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, 



