FOREST COMMISSIOXER's RErORT. 89 



Avhich produce it. When it first appears it usually increases 

 in diameter at about the same rate that the tree is growing, but 

 later spreads more rapidly and encroaches on the white sap- 

 wood. Mature trees usually show from 2 to 4 inches of sap- 

 wood outside of the heartwood. The heartwood has the same 

 general shape as the tree itself and decreases in diameter from 

 the base up in about the same proportion as the bole, so that 

 there is none in the upper part of the tree. 



When the heart first appears it is sound and may remain so 

 for many years or may decay very soon, depending on the local- 

 ity and the general health of the tree. In an average stand, at 

 maturity, perhaps 10 or 15 per cent of the trees are affected by 

 heart rot, but as soon as the stand becomes overmature the pro- 

 portion increases very rapidly. 



SOIL. MOISTURK, AND LIGHT KLQUIRLMENTS. 



Paper birch is not particularly exacting in respect to its de- 

 mands upon soil and moisture. It prefers a fairly deep, loose, 

 well-drained, sandy loam, and attains its best development on 

 soils of this character. It is not likely to grow on dry, sandy 

 barrens or in submerged swamps, but is found in practically 

 all situations intermediate between these two extremes. In the 

 White Mountains it is abundant on the thin-soiled rocky slopes 

 above 3,cxdo feet. This is not, however, because it prefers such 

 situations, but because it is able to grow there while many other 

 trees are not. On IMount \\'ashington it has ])een found at an 

 elevation c^f 5.700 feet as a prostrate shrub. 



In its light requirements paper birch is much more exacting. 

 It is one of the most intolerant species of the Northeast, and 

 in this respect is exceeded among the broadleaf trees with which 

 it associates only by the aspens. Young seedlings will grow 

 under verv lii-ht shade but will not survive under a crown cover 

 as dense as that usually found in virgin forests. Saplings and 

 poles when shaded from the side will sometimes struggle along 

 for a while as poorly developed, suppressed individuals but must 

 have absolutely free crowns in order to thrive. Suppressed 

 trees do not recover after being released from suppression, as 

 do spruce and balsam. 



