SILVICS 9 



mineral soils, poplar, bird cherry, and birch spring up freely; 

 on the abandoned fields, pine and spruce are especially apt to 

 appear; and on the moss-covered decaying logs of the northern 

 woods, little hemlocks, fir, and spruce seedlings start. In a few 

 years the growth and very existence of these seedlings are in- 

 fluenced and other changes take place. The tops of the old 

 trees may spread out and shade the openings, so that only the 

 tolerant seedlings can endure; or the remaining old stand may 

 be felled and an unlimited amount of light admitted. Under 

 this impetus certain rapid-growing species may push up to the 

 entire exclusion of the others, or these others if they are tolerant 

 of shade may be relegated to an understory of secondary im- 

 portance. On an acre of forest soil thousands of such seedlings 

 may start, but in the course of their various struggles and under 

 innumerable external influences their ranks are so thinned that 

 only a few hundred grow into fair-sized trees. A stand of 

 looo trees per acre 20 feet high by the time it- is 80 feet high 

 and correspondingly large will seldom contain over 400 trees to 

 the acre. The trees that are killed in this constant strife fall 

 to the ground after a few years, decay, and add to the humus of 

 the soil. As the forest grows old gaps are caused by these 

 deaths and reproduction again takes place. Whether the same 

 kind of trees will come in depends upon the conditions at that 

 time. 



In every region certain combinations of forest trees and under- 

 brush are characteristic of different sites. As already explained, 

 this community of tree life is called a ''type," and is influenced 

 by soil, exposure, elevation, moisture, and other factors. As 

 examples of types, we may mention the ''spruce slope type," 

 characteristic of the steep mountain slopes of northern New 

 England, and "chestnut oak type," of the trap ridges of Connec- 

 ticut. Under natural conditions such a forest type maintains 

 itself century after century on the site to which it has become 

 adapted. Accordingly it is known as the permanent type of 

 that particular site. Circumstances not in the regular course 

 of nature may entirely change the conditions and cause the 



