated with
wood preservatives that protect the structure from decay and deterioration
for many years (Chapter 4).