16 FOREST TREE DISEASES. 



Heavy storms may injure a tree in various ways. 

 Tops or limbs or the trunk are broken off. Sometimes 

 part of the roots of a tree which is poorly anchored in 

 the soil may be torn off and the tree completely up- 

 rooted and blown over. In falling such trees fre- 

 quently injure their neighbors. 



When a cloudburst displaces the ordinary bed of a 

 mountain stream, any trees caught in the torrent, even 

 if they withstand the pressure of the water, suffer more 

 or less from the action of rolling bowlders, which rub 

 off the bark, and later from the changes brought about 

 in their life conditions by the thick layer of debris left 

 after the stream has receded. 



Other causes of disease are smelter fumes and smoke, 

 frost, sunscald, drought, and excess of water in the soil. 

 In the neighborhood of smelters and other large indus- 

 trial plants which burn great quantities of coal the for- 

 ests, particularly in the direction of the prevailing 

 winds, are often very seriously injured by the poison- 

 ous gases escaping either from the ore or from the coal 

 used. 



Winter frost sometimes causes long internal cracks 

 in older forest trees, often corresponding to rather con- 

 spicuous perpendicular ridges on the outside of the 

 trunk. Late frosts are likely to kill young shoots (see, 

 however, white fir, p. 37). In long, dry winters the 

 foliage of coniferous trees is often stimulated on 

 warmer days to transpiration of water, which the root 

 system is not able to restore from the frost-bound soil. 

 The needles then die and turn red. 



