is, moreover, covered with a heavy undergrowth and thus behaves like
    a sponge, absorbing the water that falls upon it and then permitting
    it to ooze out gradually to the valleys and rivers below.  A forest
    soil will retain one-half of its own quantity of water; i.e., for
    every foot in depth of soil there can be six inches of water and,
    when thus saturated, the soil will act as a vast, underground
    reservoir from which the springs and streams are supplied (Fig.
    125).  Cut the forest down and the land becomes such a desert as is
    shown in Fig. 126.  The soil, leaves, branches and fallen trees dry
    to dust, are carried off by the wind and, with the fall of rain, the
    soil begins to wash away and gullies, such as are shown in Fig. 127,
    are formed.  Streams generally have their origins in mountain slopes
    and there, too, the forests, impeding the sudden run off of the
    water which is not immediately absorbed, prevent soil erosion.

[Illustration:  FIG. 128.—­Flood in Pittsburgh, Pa.]

Where the soil is allowed to wash off, frequent floods are inevitable.  Rain which falls on bare slopes is not caught by the crowns of trees nor held by the forest floor.  It does not sink into the ground as readily as in the forest.  The result is that a great deal of water reaches the streams in a short time and thus hastens floods.  At other periods the streams are low because the water which would have fed them for months has run off in a few days.  The farms are the first to suffer from the drouths that follow and, during the period of floods, whole cities are often inundated.  Fig. 128 shows such a scene.  The history of Forestry is full of horrible incidents of the loss of life and property from floods which are directly traceable to the destruction of the local forests and, on the other hand, there are many cases on record where flood conditions have been entirely obviated by the planting of forests.  France and Germany have suffered from inundations resulting from forest devastation and, more than a hundred years ago, both of these countries took steps to reforest their mountain slopes, and thereby to prevent many horrible disasters.

[Illustration:  FIG. 129.—­Planting a Forest with Seedling Trees on the Nebraska National Forest.  The man on the right is placing the tree in a slit just made with the spade.  The man on the left is shoveling the dry sand from the surface before making the slit for the tree.]

[Illustration:  FIG. 130.—­Diagrammatic Illustration of a Selection Forest.]

How forests are established:  New forests may be started from seed or
    from shoots, or suckers.  If from seed, the process may be carried on
    in one of three ways: 

    First, by sowing the seed directly on the land.