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PRINCIPLES OF AMERICAN FORESTRY. 



as near as possible to one central shaft, or the longer 

 branches shortened so that they \\ill not exert too great 

 a leverage, the losses may be reduced to a minimum. Trees 

 having brittle wood or weak crotches, as the Soft Maple, 

 are much more liable to this injury than those with tough 

 wood, as the Willows, Oaks, and Elms, and need more 

 pruning on this account. Evergreens are likely to be 

 broken by heavy snows that freeze on the leaves. This 



Fig. 50. — ^Trees heavily loaded with ice after a sleet-storm. 



may be prevented on lawn and shade trees by shaking 

 the snow off from them before it freezes. 



Frost-Cracks are a rather infrequent injury caused 

 by the cracking of trees from centre to outside, due to un- 

 even contraction in very cold weather. They are generally 

 accompanied by a loud report. Such cracks are often 

 eight or ten feet long, and occasionally longer. They 

 generally close up again when the wood thaws out, and 

 during the following summer grow over only to burst 

 open again the next winter. This alternate bursting 



