Everything You Ever Wanted to Know About Tree Rings

The world around us is full of patterns.  Natural phenomena such as the weather, planetary movements and even our own bodies all have a rhythm or pattern to them. It’s not surprising that people in different places and cultures throughout history have noticed these rhythms and used them in their daily lives.

Consider trees for a moment.  These great organisms show a wide variety of patterns which we can use to learn about past conditions. These rhythms or rings tell us much more than just how old the tree is. Tree rings contain clues as to what the weather was like, if fires occurred and even what the surrounding landscape looked like!

Each year, as trees grow, they produce a ring. Within these rings, tree-ring scientists (or dendrochronologists) can count the number of rings to determine how old a tree is.  A cross section of the trunk would reveal the age of the tree, but doing that for every single tree in an area is quite time consuming! Imagine if you wanted to count the rings of every tree in your neighborhood. Instead, scientists take core samples from trees (usually no more than a couple inches in diameter) and count the rings under a microscope. It is much easier to do this on dead wood rather than cutting down healthy trees!

Acacia tree rings

Tree rings are so important that there is even an entire branch of science devoted to studying them: dendrochronology. (Hence the dendrochronologists mentioned above.)  By looking at the spacing of rings and the thickness of each ring scientists can determine what conditions were like in a particular year. For instance, if the rings are narrow one year and wide the next it means that there was more rain than usual.

Scientists all over the world use tree rings to answer all sorts of questions. They look at the rings of trees near areas where wars have been fought and can determine how intensely those wars were fought by whether the tree was growing or not during those war years. Tree-ring scientists study wood from old structures such as bridges, ships and even entire buildings in order to figure out when they were built.

In addition to looking at the spacing of rings, scientists can also look at the color of a tree ring. In conifers such as pines and spruces, there is a layer in each ring where the cells contain no green chlorophyll (the molecule which plants use to turn sunlight into sugar). This is called a ‘line of weakness’. The lack of green cells makes that section look darker than the rest. Usually, the line is easy to see and scientists can simply count how many dark lines appear in a particular year.

But what about trees such as oak and birch which don’t form these visible lines? For these species scientists use another method called ‘needlecasts’. A ‘needlecast’ is a group of microscopic structures called tracheids which grew when the tree was stressed. The rings formed by tracheids do not have lines of weakness and so cannot be counted like other trees, but they can tell scientists about past climates.

What kinds of environmental stress might cause these groups of tracheids to grow? Droughts are the most common cause, but scientists have also found that low temperatures or high winds can cause needlecasts. Knowing how conditions were in the past may help us understand what climate change could be like in the future.

Often times it is not drought or cold which causes a tree to form these groups of cells, but fire! These are called ‘fire scars’ or ‘light rings’. If a tree is growing in an area which had fires every year it would form many of these light rings. But if there was less frequent burning the number of light rings would decrease.

Fire scars are not only found on trees which are still alive, but also on long dead stumps! By looking for these scars, scientists can determine how frequently a particular area burned.

How does this help us understand the past? Dendrochronologists have found that fire frequency was lower before humans started to regularly burn areas to improve hunting and farming. In some places, controlled burns are done today in order to reduce the intensity and frequency of wildfires and it is important to understand how frequently these fires burned in the past.

Dendocrhonologists have also been able to learn about human history by studying old trees. In the American Southwest they have found trees which were growing at the time of Columbus’ arrival in North America. These trees had narrow rings from 1493 to 1591, a period of war and famine known as the ‘Little Ice Age’.  By looking at cross sections of dead stumps it is possible to determine how old they are and sometimes even their cause of death. All in all, trees which lived for hundreds of years provide us with a lot of interesting information!

Adam Zavesky is a research technician at the University of Arizona’s Laboratory of Tree-Ring Research. The lab was founded under the direction of A.E Douglass in the 1920’s and has been run by scientists of the University of Arizona ever since.

The lab studies tree rings from all over the world in order to look at how changes in climate affect trees and other aspects of nature such as animals, plants and even humans! Recent subjects have included the 2003 blackout in North America, forest fires on Vancouver Island.

All in all, there are many things that can be learned by studying the rings of trees! And because trees are living things they can help us understand not only what happened in the past, but also provide insight into the future.

The University of Arizona has a webpage with more information about the lab and some of their latest research! You can check it out here:   http://www.ltrr.arizona.edu/