The thin and the thick of it – tree rings

I found myself needing to very accurately age some trees. The margin of error required for the work was plus or minus months, not give or take a few years. After trying every conceivable option, including photos, sales records, comparative plantings and saying no, I found an increment borer, made some holes and pulled some cores.

When it comes to counting tree rings – ageing a tree by counting annual growth increments; generally speaking, the wood that is formed in early part of the season (early wood) appears lighter in colour in colour compared to the wood that formed towards the end of the season (late wood). This is because the environmental conditions are typically least limiting in the early part of the growing season, and most limiting in the latter part of the season. Less-limiting conditions tend to result in rapid growth, characterised by larger cells with smaller cells formed towards the end of the season as growing conditions become less favourable.

The contrast between small, thick-walled cells formed at the end of one season and large, thin-walled cells formed at the beginning of the next growing season results in a rather distinct line (or tree ring). So far, so good. If environmental conditions vary, it is possible that there will not be a period of rapid growth, or there may be one or more periods of rapid growth within a single growing season. It is not possible for a growth ring to be completely absent, but it is possible for more than one growth ring to be formed in a single growing season. Additional rings that are formed in a single growing season are referred to as false rings. So not every growth ring represents a year.

That’s not too complicated. If all things were equal and Mother Nature didn’t have a warped sense of humour, counting trees rings would be as easy as that – but it’s not, it never is. Not all wood is the same.

There are three general types of wood, and these are distinguished based on the presence, absence, or types of water-conducting vessels. The wood of conifers (gymnosperms/softwoods) does not contains vessels, or pores, and is referred to as nonporous. The growth rings of many nonporous woods are characterised by a distinct colour difference between latewood and earlywood.

The wood of angiosperms (flowering plants/hardwoods) has pores. Angiosperm wood can be divided into diffuse-porous and ring-porous types. And then there is semi-ring-porous wood… in fact, consider angiosperm wood as being on spectrum, the Ring Porosity spectrum.

At one end, ring-porous woods form a band of very large pores at the beginning of the growth season and the rest of the ring (latewood) contains fewer and smaller pores. So, there can be a detectable difference between latewood and earlywood in ring-porous trees. At the other end of the spectrum, diffuse-porous woods tend to form pores of a relatively consistent size throughout the growing season, so there is often very little colour or texture variation across the entire ring. Growth rings in diffuse-porous trees can be difficult to distinguish.

So, it turns out that there is a bit of science to ageing a tree by counting annual growth increments – who would have thought? Close examination is required. For me, that meant sitting in a laboratory looking through a microscope – I must acknowledge and thank the University of Otago Botany Department. They happily let me borrow a lab-coat and gave me access to all sorts of expensive things that I couldn’t remember how to use…

Through a microscope, true boundaries become easier to see. There is an abrupt change in appearance between the last-formed latewood of one year and the first-formed earlywood of the next year. The transition from rapid growth to slow growth in a false ring is much more gradual, and even though my wood was diffuse-porous, the growing environment down here (the definite winters that we have) produced a detectable seasonal difference.

Job done.