METEOROLOGICAL FORCES.
Fig. 74. Effect of Wind, July, 1902, Cass County, Minnesota. U. S. Forest Service.
Wind. "Windfalls" are not an uncommon sight in any forest. Frequently only small areas are blown down, one large tree upsetting a few others, or again a vast region is destroyed by great storms, Fig. 74. An area of many square miles in Florida covered with long-leaf pine was thus destroyed several years ago. The "slash" thus formed, when well dried, is particularly liable to catch fire and burn furiously. Windfalls are especially common among shallow-rooted trees, as hemlock, basswood and spruce, on sandy soil and on shallow soil underlaid with solid stone, especially where open spaces give the wind free sweep. It follows that an unbroken forest is a great protection to itself. The only precautions against wind therefore, that can be taken by the forester, are to keep the forest unbroken by selecting only the larger trees for felling or to cut down a given tract by beginning at the side opposite the direction of prevailing storms and working toward them.
In sandy regions, the wind does immense harm by blowing the sand to and fro in constantly shifting dunes, Figs. 75 and 76. These dunes occupy long stretches of the Atlantic coast and the shore of Lake Michigan. Such dunes have been estimated to cover 20,000 square miles of Europe. Along the Bay of Biscay in France, the sand dunes formerly drifted in ridges along the shore, damming up the streams and converting what was once a forest into a pestilential marsh. This region has been reclaimed at great expense by building fences along the shore to break the wind and thus keep the moving sand within limits. In this way a million acres of productive forest have been obtained.
Fig. 75. Sand-dunes, Cape May, New Jersey. U.S. Forest Service.
Fig. 76. Sand-dune. Oregon. U.S. Forest Service.
On the other hand winds are beneficial to the forest in scattering seeds, weeding out weak trees, and developing strength in tree trunks.
Drouth both injures the foliage of trees and causes defects in the grain of wood, the latter appearing as "false rings." These arise from the effort of the tree to resume growth when the water supply is restored. See [p. 19].
Water. Certain trees have become accustomed to living in much water, as cedar and cypress have in swamps, and certain trees have become accustomed to periodical floods, but other trees are killed by much water. So when lumbermen make a pond which overflows forest land, the trees soon die, Fig. 77.
Fig. 77. Effect of Flooding. First Connecticut Lake, New Hampshire. U. S. Forest Service.
Lightning frequently blasts single trees, and in dry seasons may set fire to forests. This is a much more important factor in the west than in the east,—in the Rockies, for instance, where there are electrical storms without rain.
Fires will be considered later under man's relation to the forest.
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.