Forests and Soils 



117 



tainous country, the quantity of mois- 

 ture available to a tree varies with its 

 position on the slope. Trees on lower 

 slopes normally have more available 

 moisture than trees on otherwise sim- 

 ilar upper slopes. Deep soils that have 

 adequate water-holding capacities keep 

 trees growing at maximum rates if 

 other factors are not limiting whereas 

 shallow soils not having adequate 

 water-holding capacity do not. 



Some tree species grow well under 

 many soil conditions, others do not. 

 Black locust, for instance, can grow on 

 deep or shallow, moist or dry soils. 

 True, it does not grow equally well on 

 all situations, but it does persist. Other 

 species, such as yellow-poplar, occur 

 on only a few deep moist soils and 

 usually do not become established on 

 shallow dry soils or on tight claypan 

 soils. Segregation of species within 

 their ranges is therefore often due to 

 differences in soils. 



Forest types or associations of tree 

 species depend somewhat upon rela- 

 tive tolerance of the several associated 

 species to shade. Some stand more 

 shade than others. For instance, pon- 

 derosa pine, cottonwood, and black 

 locust are less tolerant of shade than 

 yellow-poplar, white oak, beech, and 

 hemlock. Some species appear to be 

 more tolerant under some soil condi- 

 tions than under others. Two factors, 

 then, chiefly determine forest types in 

 any climatic province: First, the in- 

 herent capacity of a species to with- 

 stand the shade and, second, the soil 

 conditions. 



FORESTS AFFECT THE SOIL most of 

 all through litter. Litter breaks the im- 

 pact of rain, retards runoff, and filters 

 rain water into the soil without disturb- 

 ing soil structure. In dry weather, litter 

 reduces surface evaporation. When 

 litter decays, it provides mineral ele- 

 ments for tree growth. It shelters mi- 

 crobiotic life, which breaks down many 

 kinds of complex substances into sim- 

 ple forms, and it shelters worms that 

 help to keep the soil granular and 

 mellow. In extremely cold weather, 



the forest litter acts as a blanket 

 through which the heat from the soil 

 cannot escape rapidly. Litter therefore 

 reduces the depth of freezing of forest 

 soils. When a forest soil does freeze, it 

 tends to honeycomb and is therefore 

 permeable to sudden rains that may 

 come in late spring. 



Litter is the source of the humus 

 horizon of a forest soil, and the humus 

 layer is the part of a forest soil that 

 distinguishes it from an agricultural 

 soil. Cultivated soils contain humus, 

 too, but it chiefly comes from a humus 

 layer previously formed under grass 

 or forest. The humus of a farmed soil 

 is maintained only by extraordinary 

 methods of crop rotation and fertiliz- 

 ing, whereas the humus layer of a 

 forested soil is maintained by the yearly 

 leaf fall. When bare fields are planted 

 to trees the humus layer increases in 

 thickness. This increase is a good in- 

 dex of site recovery. 



Forests help prevent peak floods 

 through their effect on the soil. A po- 

 rous, permeable soil absorbs rainfall 

 faster than a cultivated soil. A soil 

 covered with litter, brush, and tree 

 stems retards runoff of much surface 

 water that may not be absorbed 

 quickly. Experiments show that from a 

 40- to 50-inch rainfall in Ohio, forests 

 store about 6 inches more water than 

 fields in cultivated row crops. 



The forest intercepts much of the 

 force of wind-driven rain, and thus 

 prevents beating of the protective lit- 

 ter and soil. It protects the soil from 

 excessive heat, light, and drying winds. 

 Its roots hold the soil in place. They 

 have grown, died, and decayed through 

 centuries, and have made the soil more 

 and more porous and permeable. As 

 they decay, they leave deep channels 

 through which water may percolate 

 and air may move. 



Rain water dissolves salts of calcium, 

 potassium, and magnesium from the 

 soil, causing it to become sour, but in 

 the hardwood region these bases, re- 

 plenished in the litter, tend to preserve 

 a "sweet" soil. These elements, to- 

 gether with organic matter, keep the 



