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.