A PRIMER 'OF FORESTRY. 



only 60, while another place, with an average of 70, may 

 have occasional frosts. Trees which could not live at all 

 in the second of these places, on account of the frost, 

 might flourish in the lower average warmth of the first. 

 In this way the bearing of trees toward heat and 

 cold has a great deal to do with their distribution over 

 the surface of the whole earth. Their distribution 

 within shorter distances also often depends largely 

 upon it. In the United States, for example, the Live 



Oak does not grow 

 in Maine, nor the 

 Canoe Birch in Flor- 

 ida. Even the op- 

 posite sides of the 

 same hill may be 

 covered with two 

 different species, be- 

 cause one of them 

 resists the late and 

 early frosts and the 

 iierce midday heat 

 of summer, while the other requires the coolness and 

 moisture of the northern slope. (See fig. 24.) On 

 eastern slopes, where the sun strikes early in the day, 

 frosts in the spring and fall are far more apt to kill 

 the young trees, or the blossoms and twigs of older 

 ones, than on those which face to the west and north, 

 where growth begins later in the spring, and where 

 rapid thawing, which does more harm than the freez- 

 ing itself, is less likely to take place. 



REQUIREMENTS OF TREES FOR HEAT AND MOISTURE. 



Heat and moisture act together upon trees in such a 

 way that it is sometimes hard to distinguish their 



FIG. 23. A forest of Palms in southern Florida. 



