In general, the conifers need less soil and atmospheric moisture than do 

 the broadleaf trees; for that reason conifers are the main species in the drier 

 regions and broad leaves in more humid regions. 



Proper temperature. — Some species of plants and animals can live with little 

 heat; others require much. Some kinds of trees can grow in central Alaska, 

 whereas others require the heat of the Torrid Zone. The conifers usually 

 occupy the colder regions while the hardwoods occupy the warm regions 

 but some kinds of conifers live in the southern parts of the United States 

 and some broadleaf trees live in Canada. 



Apparently there are certain critical temperatures above which and below 

 which a tree cannot survive during its growing period. Some tropical 

 varieties are killed by temperatures of 41° F. Length of growing season is 

 another factor determining the kinds of trees that grow in a region. 



Strains of the same species become adapted to different temperatures. 

 If seed collected from black oak in Oklahoma is planted in Connecticut, 

 the seedlings will rarely live through the first winter, although black oak 

 is native to Connecticut. 



Northern varieties of trees encounter a southern limit beyond which they 

 cannot survive. Although they gain some advantages in increased temper- 

 ature and often increased moisture, other things interfere with their growth. 

 The absence of a winter rest period required by some varieties may be the 

 limiting factor. Others produce infertile seed even though they may grow 

 south of the area to which they are adapted. If nothing else hinders them, 

 competition with the trees that are at home in the southern climate crowds 

 out the northern trees (fig. 2). 



Altitude and slope. — The northern varieties of trees may extend south- 

 ward by occupying the higher altitudes and the northern slopes. The 

 northern forest region typical of the New England and northern Lake 

 States, for example, extends as far south as northern Georgia by following 

 the colder temperatures of the Appalachians at an altitude of about 2,000 feet. 



The temperature on the peaks of some of the higher mountains is so low 

 trees cannot live there. The upper limit of tree growth on the mountain- 

 side is called the timber line. This line becomes established at lower levels 

 toward the poles and at higher levels in warmer climates. 



Direction of mountain ranges. — Europe has only about 100 species of trees. 

 North America has more than 800 native species. The explanation: Gener- 

 ally the mountain ranges of Europe run east and west and those of North 

 America north and south. When the glaciers moved southward, the pre- 

 glacial forests north of the European mountains were buried. Only a few 

 species were able to cross these barriers to the south and back again because 

 of the cold temperature of the high altitudes. In North America without 

 such barriers many of the northern trees moved southward fast enough to 

 keep ahead of the glaciers, if they were not already distributed to the south. 

 It would be only natural for them to regain their lost territory as the ice 

 packs receded to the north, especially since they had no higher altitude to 

 cross than was suited to their temperature needs. 



Ocean currents. — Tree growth extends as far as 70° north latitude on the 

 coast of Alaska because of the warming influence of the Japan current. On 

 the eastern side of Canada tree growth extends only to about 50° north 

 latitude because there are no warming ocean currents. The southern forest, 

 typical of the Gulf States, extends as far north as Maryland along the lower 

 lands close enough to the coast to be influenced by the tempering effect of 

 the ocean. 



