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Why can you count the years in the rings of a tree?

The fact of being able to infer the age of a tree at the time it was felled , by counting the rings that we see on its trunk, is a knowledge that has almost been incorporated into popular culture ; or at least that’s how I learned it. Being such a well-known curiosity, why in many cases do we not know what its explanation is? In other words, why do trees show us their age inside their trunk?

First of all, it is important to know roughly the different elements that are part of the section of a log:

  • The outermost and darkest circumference corresponds to the crust . A double layer, formed by the inner cortex, formed by living cells, and the outer, by dead cells. In its innermost part, we find the phloem , a layer of cells responsible for transporting organic compounds, created during photosynthesis, throughout the plant.
  • Next, the vascular cambium is a thin reddish layer that is responsible for creating new cells , both inward (xylem) and outward (phloem), allowing the tree to increase its thickness each year (secondary growth).
  • The next layer, whiter, is the sapwood or xylem, it corresponds to the youngest wood of the tree and is responsible for transporting water and nutrients upwards, from the roots to the branches and leaves.
  • Next, of an increasingly darker color, we find the heartwood . They are dead cells that were once sapwood. It is the part of the trunk that is used as wood. Being so hard and resistant, it is the part in charge of providing support to the tree.
  • Finally, in the central part of the trunk we find a small dark area called the medulla . It is made up of living cells, responsible for transporting essential nutrients and distributing them towards the periphery through the medullary rays, whitish streaks that emerge from the center towards the periphery of the trunk.

Now that we know how to recognize the different parts of the trunk section, let’s see how the rings are formed. Do you remember the vascular cambium? This is the layer of cells responsible for the creation of the rings (formed by xylem) and, in fact, it does not create just one per year, but two: one lighter, during spring, and another darker, during late summer. and early fall. Each of them represents a growing season of the tree and its thickness will depend on the amount of water available to the tree. Thus, the cambium will create cells of greater diameter during spring, when water is abundant, giving rise to the clearest rings. On the contrary, during the summer and early autumn, when water is scarce, narrower cells will be created, corresponding to the darker rings. As the tree grows in thickness, the rings will be printed inside the trunk.

In addition, tree rings can also have scars, which in most cases are due to the impact of lightning, but also to blows or even infections. In the latter case, we may observe what is known as a knot in the trunk.

As a curiosity, there is a science called dendrochronology , which corresponds to the study of tree rings to obtain information on climates of times past through the life history of the tree, such as variations in rainfall, printed on the thickness of the trees. lighter rings. So the trees show not only their past, but everything that lived around them.

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