38 PRACTICAL FORESTRY IN 



but air solidified by sunshine. A tree's immense and com- 

 plicated foliage system is the laboratory with which it effects 

 this transformation. 



Since air exists everywhere and the chemical quality of the 

 soil is comparatively unimportant, the requirements of dif- 

 ferent species for light, heat and moisture are what mainly 

 determine their distribution and habits of growth. And since 

 heat and moisture are largely climatic factors and fairly uni- 

 form in given localities, it follows that the demand of a 

 species upon light may practically fix its habits and possibili- 

 ties in those localities. The very great variance of species in 

 light requirement accounts to a large extent for the composi- 

 tion of most primeval forests. It is of peculiar importance 

 in the management of forests by man because he cannot con- 

 trol it as he may be able to control some of the other agencies 

 which affected the primeval forest, such as fire or seed supply. 



SELECTION FORESTS 



It would be unprofitable to discuss here all the many meth- 

 ods of forest management which have proved to be best, tech- 

 nically, for given species and combinations of species. Where 

 market and transportation facilities are highly favorable, as 

 in Europe, the timber owner can adopt the method which will 

 bring the best results, but here he has no such choice. He 

 can but bear in mind certain fundamental principles, uni- 

 formly applicable to large areas for considerable periods of 

 time. Roughly, however, our Western forests can be classi- 

 fied by their adaptability to the two directly opposite systems, 

 known as clean cutting and selection cutting, of which almost 

 all methods are modifications. 



A selection forest is one in which all ages of trees exist, 

 from seedling to maturity. It is the natural growth of species 

 which are tolerant of shade. In a natural state, undisturbed 

 by cutting, it maintains much the same aspect continuously, 

 for as the oldest trees die, their place is taken by younger 

 ones. Obviously such a forest must be composed of species, 



