All of us, at one time or another, have probably counted the number of growth rings in a tree in order to determine its age.
Ring width is associated with the growing conditions, which can vary significantly from year to year; take a look at Fig 1. External events like grass fires, drought, or insect attack can sometimes cause false rings to occur as tree growth falters and then restarts. Generally though, if you assume one growth ring per year, then you will arrive at a good estimate of tree age.
The cells in the lighter coloured part of the growth ring have relatively thin cell walls and large lumina (the void in the centre of the cell) and are called early wood or spring wood. Conversely, the darker wood, known as late wood or summer wood, has thick walls and relatively small lumina.
The various machining operations needed to make wood products will cut through the early and late wood bands, creating figure. If the cut is at a tangent to the growth rings then figure like that in Fig 2 is produced. This same kind of figure is often seen on the faces of plywood, because the relative motion of the knife in the lathe is at a tangent to the spinning log.
You will notice that the growth rings at the top of Fig 1 have a rather sudden transition between early and late wood, whereas the growth rings near the bottom have a more gradual transition. So this begs the question, when does early wood become late wood? The transition between early and late wood in softwoods is most often defined using Mork’s index. The early wood/late wood transition is considered to occur when the cell wall thickness exceeds a quarter of the lumen width, which is equivalent to when Mork’s index exceeds 1: M = 2t dw L
Mork’s index, where tdw = double cell wall thickness and l = lumen width.
Hardwoods can be classified into ringporous, semi-ring porous and diffuse porous. In the first two groups, the vessels tend to have larger diameters and/or be more numerous in the early wood compared to the late wood, so the growth rings are generally clearly defined. The growth rings of diffuse porous hardwoods are sometimes more difficult to see because the vessel diameter does not really change. However, the end of the growing season can be marked by a ring of late wood tracheids and fibres, creating a thin line.
Growth rings can be found in tropical trees too, but they are much more difficult to see due to the fairly continuous growth. In addition, the growth rings may not be annual, but linked to weather conditions or other external influences. The anatomical variety of trees brings about the beauty of wood and makes every single piece of wood unique. So appreciate the wood based panels in front of you, because they are exclusive!
