The natural durability of wood, as interpreted in this text, signifies its ability to resist the attacks of foreign organisms, i.e., fungi, insects, and marine borers. Although no wood is entirely immune to the attack of such organisms, a number possess superior resistance.

Chapter 13 Natural durability of wood 木材的天然耐久性

I. Durability of sapwood and heartwood

II. Variations in durability within and between trees

III. Effect of density and rate of growth on durability

IV. Silvicultural and genetic effects on durability

V. Effect of climate on durability

VI. Effect of season of cutting on durability

VII. Relative durability of wood from living and from dead trees

VIII. Effect of various treatments on durability

Topical highlight

I. Durability of sapwood and heartwood

Generally, the durability of heartwood is much higher than that of sapwood of the same species.

The greater durability of heartwood, in comparison with sapwood of the same species, is attributable largely to the presence in the former of a wide variety of toxic extraneous materials, such as essential oils, tannins, and phenolic substances.

The sapwood of all wood species, even those in which the heartwood is highly durable, is susceptible to deterioration by biological agents, because it lacks extractives in sufficient quantity or toxicity to inhibit the growth of microorganisms. In fact, the presence of reserve foods in the parenchyma cells of sapwood may increase its susceptibility to decay and particularly to bacteria and fungal staining.

II. Variations in durability within and between trees

1. Variations in durability of heartwood within trees

The decay resistance of heartwood generally tends to be at a maximum in the outer heartwood and decreases inward to the pith. This pattern is particularly marked in the butt log of old tree in naturally durable softwoods and in hardwoods of both temperate and tropical regions with heartwood that is resistant to deterioration.

The natural durability of the wood of individual trees of the same species may vary within wide limits. Such variability is thought to be largely genetically controlled, although tree vigor and the fertility of the soil on which the trees are grown are known to influence fungal resistance of the heartwood. In addition, size and age of trees also have some effect, as was indicated in the previous section.

2. Variations in durability between individual trees

1. Effect of density

Nor is there any conclusive evidence that variations in density within the species or a given tree have much effect on wood durability, unless the higher durability is correlated with the greater accumulation of toxic substances.

III. Effect of density and rate of growth on durability

All attempts to correlate growth rate with durability have failed, since changes in the rate of growth entail （留下） variation in the amount of early wood and late wood in a growth increment. These tissues are decidedly different in density; hence variations in their amounts result in variation in density of the entire piece. But it has already been shown that density itself is no measure of the durability of wood.

2. Effect of growth rate

IV Silvicultural and genetic effects on durability

There is no evidence that the geographic range of a tree species can be correlated with durability of that species, but it may be assumed that they are due to genetic variations.

The effect climate may have on durability is determined largely by the amount of precipitation and by the temperatures and the relative humidities that prevail in region. In a locality where the moisture is too scanty to promote the growth of fungi and/or the temperatures are unfavorable for their growth, a wood may be durable; elsewhere, where the conditions are different, the reverse may hold true.

V. Effect of climate on durability

The statement is sometimes made that wood from trees felled during the winter months is more durable than that from trees cut at other seasons of the year. Winter felling does offer certain advantages; e.g., the climatic conditions prevailing during the colder months are unfavorable for the activities of fungi and insects, thus making less critical the problem of caring for the lumber before it is used.

VI. Effect of season of cutting on durability

Contrary to a rather widely held opinion, wood from a dead tree, if sound, is fully as durable as that from a living tree. But in general, timber from dead trees requires more careful inspection than that from living trees because the likelihood of the presence of decay and insects is greater in the first instance.

VII. Relative durability of wood from living and from dead trees

Considerable evidence exists that the natural durability of a species of wood may be seriously affected by the treatment to which this wood is subjected. Methods of drying, prolonged soaking in water, and the application of heat during wood processing may have reverse effects on the natural durability of wood.

Ⅷ. Effect of various treatments on durability

Reflection and practice:

1. For most tree species, the natural durability of heartwood is better than that of sapwood. Why?

2. Generally where is there the highest natural durability in a log from mature tree?

3. Why may climate has effects on natural durability of wood?

4. Do you think the saying “wood from a dead tree is less durable” true or false? Why?

5. Does the season of tree cutting have effect on natural durability of wood?