SYLVIC CHARACTERISTICS. 13 



fertile slopes and coves. The leader of yellow poplar is rarely broken by 

 sleet. Both old and young trees are very windfirm. 



Properties and uses of the wood. The wood of yellow poplar is soft, 

 light, stiff, and rather brittle, but close-grained. The heartwood varies in 

 color from nearly white to a yellowish brown ; the sapwood is white. The 

 heartwood shows only a slight tendency to warp and shrink, and is dur- 

 able when exposed to the weather, but only moderately so in contact with 

 the soil. The sapwood is much less durable than the heartwood when ex- 

 posed ; and shrinks, warps and checks more, but if carefully kiln-dried the 

 tendency to these defects is reduced. Air-dried, the wood has a specific 

 gravity of .42 and weighs from 26 to 28 pounds* per cubic foot. Rough, 

 kiln-dried lumber weighs about 2,800 pounds to the 1,000 board feet, but 

 the green wood is much heavier. While slightly heavier than white pine, 

 poplar has about the same relative strength (Modulus of elasticity 1,716,- 

 000; modulus of rupture, 10,850) f, and is consequently adapted to nearly 

 the same uses. It withstands end-compression well, has high shearing 

 strength and abrades quite even and slowly. While it does not split 

 easily, the fracture is straight and clean. 



In the center of thrifty second-growth trees there are frequently as 

 few as three rings of annual growth to an inch of radius ; while the peri- 

 phery of young trees, stimulated to growth by isolation, may be equally 

 as coarse-grained. In old trees, twelve rings per inch of radius is usual, 

 and commercial timber will generally show from ten to fifteen rings per 

 inch. There is a little difference in density between the early and late 

 wood in the annual band. The numerous pores are small and uniformly 

 distributed through both early and late wood of the annual band. The 

 pith rays or medullary rays are of uniform size, nearly invisible to the 

 naked eye, and scarcely raised when the wood is split radially. While the 

 grain of the wood is as a rule extremely straight, it is occasionally wavy 

 or curly, a characteristic which is usually limited to one face or one side 

 of a tree. Occasionally trees are burly or bird's eyed. 



The proportion of heartwood varies with age, and is much greater in 

 old trees. It begins to form at from fifteen to thirty-five years of age, 

 earlier on good than on poor sites. Its formation, however, does not con- 

 tinue regularly and at the same rate as the growth of the tree, but more 

 slowly, since a tree seventy-five years old will usually have more than 

 forty rings of sapwood. Young trees on average soils which are fifty 

 years old or less, are for this reason largely sapwood and have only a 

 small core of heartwood. In young trees this large proportion of sap- 



*Sargent, Tenth Census. 



fTests by Branch of Products, U. S. Forest Service. 



