1. Where Does a Tree Add New Wood?
Just under the bark of a tree there is a thin layer of cells that keep dividing. It is called the cambium. Each year it adds a new ring of wood (the tissue that carries water) on its inner side, and the bark sits outside it [1]. Because the new layer is added under the bark, the newest ring is always the outermost one, and the trunk gets wider each year [1].
So the oldest rings are near the middle, and the youngest are at the edge. A simple way to picture it is a snowman that gets bigger because someone keeps packing fresh snow around the outside. The tree is not made of snow, of course. Its new layer is built by living cells.
2. Why Does One Ring Usually Mean One Year?
A tree does not grow at the same speed all year. In the places where tree rings are easiest to read, growth stops and starts with the seasons, and that leaves visible bands of wood around the trunk. NOAA's education team puts it simply: each ring corresponds to one year of growth [2].
The bands show up because the wood grown early in the season is different from the wood grown late. In many conifers (the pines, firs and other cone-bearing trees), the first part of a ring, called earlywood, has large cells with thin walls and looks light. The last part, called latewood, has small cells with thick walls and looks dark [3]. In these trees, one light band followed by one dark band makes one ring. Other kinds of trees can look different.
That is why people say you can tell a tree's age by counting its rings. For many trees in places with clear seasons this works fairly well, but there is a catch, which we come back to in section 4 [2].
3. What Do Wide and Narrow Rings Tell Us About the Weather?
Rings are not all the same width. A ring records how well the tree grew that year. If the rings are wide, conditions were probably good for growth. Narrow rings may point to drought, disease, or other conditions that were bad for growth [2].
Which weather matters most depends on where the tree lives. NOAA explains that trees that depend heavily on moisture during the growing season tend to have wider rings in rainy periods, while trees limited mainly by temperature tend to have narrow rings during cold periods [4]. In dry regions, such as the US Southwest, rings often record wet and dry years. In some cooler regions, such as high latitudes and high elevations, ring width can help researchers work out past temperatures [5].
A narrow ring is a clue, not a verdict. The tree might have been damaged by something other than weather, such as insects, wind, or competition from nearby trees [5]. Scientists therefore sample many trees and compare the rings with weather-station records before they trust a pattern as a record of climate [4]. Done carefully, this lets them extend climate records back to a time well before weather instruments existed [4].
4. Why Don't Scientists Just Count the Rings?
If every ring were exactly one year, counting would be enough. But the University of Arizona's Laboratory of Tree-Ring Research says that counting rings does not ensure the correct date for each individual ring [3]. Part of the trouble is that a ring can be missing from one side of a tree, and a false band can look like a ring [3]. NOAA gives the same warning: some parts of a tree may have extra rings, and others may be missing rings [2].
The fix is called crossdating. Scientists match the pattern of wide and narrow rings across many samples from the same area, so that each ring can be assigned a single calendar year [3]. The pattern works a little like a barcode: trees that lived through the same years share ups and downs in their rings. Because patterns from dead wood can be matched to patterns in living trees, a record can be pushed back beyond the life of any one tree [4].
The same matching can date old wooden objects. Archaeologists have used the ring patterns in the timbers of buildings to date famous structures, including Mesa Verde in Colorado and the Church of the Nativity in Bethlehem [5].
A very old tree shows what can go wrong with counting. A bristlecone pine nicknamed Prometheus, in what is now Great Basin National Park in Nevada, was cut down in 1964 during a scientific study. Its rings were counted at 4,862, and its age was estimated at about 4,900 years. The park explains that the tree grew in harsh conditions, so a ring likely did not form every year, and the count is probably lower than the true age [6].
5. How Old Can a Tree Get?
Another bristlecone pine in the western United States is nicknamed Methuselah. The New York Botanical Garden says Dr. Edmund Schulman found it in 1957. Scientists took core samples, which are thin pencil-shaped pieces of wood drawn from the trunk without cutting the tree down, and estimated its age at about 4,700 years. To protect it, its exact location is kept secret [7].
Notice the word "estimated." Even for the most famous old trees, the age comes from careful reading and matching of rings, not from a simple tally. That is part of what makes tree-ring science a careful craft rather than a counting game.
Look at a Stump or Slice of Wood Yourself
If you find a cut log end or a stump where it is fine to look, trace the rings with your finger from the bark inward. The outer rings are the newest. Can you find a few that are clearly wider or narrower than their neighbors? Do the narrow ones seem to come in groups? You can look at the rings without being able to say what the weather was, which is the point: scientists need many trees and weather records to know that. Please do not cut down or damage a living tree to look inside.
To read more, the NOAA pages in the sources list below explain how rings are used to study climate [4] [5], and the University of Arizona's Laboratory of Tree-Ring Research explains how rings are dated [3].