PROTRCTION OF FORESTS FROM FIRE 



517 



Injury to the Soil 



A surface tire burn^ the dry leaves, 

 and usually the humus which lies on the 

 surface of the ground. If the trees are 

 all killed by the fire, the crown cover, as 

 well as the layer of litter and humus, 

 is destroyed, and injury to the soil fol- 

 lows this exposure to the wind and sun. 

 If the canopy is not seriously inter- 

 rupted by the fire and only the surface 

 litter and humus are burned, the extent 

 of the soil injury from one burning is 

 not serious. A very light surface fire 

 that merely burns otT the dry litter 

 formed by one or two years' fall of 

 leaves has little influence on the soil : 

 and probably no single fire, even if it 

 burned the entire humus and layer of 

 litter, would so injure the soil as se- 

 riously to afifect the growth of well- 

 established trees. Normally in every 

 forest a certain amount of humus is 

 mixed with the mineral soil. This is 

 of value, both physically and chemically. 

 If a forest is burned over repeatedly, 

 however, the humus in mixture grad- 

 ually disappears, and since the leaves 

 which fall are destroyed, and no new 

 liumus is formed, the soil is injured. 

 While the soil loses its supply of nitro- 

 gen and the physical benefits of humus, 

 the mineral ashes are not lost except as 

 they are subsequently leached away. 

 Nevertheless, repeated fires are very in- 

 jurious to the forest. 



Besides the direct injury to the soil 

 through changes in its chemical content 

 and physical quality, fires do further 

 damage through opening the way to soil 

 erosion. A leaf litter reinforces the 

 forest canopy in ])rotecting the soil 

 against the impact of falling rain, and 

 the network of roots which fill the 

 ground hold 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 in- 

 crease 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 number 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 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 some- 

 times possible to prevent loss by cutting 

 directly after the fire. Often, however, 

 it is not practicable to make a cutting 

 in a given part of a forest just when 

 desired. 



When some of the trees in an imma- 

 ture stand are killed or injured there 

 is always a loss. If the stand is cut, 

 there is a loss through cutting trees 

 which are in full productive growth. If 

 the stand is allowed to grow, the final 

 yield is reduced nearly in proportion to 

 the reduction in the number of trees 

 killed in the dominant or leading class. 



An owner is often confronted with 

 the problem of dealing with an imma- 

 tm"e stand in which a part of the trees — 

 say thirty to sixty per cent — are killed 

 or injured by fire. If the remaining 

 trees are sound and thrifty, the best 

 plan is usually to cut out the dead and 

 damaged individuals, utilizing such as 

 are marketable, and permit the re- 

 mainder to mature, provided enough 

 can be realized to cover substantially 

 the cost of the work. 



Effect on Reprodnction 



Reproduction in the forests of this 

 cotmtry has been more influenced by 

 fire than by any other one factor. The 

 present composition, form, density, and 

 vield of a great many stands are due to 

 the influence of fires on reproduction. 



Repeated fires prevent reproduction 

 b)- destroying the seed and killing the 

 seedlings. This is well illustrated in 



