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. [Illustration: Fig. 75. Sand-dunes, Cape May, New Jersey. _U.S. Forest Service._] [Illustration: 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. [Illustration: 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. [Illustration: 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.