196 Forestry Quarterly. 



terioration of the site. The indirect effect is long continued and 

 difficult to estimate. 



The several factors of varying importance influencing the re- 

 sistance of sound trees of given age to fire are : 



1. Thickness of bark. 



2. Water content of tissues. 



3. Season at which the fire takes place. 



4. Character of the root system. 



5. Origin, seedling or sprout. 



6. Vigor. 



7. Size, 



8. Character of wood. 



Thickness of Bark : This is the most important of all fac- 

 tors influencing resistance of a tree or species to fire. Trees 

 with a thick, heavy or non-combustible bark have their living 

 tissue protected, which enables them to withstand severe fires, 

 while others having a thin bark, have their living tissue killed by 

 the same fire. Thick bark protects a tree in two ways: it keeps 

 the fire itself from the living tissue and being a poor heat conduc- 

 tor prevents dessication that follows high temperature, which is 

 as likely to cause death as direct burning. Since bark thickness 

 increases with age, fire resistance increases likewise. A hard, 

 non-combustible bark acts as a thick one although not as efficiently. 



Water Content of Tissues: Certain species normally have 

 more water in their tissues than others and the amount 

 varies with the seasons, being highest in the spring. The bark of 

 a tree with much moisture in its living tissue is easily scalded and 

 blistered by fire, but moist wood burns with difficulty. Thus in 

 swamps where the moisture content of vegetation is high, fires are 

 often fatal, but destroy little of the timber killed. 



Season at Which the Fire Takes Place: Since the wa- 

 ter content of trees varies with the seasons, fires of equal 

 intensity are more harmful at some seasons than at others. In 

 spring when the cambium is rapidly dividing, the newly formed 

 tissue is tender and easily succumbs. Later when hardened it 

 withstands much greater heat. A mid or late summer fire does 

 not allow time for sprouts or healing tissue to become hard enough 

 to resist frost. The season of a fire is more important to young 

 trees where injury is to the living portions. 



