INJURY CAUSED BY FOREST FIRES 247 



Injury to the Soil. — Forest fires in their action on the soil 

 affect its physical rather than its chemical properties. Min- 

 eral soil itself is rarely changed chemically by the action of 

 forest fires. 



Its physical properties may be radically altered. From the 

 standpoint of tree growth the physical properties of the soil 

 are recognized to be of more importance than the chemical. 

 Physical properties of soils are influenced by fires through 

 decrease in the humus content. Humus is in fact the key to 

 good physical condition of the soil. 



A severe fire which kills most of the trees opens the forest 

 canopy, bums off the litter and exposes the soil to the drying 

 influences of sun and wind. The humus in the soil disappears 

 and without litter or trees to furnish litter no more htunus is 

 formed. The soil on bare burned areas consequently dete- 

 riorates in physical condition.' 



Heavy soils become dry, hard, impervious to water and 

 often crack open. Sandy soils become hotter, more porous 

 and leachy. The mineral salts left in the wood-ashes are 

 apt to be blown or washed off the surface or leached away 

 through the soil. 



A fire of the lightest type which consiunes the litter but 

 does not directly injure the standing trees causes little or no 

 injury to the soil. The loss of the accumulated litter does 

 not prove serious provided a forest canopy exists to provide 

 more without delay. Light fires burning over the same 

 ground every year consume the annual leaf-fall and thus keep 

 the ground bare of litter. Under these circumstances, when 

 the humus in the soil is once exhausted the soil deteriorates. 

 Hence repeated light fires seriously injure the soil. 



In some places forests are growing with boulders or solid 

 ledges of rock only a few inches below the surface. Here the 

 soil consists largely of organic material and may be completely 



