NORMAL '(Some protection) J 

 VELOCITY | Distance=IO | 

 ■h times height of 

 I windbreak. 



Distonce 20 times height ot windbreok. 



Figure 4. — How and to what extent do field windbreaks affect the conditions within an area? 



contributes to an improved appearance of farmsteads and the landscape 

 and to more agreeable living conditions generally. 



Forests and streams. — The close relation of forests and rivers appears when 

 a few common observations are made. The force of rain falling in the for- 

 est is broken by the leaves and branches of the trees. The water reaches 

 a soil covering of fallen leaves, small plants, and twigs if this covering 

 has not been destroyed by fire. Much of the water is absorbed as by a 

 sponge or held temporarily as if by millions of small dams. The forest 

 thus checks rapid run-off of the water and the accumulation of silt in 

 streams. 



The forest has other important functions to perform with water. The 

 shade of the trees protects the ground from direct sunlight that hastens 

 evaporation of the water stored in leaf litter and soil. The trees also pro- 

 tect this stored water from winds that would pick up much of it. Snow 

 melts later in the shade than in the open. The forest thus contributes to 

 the steady flow of streams. 



Water seeping through the spongy forest floor is clean, and the soil 

 remains porous because there is little sediment to plug the natural runways. 

 Some of the water trickles downward through subterranean channels to 

 find outlets in springs or through seepage at lower levels and thus helps 

 keep the rivers flowing when there is no rain. Much of it may find a way 

 into underground reservoirs beneath lands far removed from the forests, 

 thus helping to maintain the level of their water table. And some of the 

 water is used by the trees and is released through the roots, trunk, branches, 

 and leaves. The forest soil in various ways is thus partly emptied of its 

 water so that when the next rain comes it can readily absorb more water 

 and prevent excessive run-off. 



A relief map of the United States shows a network of rivers and streams. 

 Many of these rivers flow out of the great reservoir of forest land. Nearly 

 three-fourths of the forest area in the 17 drainage basins of the United 

 States exerts a moderate to major influence on the protection of soil from 

 erosion, in the reduction of rapid run-off, and in moderating floods (fig. 5). 

 The evaporation of water from the leaves also increases the humidity of 

 the air in the immediate locality. 



Forest types. — Forests of the world are of three main types: Coniferous, 

 temperate hardwood, and tropical hardwood. Coniferous forests occupy 

 more than one-third of the forest area of the world, temperate hardwoods 

 occupy one-sixth, and tropical hardwoods nearly one-half. An important 

 fact to remember in this connection is that 95 percent of the coniferous 

 forests, upon which the world chiefly depends for its wood construction, 



