CHOICE OF SPECIES IN ARTIFICIAL REGENERATION 25 



are buried to considerable depth in the soil, usually reproduce 

 well in the open. Birch and aspen, although small-seeded, germi- 

 nate quickly, grow rapidly, and often reproduce well on open 

 land. Small-seeded species like spruce, fir, and hemlock, which are 

 more or less tardy in germination and grow slowly in the juvenile 

 stage, seldom reproduce well in the open. In general, the greater 

 the proportion of root to shoot surface and the more rapid the 

 depth of root penetration, the more successful is artificial re- 

 generation in the open. Species like white pine, red pine, and 

 tulip often can be safely seeded on soils which retain an excess of 

 moisture in the surface layers, while attempts to regenerate them 

 by seeding in the open on loose, sandy, or shale soils are likely 

 to result in failure due to the effect of direct sunlight in over- 

 drying the surface layers of the soil. Red cedar reproduces re- 

 markably well in the open and on dry, sandy soils. This appears 

 to be due chiefly to the very long, deeply penetrating root which 

 develops quickly after germination and to the small surface ex- 

 posed to the sun and air. 



One of the chief advantages of an overwood in artificial regen- 

 eration is the effect of the cover in rendering the surface soil 

 more uniform in soil moisture. Therefore, slow-germinating 

 and slow-growing species and those with a shallow juvenile root- 

 system or a large proportion of shoot to root can usually be 

 more successfully reproduced under an overwood. As soon as 

 the trees are well established and their roots well below the super- 

 ficial soil layers the overwood is no longer beneficial and, in most 

 cases, is harmful because of its shading effect upon the young 

 stand and the consequent decrease in carbon assimilation. 



12. Humidity in Its Relation to the Choice of Species. All 

 trees require a certain amount of moisture both in the air and in 

 the soil. The air moisture is humidity, while the soil moisture is 

 water content. Trees are continually subject to the action of both. 

 There is no other factor which is so thoroughly clear in its in- 

 fluence upon tree life as water. We can follow it step by step from 

 its entrance through the root to its exit through the shoot. Al- 

 though the stimulating effect of humidity is confined chiefly to 

 the shoot and that of water content to the root, they work to- 

 gether in determining growth and, in reality, are two phases of the 

 same stimulus. The various modifications in form and structure 

 due to the degree of absorption cannot be directly referred to 



