46 Timber 



considered unsuitable. The use of the less durable timber is justified, 

 to a large extent, by the greater perfection which has been obtained 

 in the preservative treatment of timber; this has increased enor- 

 mously the durability of the timber produced by many species, and 

 also of the sap-wood of the more durable kinds. Information on 

 the preservation of timber is given in Chapter VIII. 



The main factors affecting durability are : 



(1) Climate. A climate where a short spring prevails is usually 

 productive of a small zone of the light-coloured spring wood; 

 consequently, the annual ring contains a larger proportion of summer 

 wood, which is more durable. But it is necessary for the production 

 of good timber that the species must be suited to the soil and to the 

 climate, for the thickness of the cell walls composing the timber is 

 dependent to a large extent on the energy of growth. 



(2) Situation. Isolated specimens, such as are found in hedge- 

 rows (other conditions being equal), produce more durable timber 

 than trees grown in a dense wood; in the latter case, however, if 

 thinning is rationally conducted, this difference is more than 

 counterbalanced by the cleanness of the timber produced from 

 trees grown in woods. (See p. 34, Thinning.) 



(3) Age of trees. Species noted for their durability when 

 mature, are often of little value when young. This is due to the fact 

 that in species such as oak, Scotch pine, etc., there is only a slight 

 development of heart-wood when young, and it is this portion 

 of the trunk, especially, which is durable. 



(4) Season of felling. The season of felling has a considerable 

 effect on the resistance of a timber to decay. Winter, in all cases, 

 is the most suitable time, for the wood seasons more thoroughly if 

 felled at that time, as it contains less sap and moisture than 

 in summer. An example is often seen in the case of oak felled in 

 spring for the sake of the bark, as contrasted with the same species 

 felled in winter. 



(5) Species. It is common knowledge that different species of 

 timber differ in their durability. The following tables, taken from 

 Schlich (p. 94), indicate the relative durability of the common species 



