When most people hear the word “durability” in relation to wood, they immediately think of its ability to withstand dents and scrapes. However, in this context it specifically refers to a wood’s ability to resist elemental and natural forces of decay. (The former notion of durability equating to physical toughness would be better explored through Janka hardness and Modulus of Rupturevalues.)
Degradation can occur from fungus (caused by cycles of rain/moisture), or from termites or other boring/destructive insects. An overall chart defining the terms used to describe a wood’s durability in direct ground contact:
This durability assessment is only based on the tree’s heartwood, and not its sapwood—as only the heartwood, due to its extractives, has any appreciable degree of durability; in nearly all instances, sapwood should be considered perishable.
Some genera of Bamboo are only expected to last 6 months to 3 years in direct ground contact. On the other end of the spectrum is wood such as Teak, which is well-known for its durability, and is frequently used in boatbuilding and other outdoor applications.
In addition to the length of time the wood can physically maintain its structural integrity, there’s also the matter of a wood’s weathering characteristics. Weathering can’t be as clearly expressed in a single number or measurement, but overall, woods with good weathering characteristics exhibit limited photo-degradation (caused by UV rays in sunlight), as well as above-average resistance to contraction and expansion, warping, and surface checking due to seasonal changes in temperature and humidity.
Because of this vague definition, only woods that have notably good (or notably poor) weathering characteristics will be noted. (Again, Teak is noted for both its excellent durability and its superb weathering characteristics.)