31 



near all kinds of industries that consume large quantities of 

 coal. Coniferous species are most sensitive because of their per- 

 sistent leaves. The degree of resistance is nearly proportional to 

 the length of time that the leaves remain on the tree. Spruce and 

 fir are very sensitive, pine less so, and larch the least of all coni- 

 fers. Among broadleaved species oak and chestnut are the least 

 sensitive; maple, ash, and elm more so; and beech, poplar, and 

 cherry very sensitive. The relative humidity of the atmosphere 

 when laden with the gas very materially influences the degree of 

 injury, all species being much more sensitive in damp, foggy 

 weather. 



17. THE EFFECT OF SOIL FACTORS ON THE CHOICE OF SPECIES 



The soil gives stability and physical support to the tree. It is 

 also the medium through which adequate root expansion is at- 

 tained and necessary moisture and soil nutrients reach the roots 

 to supply the various vital processes. The more perfectly the 

 soil meets the requirements of the tree for support and nutrients, 

 the more completely will the tree perform its various life processes 

 and the better will be its growth. All trees demand that the soil 

 provide these essentials but in different degrees. A loose, shallow 

 soil endangers the stability of trees. However, species such as oak 

 and hickory with deeply penetrating roots are much more stable 

 than shallow-rooted species like spruce which spread out their 

 roots in the surface soil. Thinnings must be undertaken with 

 due foresight, or the crop left standing will be thrown by the wind. 

 A shallow soil on an impervious subsoil endangers the stability 

 of trees in a similar manner and also restricts root expansion. 

 A compact, heavy soil also restricts root expansion and is par- 

 ticularly harmful in the seedling stage of tree growth. Spring- 

 fed swamps and northern slopes decrease soil temperature and 

 consequently affect gro\vth. Sandy soils and southern slopes 

 usually mean increased soil temperature. Certain species like 

 spruce and hemlock germinate at a much lower temperature than 

 their hardwood associates and are able to start earlier in the 

 spring. When such species grow in mixture with hardwoods, their 

 germination at lower temperature may account for their occupy- 

 ing the cooler sites. It is obvious that if the soil is deficient in 

 water and soil nutrients tree growth cannot exist at all. A coarse, 



