PROTECTION OF FORESTS FROM FIRE 241 



formed, the soil is injured. While the soil loses its sup- 

 ply of nitrogen and the physical benefits of humus, the 

 mineral ashes are not lost, except as they are subse- 

 quently leached away. Nevertheless, repeated fires are 

 very injurious to the forest. 



Besides the direct injury to the soil through changes 

 in its chemical content and physical quality, fires do fur- 

 ther damage through opening the way to soil erosion. 

 A leaf litter reinforces the forest canopy in protecting the 

 soil against the impact of falling rain, and the network 

 of roots which fills the ground holds the soil in place. 

 The greater the humus content of the soil, the more 

 absorptive the soil is. Fires leave the soil in condition 

 to be easily borne away by running water, and increase 

 the amount of water which runs over the surface instead 

 of sinking in. If the slopes are steep and the soil easily 

 borne away, erosion is sure to follow fires. In mountain 

 country, if the rainfall is heavy, thin soils may be so 

 badly washed as to be no longer capable of supporting 

 forest growth. 



Reduction of Density. — Most fires kill a certain num- 

 ber of trees, or injure them so that they either die or 

 deteriorate in value before the forest can be cut. This is 

 particularly the case with immature forests. The result 

 is a reduction in the number of trees which will come to 

 maturity, and hence a reduction of the total increment 

 and the final yield. 



If a stand is mature and a part of the trees are injured 

 or killed, it is sometimes possible to prevent loss by cut- 



