Where any of these conditions exist, the woodland requires immediate attention.  Otherwise, as time goes on, it deteriorates more and more, the struggle for space among the crowded and suppressed trees becomes more keen, the insects in the dying trees multiply and disease spreads from tree to tree.  Under such conditions, the soil deteriorates and the older trees begin to suffer.

[Illustration:  FIG. 141.—­Second Stage of Deterioration.  The Surface Soil of the Wooded Area Has Washed Away and the Trees Have Died.]

    The attention required for the proper care of woodland may be summed
    up under the four general heads of soil preservation, planting,
    cutting, and protection.

Improvement by soil preservation:  The soil in a wooded area can best be
    preserved and kept rich by doing two things; by retaining the
    fallen leaves on the ground and by keeping the ground well covered
    with a heavy growth of trees, shrubs and herbaceous plants.  The
    fallen leaves decompose, mix with the soil and form a dark-colored
    material known as humus.  The humus supplies the tree with a
    considerable portion of its food and helps to absorb and retain the
    moisture in the soil upon which the tree is greatly dependent.  A
    heavy growth of trees and shrubs has a similar effect by serving to
    retain the moisture in the soil.

Improvement by planting:  The planting of new trees is a necessity on
    almost any wooded area.  For even where the existing trees are in
    good condition, they cannot last forever, and provision must be made
    for others to take their place after they are gone.  The majority of
    the wooded areas in our parks and on private estates are not
    provided with a sufficient undergrowth of desirable trees to take
    the place of the older ones.  Thus, also, the open gaps must be
    planted to prevent the soil from deteriorating.

Waste lands on farms which are unsuited for farm crops often offer areas on which trees may profitably be planted.  These lands are sufficiently good in most cases to grow trees, thus affording a means of turning into value ground which would otherwise be worthless.  It has been demonstrated that the returns from such plantations at the end of fifty years will yield a six per cent investment and an extra profit of $151.97 per acre, the expense totaling at the end of fifty years, $307.03.  The value of the land is estimated at $4 per acre and the cost of the trees and planting at $7 per acre.  The species figured on here is white pine, one of the best trees to plant from a commercial standpoint.  With other trees, the returns will vary accordingly.

[Illustration:  FIG. 142.—­A Farm Woodlot.]