FORESTRY AND CONSERVATION 329 



In a vigorously growing tree the solid wood substance itself 

 will contain half its weight in the form of water chemically com- 

 bined, and the tree, in addition, will contain from 40 to 65 per cent 

 and more of its dry weight in water mechanically or "hygroscopic- 

 ally" held. This last when the tree is cut very largely evaporates, 

 yet well-seasoned wood still contains 10 to 12 per cent of such water. 

 The weight of a green tree, a pine, for instance, is made up, in round 

 numbers, of about 30 per cent of carbon and 70 per cent of water, 

 either chemically or hygroscopically held, while a birch contains a 

 still larger percentage of water. 



The largest part of the water which passes through the tree is 

 transpired i. e., given off to the air in vapor. The amounts thus 

 transpired during the season vary greatly with the species of tree, 

 its age, the amount of foliage at work, the amount of light at its dis- 

 posal, the climatic conditions (rain, temperature, winds, relative 

 humidity), and the season. These amounts are, however, very large 

 when compared with the quantity retained ; so that while an acre of 

 forest may store in its trees, say, 1,000 pounds of carbon, 15 to 20 

 pounds of mineral substances, and 5,000 pounds of water in a year, 

 it will have transpired taken up from the soil and returned to the 

 air from 500,000 to 1,500,000 pounds of water (one-quarter to 

 one-half as much as agricultural crops). 



Mineral substances are taken up only in very small quantities, 

 and these are mostly the commoner sorts, such as lime, potash, mag- 

 nesia, and nitrogen. These are carried in solution to the leaves, 

 where they are used (as perhaps also on their passage through the 

 tree), with a part of the water, in food preparation. The main 

 part of the mineral substances taken up remains, however, as the 

 water transpires, in the leaves and young twigs, and is returned to 

 the soil when the leaves are shed or when the tree is cut and the 

 brush left to decompose and make humus. 



Hence the improvement of the fertility of the soil by wood 

 crops is explained, the minerals being returned in more soluble 

 form to the soil ; as also the fact that wood crops do not exhaust the 

 soil of its minerals, provided the leaves and litter are allowed to re- 

 main on the ground. For this reason there is no necessity of alter- 

 nating wood crops, as far as their mineral needs are concerned ; the 

 same kind of trees can be grown on the same soil continuously, 

 provided the soil is not allowed to deteriorate from other causes. 



As the foliage can perform its work of food assimilation only 

 when sufficient water is at its disposal, the amount of growth is also 

 dependent not only on the presence of sufficient sources of supply, 

 but also on the opportunity had by the roots to utilize the supply, 

 and this opportunity is dependent upon the condition of the soil. 

 If the soil is compact, so that the rain water can not penetrate 

 readily, and runs off superficially, or if it is of coarse grain and so 

 deep that the water rapidly sinks out of reach of the roots and can* 

 not be drawn up by capillary action, the water supply is of no avail 

 to the plants; but if the soil is porous and moderately deep (depth 

 being the distance from the surface to the impenetrable subsoil, 



