62 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



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 

 sweeping 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 Rocky Mountains, where forests of 

 different species but similar aspect predominate); and many great 

 fires involving loss of life and much property have become historic,^ 

 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 traveller at every 

 turn ;" 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 piehistoric times it must have 

 been the principal cause.^ 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 life- 

 time 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 

 re-establish themselves. When the foliage of the conifers is 



' See Pinchot's " Primer of Forestry" (U.S. Forestry Bulletin 24), part i, 

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

 especially map on page 22. 



- Several such posters are reproduced in colours in American Forestry 

 for November 1913. 



" See papers by Dr Robert Bell in Forest Leaves for October 18S9, and the 

 Scottish Geographical Magazine for June 1897, and Bulletins in 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 mucli valuable information 

 ■on other subjects than fire. 



