The years of trees can be known by counting the circles inside their trunk. We haven’t revealed a great mystery to you, have we? We all know that’s how it works. Every year corresponds to a circle, then in short, one plus one plus one and we know how long the tree has been alive. But wait a minute, is that really all there is to it? Definitely not. Just as within the years of our life we can find all the traces of what we have experienced, this is also the case with trees.
Inside the circles you can read much more about its age, you can understand how the tree lived, what climates it had to overcome. In short, a series of events that lead us to have a much more accurate chronology.
The system that this study brings to light is called “dendrochronology” and today we are going to look at it a little more closely!
DENDROCHRONOLOGY
Let’s start with the name. “Dendrology” is far from a simple term; it encompasses many extremely complex concepts. Don’t worry, we’ll delve into the core of this subject to help you better understand it without getting lost in the details.
The word “dendrology” originates from the Greek words: “dendron” meaning tree, “chrono” meaning time, and “loggia” meaning study. This clarifies its primary purpose: studying the progression of a tree’s life cycle. Therefore, dendrology is a dating system.
This system was established by Andrew Ellicott Douglas in 1906, but actually, it was our nation’s genius—Leonardo da Vinci—who first observed and understood the patterns in tree rings.
Dendrology is not only applied to botany but also to other fields such as archaeology. In the early 20th century, it was used to study the relationship between solar activity and Earth’s climate, and years later, it was used to analyze the importance of fire in the North American ecosystem.
DENDROCHRONOLOGY IN TREES
Earlier we underlined that this system does not only concern the years lived by the tree. But what else? First of all it gives us rather specific references on the humidity and heat of a specific period.
Thicker tree rings indicate favorable climatic conditions (abundant rainfall, suitable temperatures), resulting in abundant harvests. In such cases, cell production is higher, and the tree rings are more prominent. Conversely, very thin or unusually thick tree rings indicate unfavorable climatic conditions, with trees experiencing stress. By studying these rings and comparing them with those of other trees in the same region, it is possible to reconstruct climatic trends for a specific period.
Using this method, Douglas was able to reconstruct the evolution of solar activity and weather fluctuations over approximately 2000 years. This pioneering research has been continued in recent years, now extending back 8000 years.
In trees, tree rings can also reveal whether the tree grows at an angle (if it does, the rings are thicker on one side than the other). They also help us understand tree type; the thickest part of the trunk in gymnosperms points downwards, while in dicotyledons it points upwards.
Furthermore, in some species, there is a color difference which is divided into:
- Heartwood
- Darker color shade due to some substances produced by the plant which have the task of defending the tree from fungi and bacteria.
- Sapwood
- Outer rings characterized by a lighter color.
This chromatic difference is especially noticeable in very long-lived trees.
GROWTH RINGS
The increase in size of a tree occurs in both height and width and is marked by the production of new wood which, depending on the period of formation, takes on a well-defined color. The lighter wood is that formed during the spring months while the darker one indicates the autumn periods. With the arrival of winter the production of new cells will stop and when spring resumes the ring given by the contrast between the previous dark wood and the new light wood will emerge.
The tree structure is made up of various levels which are:
- Bark
- Book
- Edit
- Sapwood
- heartwood
- Marrow
This subdivision can help us better understand how the tree works. Let’s take a closer look at what happens in the various parts.
THE bark it is the part that we all know, the outermost side of the tree and it is also the first ring. In reality it is not a vital part of the tree and has the sole purpose of defending it from the climate and from parasite attacks.
Immediately under the bark is the bookalso known as phloem. It is very thin and has the task of limiting the humidity that comes from outside. The sap that comes down from the leaves flows inside the book.
Inside the edit instead we find the area where the new wood is created. This is precisely the point where the new rings are born. Every year new fibers develop creating the sapwood or book.
THE’sapwood (xylem) we’ve seen this before. It is made up of plant cells between which the raw sap travels, i.e. the one that rises from the roots towards the leaves. However, this function is not maintained for long, as air bubbles are created inside it which prevent the regular passage of the lymph. For this reason only the younger layers carry on the process while the older parts are absorbed by the heartwood.
The heartwood it is also known as the “heart of wood” and is comparable to the skeleton of the human body. In fact, being the thickest and most resistant part of the trunk, it has the task of keeping the plant straight. Furthermore, it is also the only wood that can be used in industry, being more resistant, hard and compact.
Did you know that a lot of work was hidden behind the bark?
THE PITFALLS
Oh yes, from what has been said it would seem that dating a plant is quite simple: just count the rings, right? No.
We often tend to think that the oldest ring is the external one, but this is not the case. The ring that forms first is precisely the one that resides in the center of the trunk. Then, as the years pass, new circles form from the bark, compressing the existing ones. For this reason, some years will be difficult to count, as the pressure will have made the older rings very thin.
There are also fake rings. A false ring occurs if the tree experiences very strong stress (attack by parasites, prolonged drought), loses its leaves prematurely and then puts out new shoots with new leaves. This leads to counting the passage of two years in a single year, as the plant has completed its complete cycle. These rings are completely similar in appearance to the others and can only be distinguished through cross-dating.
Likewise, some rings may be missing or incomplete. They will always be missing due to strong stress that causes the tree not to complete its cycle. While in the case of major lesions to the meristem (plant tissue whose cells retain the ability to create new cells) the rings will be incomplete.
AREAS OF INTEREST
As we mentioned, dendrochronology has many areas of interest and this helps to understand the importance of this dating method.
In general, dendrochronology is present in the following fields:
- Dendroclimatology
- Reconstruction of temperature during past eras. It is effective up to 1000 years from the present date.
- Archeology
- Dating of archaeological finds;
- Possibility of calibrating the dating obtained with radiocarbon, which allows the correction of errors resulting from Carbon 14 in the atmosphere. Errors caused by the fact that the concentration of Carbon 14 was not constant over time, but rather its density varied depending on solar activity.
- Artistic diagnostics
- It allows us to trace the dating of the supports of the paintings made of wood, in this way it is possible to understand the historical period during which the work was created.
DENDROCHRONOLOGY AND LEONARDO DA VINCI
This is the second article that we thought of as a tribute to Leonardo da Vinci’s five hundredth anniversary. We said it at the beginning: he was the first to observe the relationship between tree rings, atmospheric precipitation and life years.
To understand the time leap, just think that Douglass came to use this dating in 1906, while da Vinci’s intuition arrived in the 15th century. The gap in knowledge between the two eras is immense and only underlines the uncommon intellect of a historical figure who has already proven himself countless times.