Forest Preservation 75 



will admit the light required to stimulate the growth of 

 young trees, and the crowns of the growing timber will 

 furnish the shade needed for the germination of seed and 

 to prevent grass getting possession. All trees do not stand 

 shade equally well, and if the growth of light-loving trees 

 is to be encouraged, some thinning may be necessary. The 

 species which are more " tolerant" of shade often gain in 

 the struggle against those which require more light. In 

 preserving the proper balance of species the help of the 

 forester may be needed, as pure stands of a single species 

 make conditions favorable to any disease or pest to which 

 that species is subject. At times planting may be neces- 

 sary, even here, but the forest itself will usually furnish the 

 young trees. 



In Canada, however, owing to repeated fires, large tracts 

 have been practically stripped of trees which must be re- 

 placed. If the burning has been severe, the layer of mould 

 on the surface may be burned away, leaving a poor seed-bed ; 

 or if grass gets possession of the ground before trees get a 

 start, it makes the re-foresting by nature very unlikely. 

 The dry grass is likely to be burned off every few years, and 

 any seedlings will be killed. If, however, grass does not 

 take possession, the ground will likely be occupied by 

 poplar and birch. These trees, by reason of their rapid 

 growth, the spreading of their seed by the wind and re- 

 production from the root, will soon cover the ground to 

 the exclusion of all others. The aspen will spread even in 

 ground occupied by grass, and on the prairies there are 

 many groves now containing trees large enough for fence 



