The Tree Dictates the Wood
Understanding the tree might help in buying
wood online?
Structure of the Tree:
Trees are kind of amazing pieces of nature. They can stand upright
hundreds of feet into the air and yet have such a small footprint,
supporting this large canopy. The essence of this amazing engineering
feat is a series of small elongated cells that run parallel to each
other in the direction of the trunk. Studying the characteristics
of these cells can explain much about the wood strength, its overall
appearance and the amount of shrinkage to expect in the drying process…
First maybe consider a cross section of a tree trunk. It has a
number of well defined bands from the bark on the outer rim, to
the useable wood in the interior. The useable wood is usually made
up of two rather distinct colour bands, with sapwood typically lighter
and up tight against the bark and the darker heartwood forming the
rest of the core. The amount and colour variation of sapwood relative
to heartwood can vary significantly tree species to tree species.
Core of a Tree:
The very inside core of the tree is often referred to as the 'pith'.
From the perspective of a woodworker it is often of little value.
It can be quite knotty, a reminder of growth in its younger days.
In woods like basswood the pith is typically canted and used for
temporary blocking or in the north maybe just sent to the paper
mills.
Growth:
New wood is created in a very thin layer between the sapwood and
the bark. It is called the cambium. It is in this region that cell
division takes place and new wood is created, Cell growth pushes
the outer bark further out increasing the diameter of the tree trunk
and creates the distinctive fissures that we are familiar with in
the bark of larger trees.
Climate Effect:
In climates where the temperatures during the growing season varies
significantly between the beginning of the growth season and the
longer summer, the cells lay themselves down in bands. These are
alternating rings of wood formed early in the season (earlywood)
followed by thinner bands of wood evolving in the late part of the
growing season (latewood).
Often the earlywood is lighter in colour and density and the latewood
is denser and darker. This creates the distinctive growth rings
we are familiar with and probably counted at some time in our young
life to determine the age of a local tree.
It is interesting to note that trees growing in the tropics like
purpleheart or bloodwood show virtually no growth rings, but then
their climate encourage growth year round. There are rarely well-defined
annual rings in any tropical wood species, at least the ones I've
played with.
Now if we go back to our original conversation about the elongated
cells that run up and down the trunk and how this might be used
to predict a woods characteristics we can see how in this case just
the weather or rather the country of growth might suggest or maybe
dictate a woods appearance and structure. Early wood is made up
of wood with large cells and air pockets … think of sponge toffee,
and latewood is of course the opposite with thick cell walls and
creates dense wood. It is the degree of visual and physical differentiation
of these two growth cycles that can dictates the overall appearance
and structural abilities of the wood.
Sharp demarcation between the two creates strong grain patterns
in woods like BC fir and even our most common northern red oak.
The amount of earlywood vs latewood will affect a softwood's or
hardwood's density as the earlywood's more porous design will create
weak pockets in its structure. If you were to study just one species
and sourced cross sections from trees grown in the southern limits
of its range with a sample from a tree grown in its northern limits
you'd find significant differences in amount of earlywood vs latewood.
This suggests that a shorter growing season or slower growing in
general, as well as the specifics of a tree's biology can have an
impact on its physical characteristics.
Industry Application:
In fact industry capitalizes on this feature by testing some trees
within a species for exceptional strength characteristics when grown
in colder climates producing a denser, stronger lumber than traditionally
expected. It is sold on the market at a premium as a 'stress-tested'
lumber and offers a great opportunity to capitalize on a woods growing
characteristics as a way to value-add our wood resources.
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