8 BULLETIN" 556,, IT. S. DEPARTMENT OF AGRICULTURE. 



LOCALITIES WHERE GROWN. 



In the second column of the tables are listed the States in which the 

 test specimens originated. The locality of growth has in some cases 

 an influence on the strength of timber. This influence is, however, 

 usually overestimated; just as great differences exist ordinarily 

 between stands of different character grown in the same section of 

 the country as between stands grown in widely separated regions. 

 For this reason it is considered better to average the various localities 

 together. Douglas fir, however, has not been averaged in this man- 

 ner. Silviculturists have recognized that there are two well-marked 

 types * and various intergradations of Douglas fir. Strength tests 

 confirm this fact and show that there is actually a difference in strength 

 between the Rocky Mountain and Pacific Coast types of Douglas 

 fir. For this reason averages are given for the Coast and for the 

 Rocky Mountain regions rather than for the species as a whole. 



NUMBER OF TREES. 



The number of trees from which test specimens were taken is given 

 in the third column of Table 1. As previously mentioned, five is the 

 usual number from a single locality. 



NUMBER OF RINGS PER INCH. 



Rings per inch is an inverse measure of the rate of growth. It is 

 taken along a radial line on the end section of each specimen. One 

 ring, consisting of a band of springwood and a band of summerw T ood, 

 is formed by each year's growth; consequently, few rings per inch 

 indicate fast growth, and vice versa. 



Rate of growth is extremely variable, and the values given are to 

 be taken as averages of the material tested only. Rate of growth 

 has no definite relation to strength in the sense of strength being in 

 proportion, either directly or inversely, to the rate of grow T th. Tim- 

 ber of any species which has grown with exceptional slowness is 

 usually below the average of the species in strength values. In the 

 coniferous species material of very rapid grow T th is also very likely 

 to be below the average in strength. Among many of the hardwood 2 

 species, however, timber of rapid growth is usually above the average 

 in strength properties. 



i See Forest Service Circular 150, "Douglas Fir: A Study of the Pacific Coast and Rocky Mountain 

 Forms." 



2 A broad classification of timber species divides them into two groups: (1) Angiosperms, or trees with 

 broad leaves, usually deciduous, the so-called "hardwoods"; (2) gymnosperms, or trees with needle or 

 scalelike leaves, usually evergreen, most of them cone bearing, the so-called "softwoods." The two groups 

 are popularly spoken of as "hardwoods" and "softwoods," or "hardwoods" and "conifers." The terms 

 "hardwoods" and "softwoods" are therefore indicative of botanical classification and are not descriptive 

 of the quality of the wood with respect to hardness. Such "hardwoods" as basswood and aspen are low 

 in the scale of hardness; while the southern pines, tamarack, larch, and others, although called "softwoods," 

 are quite hard. 



