A 2”x6 “which is a 120 year old tree?
Some of the 2x6 lumber I purchased recently weighed differently when I picked it up and here in this picture you can see why.
But firstly, when we want to understand how old a tree is, we count the rings in a cross section of the trunk. To identify each ring, we look for the contrast between the dense cells (phloem) and the more open and larger cells of the xylem. The combination of these two shows the amount of growth the tree experienced in one year.
In the lower piece of wood one can see a dark spot in the middle which is the original stem (Year 1). Then, if we count the rings radiating from this dark centre, they are quite clearly wide for the first 26 years accounting for 2 inches of the tree’s width.
However, after 25 years, the rings suddenly become very narrow and dense so that the next 1/2” expansion of the wood takes 33 years! So the tree was, at a minimum, 60+ years old when felled.
Why is there such a dramatic slowing of growth after 25 years? This can be for many reasons, but the important observation is that the tree took 60 years to ‘provide’ a timber of only 6” width.
The top piece of wood in the photo shows a much older tree with annual rings so close together it’s difficult to count them. The 1-1/2” vertical cross section shows 70 years’ growth! And so, for this tree to reach the 6” width of the 2x6, it would need over 200 years – 200 years to produce a 2x6!
This tree was growing very slowly because of very difficult growing conditions and/or a very short growing season. Trees in regions like this have difficulty surviving their first early years while they struggle to establish their root system and so we cannot expect a full-grown tree for 2 to 300 years after a clear cut. This not only leads to high losses for the replants but to a very slow economic return for the investment and the loss of those trees to the environment’s recovery.
And when we clear cut an area, the young trees lose the support of other trees around them and are subject to the extremes of summer heat and deep winter cold.
The reason I noticed the difference in the two pieces of wood was weight – the older tree is much heavier and denser having taken in more carbon dioxide and turned it into wood fiber. Conversely the younger tree has much larger air spaces in between the xylem cells and less carbon stores.
When we include the environmental benefits of keeping older trees alive into the economic analysis (trees store much more CO2 as they age), then cutting down the old trees – especially in areas to the north – does not make sense.
Often the 300 year old specimens can’t be replaced even in 500 years from now. Suzanne Simard’s recently released book ‘Finding the Mother Tree’ is a wonderful way to understand why cutting older mother trees does not make sense on any level.