550 BIOLOGY AND HUMAN LIFE 



The use of wood for fuel is not as great per capita as it was formerly, 

 because we find it more profitable to burn coal, gas, oil, etc., and to use 

 wood for other purposes. But every forest and every wood-lot produces 

 annually large quantities of wood that cannot be used in the making of 

 paper or of other useful things, and this may well be burned. Yet 42 per 

 cent of the wood cut in this country every year is burned — a very 

 wasteful way of managing our resources. 



393. The forest and the air. Another use of the forest is 

 found in the fact that through photosynthesis fresh supplies of 

 oxygen are throv^n into the air, replacing the carbon dioxid. 

 In addition to this, the transpiration may be considered a help 

 in that it keeps dov^^n the temperature of the plants and so of 

 the surrounding air. The shade value of trees is highly appre- 

 ciated in the summer time even by city dwellers, and the effect 

 of trees in breaking the v^ind is appreciated in the winter time, 

 especially by those living in the country. 



394. The forest and water. On a bare hillside the water 

 soaks down into the soil almost as fast as it falls, or it runs 

 off, carrying particles of earth along in its course. On a hill- 

 side covered with a growth of trees the force of the falling rain- 

 drops is broken by the leaves of the trees, from which the water 

 slides down to the ground along the twigs and larger stems. 

 The rain that strikes the mulch^ soaks through slowly ; then, in 

 the entangled soil beneath, it steadily works down to form the 

 underground streams and the springs. Snow in the forests melts 

 slowly and is gradually absorbed in the spongy bed beneath; 

 from this the water slowly escapes into the springs and under- 

 ground currents. Snow upon the bare ground runs off as fast 

 as it melts. 



Actual proof of the difference was furnished a few years ago by an 

 extensive experiment conducted by the United States Geological Survey 

 in the White Mountains. Two similar areas were selected, each cover- 

 ing about five square miles. One of the regions had been entirely cut 

 down and burned over; the other retained the virgin forest (Fig. 224). 



^The mulch forms a soft, absorbent carpet consisting largely of decaying 

 leaves and other organic matter. 



