When you see a big, graceful tree in the landscape, you might wonder how old it is. Because there’s so much variety in both height and girth among different trees, you can’t really tell without a bit of counting and measurement. Get your math skills out and learn how to tell how old a tree is.
To measure the age of a living tree, there are a few different processes, most of which involve literal measurement with a good, old-fashioned tape measure or even your arms. None is foolproof, but each can provide a reasonable estimation.
How To Tell How Old a Tree is Without Cutting It
1. Tape measure for growth factor
Wrap your tape measure around the tree’s trunk at around 3.3-4.5 feet from the ground. The measurement you get is the tree’s circumference. Next, you’ll need to calculate the diameter. Do this by dividing the circumference by Pi (3.14). Then, you can multiply the diameter by the tree’s growth factor.
A list of growth factors for difference species can be found on the internet; Treehugger have a good one, but of course it doesn’t account for all species of tree.
2. Tape measure for girth
If you don’t know the growth factor of a particular tree, you can estimate its age using the girth (circumference again). Wrap your tape measure around the trunk at about 3 feet from the ground and measure to the nearest centimetre (0.39 inches, awkward I know!).
In general, one year’s growth is represented by every 2.5cm (just under an inch) of growth, so you can get a rough idea of the tree’s age by dividing the girth in centimetres by 2.5. So, a tree with a girth/circumference of 60 centimetres is about 24 years old.
If you don’t have a tape measure handy, or you’d just like to hug a tree (I can highly recommend it), you can also estimate a tree’s age by the number of hugs it takes to go all the way around it.
Your arm span from fingertip to fingertip is about equal to your height. If you know how tall you are, you know how wide your hug is.
Count how many hugs (wide, outstretched arms) it takes you to go all the way around the tree and multiply that number by your arm span. There you have the tree’s circumference. You can then use that figure for either of the two methods above.
4. When measuring won’t give you an answer
Sometimes, trees can be much older than they look. Certain practices can change a tree’s growth habit such that it grows more quickly or more slowly, and they can also mask the usual signs that you’d look for to determine the tree’s age.
Coppicing is an ancient method of woodland management which involves repeatedly harvesting the wood from suitable trees by cutting them at ground level. This results in a regrowth of new stems and can provide a sustainable and renewable supply of wood for simple building purposes — fencing, wooden products, and basic furniture — and charcoal manufacture.
Because coppicing removes stems and branches at an average rate of every eight years, coppiced trees never attain the height or girth of their unmanaged counterparts but may still attain significant ages. Coppicing can actually increase the life span of a tree by mimicking its natural shedding of branches, and is an excellent practice to improve woodland biodiversity.
Trees planted in hedges — natural, living barriers made of woven live branches — may be much older than they look. Because hedges are maintained, cut, and re-laid (re-woven), the trees that make them up don’t have the opportunity to grow very tall or very wide.
Hedges are very difficult to age as they’re usually composed of different types of tree, but a rough method is to count the number of species in each hedge. For every tree or shrub species you find, add 100 years. So a hedge with 4 different species may be around 400 years old.
Bonsai is the ancient Japanese art of growing miniature trees from seedlings or cuttings. Their growth is restricted by careful pruning and the size of their pots or, in rare cases, the natural conditions in which they grow (or are grown) in the ground.
While even the largest bonsai trees may be not more than 80 inches tall, they may be 1000 years old or more. Bonsai is usually only possible through the dedication of true masters who devote their lives to their art. (Although bonsai trees are beautiful to look at, I’m not sure how I feel about them from an ethical point of view.)
How to Tell How Old a Tree is by its Rings
If the tree has already died, its age can be estimated using dendrochronology: the process of dating a tree by counting its rings. It is also used in living trees, but the process is not without its risks, as I’ll discuss below. Archaeologists use dendrochronology to help them learn the ages of artefacts and structures with a wooden component.
Dendrochronology, or Tree-Ring Dating
To estimate a tree’s age by counting its growth rings, you need a flat, even cross-section of the tree’s trunk. In this round slice of tree, you’ll be able to see a number of rings from the center to the outer edge, dark alternating with light.
As a tree grows, it puts on new layers of wood each year. Spring growth is lighter; trees grow more quickly in spring, and as the cells that make up this spring wood are larger, they appear lighter in color. Wood grown in the summer or even the early autumn looks darker. At this time of year, trees grow more slowly, and the cells in the wood are smaller.
If you count all of either the light or the dark rings, you’ll get an approximate idea of how old the tree is. However, there are a few caveats!
Not all years are the same. It’s possible for a tree to have more than one growth cycle in a year if the weather was particularly favorable; it’s also possible for it to miss a growth cycle due to drought, another environmental event, or if the tree was damaged in some way during that year.
It’s also possible to count the tree rings on a living tree, although many arborists feel that the process is too invasive. Using an increment borer, a scientist can take a core sample of the tree: a thin cylinder of wood right from the bark to the tree’s center. This allows for the rings to be counted without the tree having to be cut down, but as it can introduce pests, parasites, and diseases into the tree, it’s often not recommended.
What Else Can You Learn from Tree Rings?
Dendrochronology can provide fascinating insights into the life of a tree beyond just an estimation of its age. The shape of the growth rings, their distance from one another, any anomalies in their color: all of these provide a window into the environmental conditions while the tree was growing.
There could be evidence of fire, another tree leaning on the tree being measured, or infection. An investigation of tree rings can give an overview to the climatic conditions in any particular year.
And you might think that it only works if you count back from this year, because if you don’t know your end point, what good is a number of rings going to be? But archaeologists and dendrochronologists maintain databases of tree rings from known periods in time, and often they can match new samples to those by comparing evidence of significant events in the wood.
To give an example, if you know there was a huge fire in 1771 in the area your tree was growing, and you can see evidence of that in its rings, then you can pinpoint that year and work back (and forward) from there.
If scientists need a more precise measurement than a tree-ring date can give for a dead tree or piece of wood, the next option is radiocarbon dating. This method, which can be used for any carbon-based substance that had its origins in something alive, measures the amount of carbon-14 (or radiocarbon) in the sample.
Every living organism exchanges carbon-14 with the atmosphere around it. But (normally) when a living being dies, this exchange stops. Radiocarbon has a half-life of 5700 years, plus or minus 30. This means that, every 5730 years or so, half of it disappears. By knowing how much carbon-14 a sample has currently and working backwards, scientists can discover how long ago the tree, plant, animal, or person died.
This method assumes that radiocarbon levels in living organisms today are the same as radiocarbon levels were in living organisms however many years ago. It won’t work on very recent material or on material older than about 50,000 years due to radiocarbon’s rate of decay, but 50,000 years is a decent span, especially for wooden artifacts!
Restrictions to Success
Although radiocarbon dating is used extensively to date wooden items, there are some problems with it, and even with this method, no date will ever be exact. There have been, and can be, variations in the amount of radiocarbon in the atmosphere; this will affect how much radiocarbon an item possessed during its lifetime. This can be mitigated in part by comparing values to those from samples with a known age.
There’s also the issue that a tree’s heartwood (the wood at its middle) will have a much older age than its sapwood (the wood at its outer edge). Because a tree does not stop exchanging carbon-14 with the atmosphere when it dies, trees are radiocarbon-dated by their rings. As each ring stops exchanging carbon-14 once it’s laid down, rings laid down first will be older than those laid down later. That can skew the date of the tree’s death and so the measurement of its age.
Still, radiocarbon dating is one of the most useful tools that scientists and researchers have for determining the age of carbon-based organisms, trees included.
How Old is That Tree?
The age and ancient status that trees can attain captivates us all. To look at a 5000-year-old yew tree, or to consider that there’s a Quaking aspen tree in Utah that could be 12,000 years old (more on that in our article How Long Can Trees Live): those amounts of time seem almost unfathomable to us. Imagine having been alive at the end of the last ice age: what amazing and incredible history you would have witnessed since then.
Trees do seem almost immortal to us sometimes. Because they can’t speak for themselves, we often forget that they have needs to, and that, if their environments become too inhospitable, they won’t be able to reach the mind-blowing ages that we associate with them.
Look after the trees in your area and the environment in general. Remember that, just like you, they need healthy food, clean air and water, and just the right amount of sunlight to live well and to live long. Treat trees like you’d treat your friends and family, and hopefully we’ll all go on living for as long as we can.
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Kira Nash lives with her family in the sunny French countryside amidst bees and swallows. A writer, editor, and artist by trade, she also teaches creative meditation. She’s passionate about nature and ecology and tries to live as green a life as possible. In her spare time, she surfs, reads, and plays with her cats, although not usually all at once. She loves tea a little too much.