FOREST TYPES AND THEIR CHARACTERISTICS 63 



percolates into the soil. Forests are therefore of vast importance for 

 the prevention of floods and the protection of watersheds. Forests 

 hold the soil and protect it from being eroded into rivers and streams 

 or into fertile fields. Planting is one of the best measures for erosion 

 control. The fertility of the soil is also increased by the forest. Trees 

 convert the chemical elements from the soil, together with carbon from 

 the air, into wood, leaves, and twigs. The wood may be removed 

 from the forest, but the leaves and twigs are returned to the soil. 

 This organic material forms the forest humus which is most important 

 for the soil fertility. Even a poor sandy soil, if mixed with forest 

 humus, may be very fertile. The humus may either be deposited upon 

 the mineral soil (this humus type is called "duff") ; or it may, by 

 means of the soil fauna, such as earthworms, be incorporated into the 

 mineral soil (this type is called "mull") . The mull is the most fertile 

 forest soil and richer in plant food than most garden soil. Duff is 

 especially found in northern climates at high elevations and under 

 coniferous forests. Mull is typical of the best hardwood forests. 



4. FOREST TYPES AND THEIR CHARACTERISTICS* 



According to a report of the Society of American Foresters, a for- 

 est type is "a descriptive term used to group stands of similar char- 

 acter as regards composition and development due to given physical 

 and biological factors, but which may be differentiated from other 

 groups of stands ... a cover type is a forest type now occupying 

 the ground, no implication being conveyed as to whether it is tempo- 

 rary or permanent." Thus there may be pure or mixed types, such 

 as a pure redwood or pure ponderosa pine type, a mixed hardwood 

 type, or a cypress and swamp hardwood type. There may be virgin 

 timber types or types based upon location, such as alpine or swamp 

 types, ridge types as the chestnut oak type, old field type as fre- 

 quently found in New England, and cove type, as in the southern 

 Appalachians. Types may also be based upon age and condition of 

 the forests, such as a young, replanted white pine type or park-like 

 type as found in the Rocky Mountains. 



Hawley and his committee, in their study of types, have accepted 

 the above definition, except that composition rather than develop- 

 ment should be the primary basis of recognition. Cover types are 

 based on the present tree cover; they are what the forester finds on 



*For further treatment of this subject, see "Forest Cover Types of the 

 Eastern United States," report of the Committee of the Society of American 

 Foresters, R. C. Hawley, chairman, Washington, D. C., 1932. 



