The title of this post is intentionally misleading. The picture I have chosen encourages misdirection, as at this point you are likely thinking about counting the growth rings as a well-known ways to determine the age of a tree, at least in temperate climates. That is good, as that is where I would like to start.
But, it is not where I am planning to be at the end of this short piece.
Why does the wood of a tree show growth rings? In temperate climates, trees lay down wood each year is a cyclical pattern, starting with the more wider-tubed spring wood and then the more thick-walled summer wood. This more or less follows the tree’s need to get moisture up into the leaves and other photosynthetic tissues early in the season, when it can also make use of stored reserves to serve as energy sources that the building blocks of its growth. Later in the season, when photosynthesis is cranking along and the tree has more, new food resources available to it, it can invest more in structural elements such as the thick-walled conductive tubes in the summer wood xylem.
And the tree shuts down for the fall and winter, to start up again the next spring.
The alternation from last year’s summer wood to this year’s spring wood is often readily apparent. And, because the cycle repeats on an annual basis but with each individual growth ring reflecting the growing conditions of that year, each growth ring also serves as a record of that year’s growth. The width of the growth ring can be interpreted as a measure of how vigorously the tree grew during that year – something of a visible diary of how much the tree prospered.
That is one part of the story.
Another important thing about trees is the fact that, particularly as they get older, most of the tree is no longer living. Much of the living parts of the tree are in the outer parts of the tree, which is simply draped over an interior tree that is no longer living. This inner tree consists largely of the wood that is left from when the tree was smaller, which, in turn, was also draped over a tree that was also much smaller back then. The elements of this interior tree are no longer physiologically active, at least in terms of containing still-living tree cells. This interior tree has also become disconnected from the outer, still living tree, in the sense that there is no longer a transfer of materials from living cells from the outer trees to this supportive but non-living inner tree.
To return to growth rings, this happens because, as each new growth ring is added, deeper into the tree an older growth ring is being “retired”. The hollow tubes that were once part of the transport system of the tree, carrying water and nutrients from the roots up to those parts of the tree active in photosynthesis, are now lined with chemicals that will resist attack by insects and fungi (think of the rich aromatic smell of cedar heartwood), while the tubes and the whole inner heartwood of the tree is flooded with water to reduce the amount of available oxygen. It is helpful to the tree to keep this inner heartwood intact, as it still functions in physically holding the aboveground parts of tree aloft – which is key to this tree getting the sunlight it needs. But, there are no living parts of the tree still within that inner tree that participate in the life processes of the tree.
Note – this does not mean that there are not living fungal cells, bacteria or other types of living cells within this inner tree. Just not living cells from the tree itself.
Looking at the above photo, it is easy to see where the interior, non-living growth rings of the oak in the photo are easy to pick out from the outer growth rings that were still physiologically active when this tree was cut down. We see this difference because the sapwood in the outer growth rings had been actively participating in the life process of this tree. Cells in the sapwood were engaged in carrying water and nutrients up to the more aerial parts of the tree, including all of those leaves and buds that are what allows the tree stay alive and keep growing. Within the sapwood, as well, the tree has been maintaining an active bank of stored starches and sugars that the tree could tap into when for its energy needs.
That is why the sapwood in this cross-section of the felled tree appears discolored in the way that it does. These stored carbohydrates are supporting the growth of fungi now that the tree can no longer defend itself from this sort of activity.
If you count the growth rings of the tree in the above photo, you will see that the tree was about 100 years old, in the conventional sense, when it was cut down. A closer look at those growth rings will show that there were about 15 layers of sapwood and 85 layers of heartwood at that point in its life.
So, could when also say that this tree was only about 15 years old when it was cut down, based on the age of its oldest layer of sapwood at the time? I think one could but, that would be an awfully narrow, teasing and pedantic thing to say, if that is all one wanted to say. But, I think there is a more important point that encourages bringing the point up.
If a tree is constantly rejuvenating itself, and if the oldest ‘living’ part of the tree is only 15 years old, as I am suggesting in this case, how does a tree ever grow old? Do trees ever grow old?
Those, I think, are interesting questions – ones I do not have the space to go into here but which I hope to return to later on.
July 4, 2023
