Wood Science – Softwoods: Structure and Growth Rings
I have a small cross section of an eastern white cedar tree. Barely 8″ wide, it’s certainly not imposing, but a closer look at the wood reveals an incredible number of small growth rings. In fact, this ancient cedar is more than 560 years old. Most woodworkers are familiar with growth rings and the fact that they can tell us the age of a tree. However, growth rings also offer clues as to how a particular wood will act when machined, sanded and finished, and as to what its final appearance and durability will be.
The way softwoods grow determines not only the architecture of their growth rings but characteristics such as texture, grain, strength and figure. Softwoods are gymnosperms, which means they produce naked seeds. They are mostly coniferous, which tells us their foliage is needle-like. Softwoods are thought to be the first trees to evolve.
After a very close inspection, the annual growth rings in this piece of eastern white cedar are incredibly narrow, telling the tale of a very slow-growing tree. The pith of this tree formed around 1460 A.D.
Though some softwoods are even more consistent in their earlywood / latewood densities, this piece of western red cedar is fairly even, making it somewhat decent for carving. Varying densities between earlywood and latewood make carving hard. (Photo by Rob Brown)
The internal structure of softwoods is simple when compared to the hardwoods. Most of the tree is made up of longitudinal cells called tracheids. Up to 95% of the wood in a softwood tree consists of tracheids. They’re usually 100 times longer than their diameter and their fibrous nature is utilized to make paper. Tracheids are dual-purpose cells; they conduct sap as well as supply mechanical support for the tree.
Tracheid diameter will vary between species and within the growth rings of an individual tree. The small diameter tracheids found in eastern white pine give that wood a fine texture. Redwood, with its large diameter tracheids, is considered a coarse-textured wood. If you’re a carver you prefer the fine-textured woods, as they will hold more detail but will also yield to a knife or chisel with more consistency.
The thickness of the tracheid cell wall also affects the density and strength of the wood. Thin-walled cells are weak and lighter in colour. Tracheid cells that have thick walls are stronger and darker in colour. This is because the thicker cell walls have more cellulose and lignin, two compounds that give the cell structural strength.
Like all trees, growth in softwoods occurs at the cambium layer. The cambium layer will continue to produce new growth as long as favourable environmental conditions are met. The cambium layer grows outward, leaving a layer of newly formed wood cells behind while also depositing new cells in front of the cambium layer that are added to the bark. The tree increases in girth and volume as the cambium migrates outward, away from the pith. This process occurs yearly and forms the annual rings that we’re familiar with.
Earlywood and latewood
In temperate regions, there is usually a period of growth followed by a dormant period. This affects the growth pattern of the tree and is revealed by distinct and visible layers in the growth rings. Early in the growth season, sap conduction is prioritized so the softwood tracheids are large and thin walled. Wood from this time is called earlywood and is weaker and lighter in colour. As the growing season progresses, smaller diameter cells with thicker walls predominate. This area of the growth ring is called latewood. What we’re interested in as woodworkers is the relative proportion of earlywood to latewood, as well as how distinct the transition is.
Let’s consider eastern white pine again. The transition from earlywood to latewood is gradual with an even grain produced from tracheids of a similar diameter. The resulting wood is consistent in how it works and wears. Southern yellow pine has a very distinct transition between the early and latewood. In fact, the latewood is three times as dense as the earlywood. The wood is unevenly grained and difficult to machine. Carvers will notice that their tools will slice through the earlywood with ease, then will require more effort in the dense latewood. The colour difference will be more pronounced, too, with the darker latewood predominating.
Growth rings also reveal clues to the overall strength of a given piece of wood. Fast-growing trees, growing either in open areas without much competition or on a plantation, produce very wide growth rings. The wood is inherently weak because much of the growth ring is made up of a higher percentage of large, thin-walled tracheids.
Trees with small growth rings, like my sample of eastern white cedar, produce stronger wood. This is because there are more of the thick-walled tracheids present, yielding a denser, stronger wood. This is why old-growth softwoods make ideal soundboards for musical instruments. They can be carved thinner without sacrificing strength, yielding a more responsive soundboard.
Sanding and finishing softwoods
Looking at the growth rings can also help us in deciding how to finish our projects. Softwoods with distinct growth lines can be difficult to sand. This is because the soft earlywood will yield when being sanded before the tougher latewood, producing a scalloped surface, especially if you sand by hand. Always back your sandpaper with a flat surface to avoid this problem.
Likewise, problems can arise when staining softwoods. When you apply a stain to softwoods, the earlywood will soak up more stain than the latewood. The earlywood will become darker and the overall colour pattern of the wood is reversed, not unlike a photo negative. I would caution against using stains on softwoods for this reason, especially if they’re wiping / penetrating stains.
If you have put pencil and paper aside, and have finalized the design of your next project, you’re now in the realm of wood selection. Selecting wood requires balancing its visual appeal and its structural nature. Learning how to read the end grain of lumber will always give important clues to help make this decision easier. By simply learning how to read the growth rings in the end grain, every piece of wood will reveal its beauty and strength, and offer clues as to best practices for working and finishing.