JANUABT 22, 1915] 



SCIENCE 



123 



small — less than 1 per cent. In others, for 

 instance beech, it is 5 per cent. Thus if a 

 certain locality receives 50 inches of rain, 

 the ground under the forest will receive 

 only 40, 30 or 20 inches. Thus 10, 20 and 

 30 inches will be withdrawn from the total 

 circulation of moisture over the area occu- 

 pied by the forest. The forest cover, be- 

 sides preventing all of the precipitation 

 from reaching the ground, similarly keeps 

 out light, heat and wind. Under a forest 

 cover, therefore, there is altogether a differ- 

 ent heat and light climate, and a different 

 relative humidity than in the open. 



The foliage that falls year after year 

 upon the ground creates deep modification 

 in the forest soil. The changes which the 

 accumulation of leaf litter and the roots of 

 the trees produce in the soil and subsoil 

 are so fundamental that it is often possible 

 to determine centuries after a forest has 

 been destroyed, whether the ground was 

 ever occupied by one. 



The effect which trees in a stand have 

 upon each other is not confined merely to 

 changes in their external form and growth 

 it extends also to their internal structure. 

 The specific gravity of the wood, its com- 

 position, and the anatomical structure 

 which determines its specific gravity differ 

 in the same species, and on the same soil, 

 and in the same climate, according to the 

 position which the tree occupies in the 

 stand. Thus in a 100-year-old stand of 

 spruce and fir the specific gravity of wood 

 is greatest in trees of the third crown class 

 (intermediate trees). The ratio of the 

 thick wall portion of the annual ring to the 

 thin wall of the spring wood is also differ- 

 ent in trees of different crown classes. The 

 difference in the size of the tracheids in 

 trees of different crown classes may be so 

 great that in one tracheid of a dominant 

 tree there may be placed three tracheid 

 cells of a suppressed tree. The amount of 



lignin per unit of weight is greater in domi- 

 nant trees than in suppressed trees. 



Forest trees in a stand are thus influ- 

 enced not only by the external physical 

 geographical environment, but also by the 

 new social environment which they them- 

 selves create. For this reason forest trees 

 assimilate, grow and bear fruit differently 

 and have a different external appearance 

 and internal structure than trees not 

 grown in a forest. 



Forestry, unlike horticulture or agricul- 

 ture, deals with wild plants scarcely modi- 

 fied by cultivation. Trees are also long- 

 lived plants; from the origin of a forest 

 stand to its maturity there may pass more 

 than a century. Foresters, therefore, 

 operate over long periods of time. They 

 must also deal with vast areas; the soil 

 under the forest is as a rule unchanged 

 by cultivation and most of the cultural 

 operations applicable in arboriculture or 

 agriculture are entirely impracticable in 

 forestry. Forests, therefore, are lagely the 

 product of nature, the result of the free 

 play of natural forces. Since the foresters 

 had to deal with natural plants which grew 

 under natural conditions, they early 

 learned to study and use the natural forces 

 affecting forest growth. In nature the 

 least change in the topography, exposure or 

 depth of soil, etc., means a change in the 

 composition of the forest, in its density, in 

 the character of the ground cover, and so 

 on. As a result of his observations, the 

 forester has developed definite laws of 

 forest distribution. The forests in the 

 different regions of the country have been 

 divided into natural types with correspond- 

 ing types of climate and site. These nat- 

 ural forest types, which by the way were 

 also developed long before the modern con- 

 ception of plant formations came to light, 

 have been laid at the foundation of nearly 

 all of the practical work in the woods. A 



