142 ECONOMICS OF FORESTRY. 



dwarfed, the esquimaux of tree growth, the treeless 

 tundra is reached, where ice and snow abound all 

 the year, the home of winter. 



Similar changes in type may be traced by ascend- 

 ing some high mountain in tropic or subtropic 

 regions. We may begin our journey under the 

 palms. As we ascend 2000 or 3000 feet, we pass 

 through the varied evergreen, broad-leaved forest, 

 into the deciduous-leaved forest, not dissimilar to 

 that of our middle latitudes. At an altitude of 

 8000 feet we enter the dominion of spruces and 

 firs. At 10,000 to 15,000 feet the forest opens, 

 the trees stand in groups, are dwarfed and tousled 

 like their northern counterparts, hugging each 

 other and the ground for protection against the 

 winter storms ; finally, the timber line is reached, 

 where killing frosts occur every month in the 

 year, and no persistent life can exist. 



Again, variation in the relative humidity, in con- 

 nection with temperature conditions, brings about 

 changes in forest types ; from the humid seashore 

 to the drouthy interior of continents, we find differ- 

 ent species adapted to the many possible combina- 

 tions of temperature, humidity, and winds, which 

 together influence that most important physiologi- 

 cal function needful in the life of the tree, tran- 

 spiration. Dry climates, like cold climates, tend 

 to diminish growth, and reduce the number of 

 species composing the forest. 



Within the geographical range of the species 



