VALUE OF FORESTS. The top- soil of forests is porous and loose. The mixture of leaves and loose top-soil that forms under the trees is known as "humus." The humus receives and pro tects young seeds and is also valuable because it assists in equal izing the How of streams.
Rain-water rolls quickly from sun-baked or otherwise com pacted soil, but humus permits the raindrops to pass through into the more or less broken and comparatively loose and porous soil below and then obstructs the free evaporation of moisture from this soil. It is not known that forests influence rainfall, but their value in regulating stream-flow is beyond estimate.' Forests Reduce or Prevent Erosion.
The humus protects the surface, and the roots contribute to the resistance offered by the soil below. Water flows with erosive force over unprotected and hardened surfaces. Quantities of soil are carried from higher elevations and deposited on lands below. Such results may be far reaching. Districts such as parts of India, China, Palestine, and Spain, that have supported considerable popula tions in the past, have been changed in this way and are now little else than deserts.
The possibilities in this direction are described further as follows (Van Hisel): "Not only so, but after the rivers are partly filled with silt, at times of flood they overflow their banks and often cover with coarse debris large areas of arable land. When this process of erosion has continued for a sufficient length of time after the removal of the forests, the steep mountains are left with nearly bare rock and little soil. When this stage of the process has been reached the violence of the floods is then further greatly increased. The rain falling upon the bare rocks is car ried down to the streams below as from the roof of a house, and unites in torrential floods. It is after this condition of affairs has come about as a result of a removal of the forests that the enormous flood losses occur to railroads, cities, and other structures of man."