Tree rings are fascinating features of the natural world. They act like a history book, telling the story of their environment. If you know how to read them, you can learn so much about the tree and its past.
These circles written in the wood are some of the first things we learn about trees as children. It seems so strange to us then that a plant knows about the years and will even write them down for us!
As we learn the science of tree rings, we find that it really is a strange and fascinating feature of the natural world.
Of course, at Gettysburg Sentinel, we honor the past and the trees that have witnessed it. That makes tree rings special to the work we do.
So let’s look over how tree rings work and what they teach us about the world we live in.
How Tree Rings Form
The Basics of Tree Anatomy
Before we dive into how trees form, we need to go back to the basics.
It might surprise you to find out that trees are not all related. What does that mean? Many species have developed their tree-like features independently.
Meanwhile, others were once trees but aren’t anymore (like strawberries).
Since there is no common ancestor for all trees, it’s better to think of it as a strategy.
What makes up that strategy? Botanists tend to agree that a tree needs to be a perennial plant that has:
roots that go into the earth
a long trunk (really a stem made of wood)
branches that come off that trunk
It’s really important to remember that this is a loose definition, and it will change depending on who you ask.
The center of the trunk is dead material surrounded by the xylem, where water is drawn up to the leaves as their water evaporates. Around this is the cambium — this is where cells are created, allowing the tree to grow.
Around the cambium is the phloem. The sugars created by photosynthesis in the leaves travel down this phloem.
And all that is protected by a layer of dead tissue called the bark.
What Causes Tree Rings?
Trees like to grow upward (what’s sometimes called primary growth), but to hold all that weight, they have to keep getting wider around (or secondary growth).
Through the year, this secondary growth changes is speed:
Early Wood: This comes early in the year, as the name suggests. It tends to be lighter in color because it is less dense.
Late Wood: Later on, during the autumn and winter, less wood is made. What is created is packed in very tight together, leading to its darker color.
As you can imagine, the late wood ends up looking like a thin line that’s much darker than the surrounding wood. And there you go! Now you have rings.
The amount of early and late wood is determined by the local conditions at any given time. During times when there is plenty of water and favorable temperature, there will be much more secondary growth. That makes the rings made in those years much thicker.
On the other hand, hardship like drought and unfavorable temperature might lead to thinner rings as less wood is produced.
What Can We Learn from Tree Rings?
The most commonly known use of tree rings is to figure out the age of a tree. Each dark line of late wood means a return to cold weather. So each of these lines represents a year in the life of the tree.
As mentioned in the beginning of this article, that can actually take us back thousands and thousands of years. How can archaeologists do this? In a given location, the relative size between the tree rings is the same for all trees. So each tree’s set of rings is like a “fingerprint” of their lifetime. If two trees overlap in their lifespan, the rings during that overlapping time will be similar — larger one year, smaller the next, and so on.
If you have samples going back far enough, you can begin to figure out what time period a tree is from by matching the relative sizes of its tree rings. That’s how dendrochronology works!
It turns out, these features can give scientists information about the ancient past. When an old building is made using wood, archaeologists can measure the sizes of the rings, figuring out the exact year the tree was cut down. How far back can they go? Up to 13,000 years!
Because tree rings differ in size depending on the weather, they work as a great measure of the climate. You can see how local conditions changed over time, everything from dry and wet years to major shifts in the local environment.
Today, scientists continue to use this method to understand how climate has changed over time. That gives us multiple insights, like:
how climate might be changing over the course of centuries and millennia
learn general climate variability
figure out how species react to climate stress
This can also help us understand mysteries from the past. For instance, we can use tree rings to observe extreme drought in the 1200s through the American Southwest. That goes a long way to explain why the peoples living in Mesa Verde abandoned their dwellings during that time.
Fire and Disturbance History
For everything they do so well, one thing trees can’t do is move away from threats. And the threats that visit them are all recorded in their tree rings.
Fire scars occur in tree rings as the tree sends sap to cover its wounds. That means you can see what years a tree suffered fire damage.
That also means we can get a better idea of how frequent fires are in an area over a long span of time. This is critical for governmental agencies tasked with managing large swathes of nature reserves.
Knowing the history of wildfires in an area gives scientists information to build fire regimes for managing forests today.
But disturbance goes beyond fire, and tree rings can tell us about all kinds of information. We can discover that many trees of the same species in an area show up around the same time, pointing to potential human intervention. Or we might see some abrupt increase in consistent growth, which could be a sign that humans took an interest in caring for the species.
In 2019, researchers used tree rings to discover that pre-colonial people in the Amazon rainforest were doing extensive management practices in the area.
Why Tree Rings Matter
As we’ve covered, tree rings are an amazing way to learn about our past. But they also open up new insights into how we see our present and prepare for our future.
When we are children, tree rings seem magical. They are a way to connect to the natural world, showing us how nature counts the years. As we get older, we can appreciate just how right we were. Tree rings not only mark the years as they pass, they also keep important information on what happened.
It’s knowledge that we are only beginning to learn how to appreciate.