The natural rapidity of growth of different species is also important, and one caring for a forest must know this rate of growth, not only as to the individual species, but also with respect to the forest as a whole. If he knows how fast the trees in a forest grow, both in height and diameter, he will know how much wood, in cubic feet, the forest produces in a year, and he can then determine how much he may cut without decreasing the capital stock. The rate of growth is determined in this way: A tree is cut and the rings on the cross-section surface are counted and measured; see Fig. 124. Each ring represents one year's growth. The total number of rings will show the age of the tree. By a study of the rings of the various species of trees on a given plot, the rate of growth of each species in that location can be ascertained and, by knowing the approximate number of trees of each species on the forest area, the rate of growth of the whole forest for any given year can be determined. [Illustration: FIG. 126.--Bottom Lands Buried in Waste from Deforested Mountains. Wu-t'ai-shan, Shan-si Province, China.] [Illustration: FIG. 127.--Eroded Slope in Western North Carolina.] Forests prevent soil erosion and floods: Forests help to regulate the flow of streams and prevent floods. Most streams are bordered by vast tracts of forest growths. The rain that falls on these forest areas is absorbed and held by the forest soil, which is permeated with decayed leaves, decayed wood and root fibers. The forest floor 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.]