Trees are amazing pieces of nature. They stand upright hundreds of feet into the air with such a small footprint, supporting a very large canopy. At the core of this incredible engineering feat is a group of small elongated cells that run parallel to each other along the trunk. Studying the characteristics and growth of these cells can explain much about the wood strength, its overall appearance and the amount of shrinkage to expect in the drying process…
Cross Section of a tree:
A tree trunk has a number of well defined bands from the bark on the outer rim, to the sapwood just inside the bark and the heartwood that forms the rest of the core. The amount of sapwood relative to heartwood can vary significantly tree species to tree species and there can be significant colour variation between the two.
Walnut is the best example with dark purple heartwood and almost white sapwood, something that might be important to know if you want to buy wood online and ensure that what you get is what you want.
New wood grows in a thin layer between the sapwood and the bark. It is called the cambium. Cell division builds wood fibre and causes the diameter of the tree trunk to expand creating the distinctive fissures that we are familiar with in the bark of larger trees.
Influence of Climate
Often wood grows in different bands around the tree as a function of variations in weather between the spring and the fall. Less dense rings of wood that form early in the season are referred to as ‘earlywood’ and darker thinner denser bands of wood evolve in the late part of the growing season creating ‘latewood’. This creates the growth rings that we all know and love, and probably counted at some time in our young life to determine the age of the backyard tree.
Trees grown in the tropics like yellowheart or bloodwood show virtually no growth rings, because their climate encourages growth all year. There are rarely well-defined annual rings in any tropical wood species, at least the ones I’ve used. Bulletwood may be a great example of this with no visible growth rings and only a very dense and very linear grain pattern.
It is quite obvious then to understand how a tree’s origin and growing environment might be used to predict a woods characteristics and appearance. Woods from temperate climates often exhibits contrasting bands of earlywood and latewood that create strong vibrant grain patterns. Think of woods like BC fir and common northern red oak.
The amount of earlywood vs latewood will also affect a softwood’s or hardwood’s density as the earlywood’s more porous design will create weak pockets in its structure. As you move further north in a tree species growth range its density typically increases as the growing season diminishes suggesting that a shorter growing season as well as the specifics of a tree’s biology can have an impact on its physical characteristics.
These are generalities though as it is pretty easy to find contradictions to these basic concepts. Keep in mind though that quite often an understanding of the sapwood vs heartwood and earlywood vs. latewood is used as a key tool in wood identification.
Visit ‘thewoodbox.com/wood’ for further information on a specific species and a larger discussion about wood, its toxicity, elasticity and more; help to become a more knowledgeable consumer if you’d like to buy wood or lumber online.
Karen Lacasse draws on 25+ years of woodworking experience, both as a woodworking teacher and industrial woodworker/owner of Lacasse Fine Wood Products. She is the author behind thewoodbox.com where you will find articles and resources to help build your knowledge to more effectively buy wood or lumber online and enjoy your woodworking passion!
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