FOREST PROTECTION 97 



or less deeply into the wood, and may even penetrate to the 

 centre of the tree. Each growing season the trees make an 

 effort to heal the cracks ; but, as a rule, they are reopened 

 during the succeeding winter. " Frost-ribs " are the result 

 of the repeated opening and closing. This injury to the 

 timber, producing what is known as frost-shake, occurs most 

 commonly with Oak and Elm, but is met with in other 

 species, especially in frosty hollows. 



Frost-lifting affects small plants on loose soil. It is most 

 common in early spring, but may occur at any time when a 

 very marked difference between the temperatures of day and 

 night brings frost and thaw alternately. Owing to crystallisa- 

 tion of the water present, the soil is heaved up, and seedlings 

 with shallow roots are raised with it ; the soil regains its 

 former position when mild weather sets in, but the small 

 plants which have been torn away from their original hold 

 upon the soil are not able to do so, and fall over on the 

 surface of the soil. 



To prevent frost-lifting, avoid weeding or otherwise stirring 

 the surface soil of nurseries in late summer or autumn, and 

 provide a covering of leaves, needles, moss, or other litter 

 over the beds or between the plants. 



Bark-scorching occurs on all species of trees which have a 

 smooth bark. Beech suffers most frequently, but Ash, Maple, 

 Sycamore, and Spruce may also be affected, when, in middle 

 life or as older trees, they are suddenly exposed by the 

 clearance of surrounding forest. Injury is generally confined 

 to the west side of the trees, for the hot rays of the afternoon 

 sun strike horizontally enough to get below the crowns. As a 

 result, the bark becomes excessively heated, dies, cracks, and 

 falls off on the side affected ; underneath the sunburned 

 cortex, the wood is then apt to be destroyed by a form of 

 dry-rot. Trees with low-branched crowns do not suffer 

 harm. 



