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 HOME WOOD LIBRARY The Tree

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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© theWoodbox.com Jan 2007