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 longleaf, shortleaf or slash pine. Because of lower strength characteristics, two other southern pine
species, Virginia and pond pine, are placed in a separate group called Virginia pine-pond pine. As
another example, for western species, larch can be mixed in with Douglas-fir.
2.1.1.2 Other Groupings. Some species groups, based on strength properties,
also have been established for hardwoods. For example, the National Wood Pallet and Container
Association has set up three species groups for the manufacture of wood pallets and containers.
These groups are based on species' density.
In other cases, only those species acceptable for a particular end use or with particular properties are
grouped together. The American Railway Engineering Association publishes a list of those species
acceptable as ties, and the American Wood Preservers' Association groups certain species according
to processing requirements for pressure treating.
2.1.2 Growth Increments. A growth increment is that layer of wood which, each year, is
added to the circumference of a tree. It is also referred to as an annual ring. The term "growth incre-
ment" is more correct since a few species, such as cypress and redwood, may form more than one
ring or "false" ring per year. The width of a growth increment may vary from a few cells in width
to as much as an inch in some fast growth woods. The rate of growth can affect the strength of the
material.
2.1.2.1 Texture. Texture is a general term which can have several meanings. It
can refer to the rate of growth, to the density, or to the degree of contrast between growth incre-
ments or parts of the growth increment. For example, slow growth hardwoods are referred to as
fine textured, meaning that the growth increments are relatively narrow (close together) and, in the
case of oak, the density is lower.
Texture can also be used to refer to the size, arrangement and appearance of the individual cells.
Oak, with its large pores, is considered coarse textured, while maple has small pores and is con-
sidered fine textured.
2.1.2.2 Earlywood and Latewood. The growth increments in some softwood
species, such as southern pine and Douglas-fir, are composed of two distinct parts. Earlywood
(sometimes called springwood) is that portion of the growth increment which is formed during the
first part of the growing season or usually in the spring. Earlywood cells are relatively large in
diameter, thin-walled and lighter colored. Latewood (sometimes called summerwood) is formed
after the earlywood (Figure 2-2). The latewood cells are smaller, thick-walled and darker. As a
result, latewood is heavier and stronger than earlywood.
The growth rate or width of the growth increment, and the percent of latewood which it contains,
are important factors in the grading of southern pine lumber where strength is desirable. For ex-
ample, a grade called Dense Southern Pine is defined as averaging, one end or the other, not less
than six annual rings per inch, and one-third or more of each annual ring is summerwood. Pieces
2-3
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