Section of a Red Oak Log
Cypress Grades • Doyle Log Scale • Hardwood Lumber Grades • Hardwood Defects and Character • Hardwood Measurements • Log Cross Section • Process Terminology • NHLA 2007 Rules Book (pdf 637kb)
The cross section of a tree shows the following
well defined features in succession from the outside to the center:
1) bark and cambium layer; 2)wood, which is most species is clearly
differentiated into sapwood and heartwood; and 3) pith, the small
central core. The pith and bark are excluded from finished lumber.
The Trunk and Its Branches
Most branches originate
at the pith, and their bases are inter-grown with the wood of the
trunk as long as they are alive. These living branch bases
constitute ingrown or tight knots. After the branches die,
their bases continue to be surrounded by the wood of the growing
trunk and thus loose or encased knots are formed. After the
dead branches fall off, the studs become overgrown and subsequently
clear wood is formed.
FAS and Selected
grades of hard-woods are cut from the outside part of the log.
Therefore, FAS and Select grades will have more sapwood than the
common grades. All growth in either diameter or length takes
place in wood already formed; new growth is purely the addition
of new cells, not the further development of existing cells.
The cambium layer is the only growing part of the tree.
Most species grown
in temperate climates (having four seasons yearly) produce well
defined annual growth rings, which are formed by the difference
in density and color between wood formed early and wood formed late
in the growing season. The inner part of the growth ring formed
first is called "springs wood," and the outer part formed later
in the growing season is called "summer wood." Spring wood is characterized
by cells having relatively large cavities and thin walls. Summer
wood cells have smaller cavities and thicker walls, and consequently
are more dense than spring wood.
The growth rings, when exposed
by conventional methods of sawing, provide the grain or characteristic
pattern of the wood. The distinguishing features of the various
species are thereby enhanced by differences in growth ring formation.
By counting the growth rings you can determine a trees age as one
ring is formed each year.
Sapwood contains living cells
and performs an active role in the life processes of the tree.
It is located next to the cambium and functions in sap conduction
and storage of food. Sapwoods size varies by species and where the
tree is growing.
consists of inactive cells formed by changes in the living cells
of the inner sapwood rings, presumably after their use for sap conduction
and other life processes of the tree have largely ceased. The cell
cavities of heartwood may also contain deposits of various materials
that frequently provide a much darker color. All heartwood, however,
is not darker.
The infiltrations of material
deposited in the cells of heartwood usually make lumber cut form
heartwood more durable when exposed to weather. Sapwood is not weather
resistant in any species.
Medullary rays extend radially
from the pith of the pith of the log toward the circumference. The
rays serve primarily to store food and transport it horizontally.
They vary in height from a few cells in some species to four or
more inches in the oaks, and produce the flake effect common to
the quarter sawn
lumber in these species.
Native species of trees and the
wood produced by these trees are divided into two botanical classes-hardwoods,
which have broad leaves, and softwood, which have needle-like or
scale-like leaves. This botanical classification is sometimes
confusing, because there is no direct correlation between it and
the hardness or softness of the wood. Generally, hardwoods
are more dense than softwoods, but some hardwoods are softer than