CONIFEROUS FORESTS 34 1 



form of evergreen tree with the same amount of wood and foliage which 

 would be less liable to injury from snow and ice than the spruces and 

 balsams. The tamarack has an additional safeguard in that it loses its 

 leaves in winter; and at the northern limit of the forests it is said to 

 grow comparatively tall and straight while the spruces around it are 

 much stunted. 



Burned areas, in which all the trees have been killed by fires sweep- 

 ing through their crowns, are and always have been, from all accounts, 

 common throughout the spruce region (not only in the East, but also 

 in the Eocky Mountains, where forests of different species but similar 

 aspect predominate) ; and many great fires, involving loss of life and 

 much property, have become historic. 3 In northern Michigan and 

 doubtless in many other places where spindle-shaped conifers abound 

 posters warning against the dangers of allowing fire to spread greet the 

 traveler at every turn; 4 and some of the western railroads print similar 

 advice in their time-tables. 



Although at the present time the origin of most of the northern 

 forest fires can be ascribed to human agencies, lightning is known to 

 cause a considerable proportion of them (estimated by Plummer at 15 

 per cent.), and in prehistoric times it must have been the principal 

 cause. 5 From all the evidence available it would seem that the normal 

 frequency of fire at any one spot in the boreal conifer forests is about 

 once in the average lifetime of a spruce tree, which may be between 

 50 and 75 years. The average extent of a single fire must be several 

 square miles. 



In the untold ages that fire has been a factor in the life-history of 

 these forests there has developed a class of plants known as fireweeds, 

 consisting of a score or more of herbs, shrubs, and short-lived deciduous 

 trees, such as birch and aspen, which quickly take possession of burned 

 areas and flourish until the dominant, but more slowly growing conifers 

 have time to reestablish themselves. When the foliage of the conifers is 

 consumed by fire the potash and other mineral nutrients stored up in 

 several years' growth of evergreen leaves is returned to the soil in readily 

 available form, and this must be a significant factor in the rapid growth 

 of the fireweeds. Quite a lengthy chapter could be written about this 



3 See Pinchot's "Primer of Forestry" (TJ. S. Forestry Bulletin 24), Part 1, 

 pp. 79-83, 1897; also U. S. Forestry Bulletin 117, by F. G. Plummer, 1912, espe- 

 cially map on page 22. 



4 Several such posters are reproduced in colors in American Forestry for 

 November, 1913. 



5 See papers by Dr. Robert Bell in Forest Leaves for October, 1889, and the 

 Scottish Geographical Magazine for June, 1897, and Bulletins 111 and 117 of the 

 U. S. Forest Service, by F. G. Plummer, 1912. The second of Dr. Bell's papers, 

 which is on the forests of Canada, contains much valuable information on other 

 subjects than fire. 



