514 



AMERICAN FORESTRY 



greater rate, particularly if the starting 

 of new fires by burning cinders is taken 

 into consideration. Even in extreme 

 cases, however, it is questionable 

 whether crown fires burn at a rate of 

 more than from six to ten miles an hour. 



The behavior of a crown fire depends 

 on the character of the crowns. Crown 

 fires are mainly confined to coniferous 

 forests, for the leaves of hardwoods are 

 not easily ignited. 



They may, however, run through for- 

 ests of mixed hardwoods and conifers, 

 and in such cases the heat generated is 

 so great that the hardwood leaves are 

 scorched or killed. The velocity of the 

 fire depends, further, on the density of 

 the stand, the thickness of the crowns, 

 and the force and steadiness of the 

 wind. ( Hher influences affect the se- 

 verity of crown fires in much the same 

 way as they affect that of surface fires. 



DAMAGE BY FIRES 



The damage done by forest fires may 

 be discussed under the following heads : 

 ( i ) Death of standing trees. 

 ( 2 ) Injury to trees that are killed. 

 ( 3 ) Injury to the soil. 



(4) Reduction of the rate of growth 

 of the stand. 



(5) Effect of reproduction. 



Death of Trees 



Crown fires kill outright most of the 

 trees in their paths. In a severe crown 

 fire the foliage of coniferous trees is 

 completely consumed. Hardwood trees 

 in mixture are generally so badly 

 scorched that the buds, leaves, and liv- 

 ing tissues in other finer parts of the 

 tree are killed, if not consumed, by the 

 heat. Sometimes, however, where the 

 fire burns somewhat irregularly as, 

 for example, where there are a good 

 many hardwoods in mixture or the fire is 

 broken by irregularities in topography- 

 single trees or groups of trees often 

 escape injury. 



Ground fires, also, usually kill all 

 trees in their way, for although they 

 burn very slowly, they generate a grea't 

 volume of heat and kill the living tis- 



sues of the roots. Sometimes the in- 

 jury is not apparent above ground, at 

 all, but the trees die and after a time 

 are blown over, because the roots have 

 been killed and weakened. 



Surface fires kill seedlings and young 

 trees with tender bark, but in a great 

 many cases do not kill outright the larger 

 trees. Nevertheless, a very severe sur- 

 face fire may kill everything in its path, 

 and, not uncommonly, hardwood forests 

 are entirely destroyed by fires which do 

 not at any time assume the character 

 and proportions of crown fires. 



Some species have much greater 

 power of resisting surface fire than 

 have others. This is usually due to 

 the character and thickness of the bark. 

 Trees with delicate, thin bark are killed 

 much more readily than those with 

 thick, corky bark. Young trees are 

 killed more readilv than old ones, be- 

 cause the bark is thin and there has not 

 been developed the layer of cork, which 

 increases in amount with age. Accord- 

 ingly, some trees which are very re- 

 sistant to fire when mature are exceed- 

 ingly sensitive when young. Good ex- 

 amples are the eastern and western 

 white pines, the red pine, the western 

 larch, and Douglas fir. The cork in 

 the bark acts as a nonconductor and 

 protects the living tissues from over- 

 heating. 



Some species exude from the bark a 

 great deal of resin, which catches fire 

 and increases the intensity of the heat. 

 A good example is lodgepole pine, which 

 often exudes resin over a considerable 

 portion of the trunk and increases the 

 damage by fire. Other trees have soft, 

 Paky bark, which catches fire readily. 

 Like the resinous trees, these are killed 

 at the point burned by the heat gen- 

 erated in this way. Shallow-rooted trees 

 may be killed by surface fires when the 

 heat of the burning humus is great 

 enough to injure the insufficiently cov- 

 ered roots. 



The living parts of a tree are more 

 sensitive to intense heat at some periods 

 of the year than at others. The most 

 sensitive period is during the early part 

 of the growing season, when active cell 

 division is taking place and new cells 



