Fig. 78. Slim Trees Bent Over by Snow; Stouter Trees Unharmed. Zurich, Switzerland. U. S. Forest Service.
Snow and ice often bring serious harm to saplings by permanently bending them over, Fig. 78, or by breaking off tops and branches.
Frost kills young plants; and sudden changes in temperature seriously affect grown timber, producing "frost checks" and "wind shakes." When there is a sudden fall in temperature, the outside layers of the tree, which are full of sap, contract more rapidly than the inner portions, with the result that the tree splits with a sudden pistol-like report, the check running radially up and down the tree. This is called a "frost check" or "star shake," Fig. 41.a, [p. 47], and such wounds rarely heal, Fig. 79.
On the other hand when the temperature rapidly rises, the outside layers of the tree expand so much more rapidly than the inside, that they separate with a dull muffled chug, the check extending in a circular direction following the annual rings. Such checks are often called "wind shakes" and "cup shakes," Fig. 41.c, [p. 47]. These injuries are found in regions where sudden changes of temperature occur, rather than in the tropics or in very cold climates.
Fig. 79. Contraction Frost Check. U. S. Forest Service.
VEGETABLE ENEMIES.
Under this head may be classed, in addition to fungi, a number of unrelated plants, including such as: moosewood and dogwood, Fig. 80, which crowd out young trees; vines, like bitter-sweet, which wind about trees and often choke them by pressure, cutting thru the bark and cambium; saprophytes, which smother the foliage of trees, of which Spanish moss, Fig. 47, [p. 201], is an example; and finally such parasites as the mistletoes, which weaken and deform the trees.
