AMERICAN SYLVICULTURE 



combined furnishes insufficient information relative to such climatic 

 needs. 



H. If there are found two neighboring species of the 



same genuo in a given climatic zone, it is safe to assume that these 

 two species were not mix?d originally, but that each had its dis- 

 tinct habitation and that the mixture is due to the action of man. 



I. In primitive forests, the species which harmonize are 



those which diflVr botanically. 



J. When two kinds are so alike as to be almost varieties 

 but have, nevertheless, different climatic needs, then they are, in 

 reality, true and distinct species (Douglas Fir in Colorado and 

 Oregon ) . 



K. Frost injury is always due to the death of the Plas- 

 modium killed by the direct action of the frost. The plasmodium 

 is most sensitive during the time of cell formation and of active 

 growth. The plasmodium in the inert stage, as in seeds, is actually 

 insensitive. 



L. All species become more hardy as they grow older. This 

 is simply due to the trees rising above the cold layers of tempera- 

 ture near the ground and to the greater thickness and mass of the 

 trunk, resisting rapid changes of temperature. 



M. The degree of moisture in the air required for forest 

 ^owtli is 50% relative humidity during the growing season. The 

 broad-leaved trees and the two and three needled Pines are the 

 species best adapted to regions of extreme dryness or of sudden 

 changes in atmospheric moisture. 



N. The association of trees into a forest has the effect 

 of increasing tlie relative humidity by not to exceed 10%. Hence 

 the necessity of maintaining forest in regions where the tension of 

 watery vapor is close to 50%. The partial destruction of a forest 

 may entail the death of the remainder rendering reforestation im- 

 possible unless it is started from the nearest adjoining forest. 

 Inside a forest the greater atmospheric humidity acts as beneficially 

 aa a moist ocean wind, lacking, however, the latter's violence. 



O. It is in moist, cool localities (mountains and northern 

 climate) that climate variations are the least extreme during the 

 growing season. It is here that the annual rings are equal, the 

 grain fine and regular, and the timber of the greatest commercial 

 utility. 



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