Tree girth measurement facts for kids
Tree girth is a way to measure how thick a tree trunk is. Think of it as wrapping a tape measure around the tree! It's one of the oldest and easiest ways for foresters (people who study and manage forests) to find out how big a tree is and how much it has grown. Different countries have slightly different rules for how to measure it. A fun use for this measurement is to compare amazing trees from different places or of different types.
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How We Measure Tree Girth
Girth is the distance all the way around a tree trunk. It's measured straight across from the trunk's center. In the United States, it's usually measured at "breast height." This means about 4.5 feet (1.4 m) (or 1.37 meters) above the ground. Other parts of the world might measure it at 1.3 metres (4 ft 3 in), 1.35 metres (4 ft 5 in), 1.4 metres (4 ft 7 in), or 1.5 metres (4 ft 11 in).
The "breast height" rule has been used for a long time because it's easy. Tree trunks often get wider at their base, like a bell. This wider part is called "buttressing" or "flare." Sometimes this flare goes up the trunk a little, and sometimes it goes up a lot. But for consistency, the measurement is still taken at the standard height. If the tree flares out above this height, another measurement might be taken higher up.
Tree girth is often measured for "champion tree" programs. These programs find and record the biggest trees. Other things measured include a tree's height, how wide its crown (the leafy top) spreads, and its total wood volume. Groups like American Forests use a special formula to give trees "Big Tree Points." A tree gets 1 point for each foot of height, 1 point for each inch of girth, and a quarter point for each foot of crown spread. The tree with the most points for its type becomes the champion!
Trees with the Biggest Girths
One tree known for its huge girth was the Glencoe Baobab in South Africa. It had a diameter of about 52.2 feet (15.9 m) near the ground. This means its girth was around 164 feet (50 m)! Sadly, this hollow tree split in 2009.
The Árbol del Tule in Mexico is another famous tree. It has a girth of 119.8 feet (36.5 m) and a height of 116.1 feet (35.4 m). Its crown spreads 144-foot (43.9 m) wide! Because this tree is very wide at its base, some people think it's actually several trunks that grew together.
The General Grant Tree in California is clearly a single-trunk tree. It has a girth of 91.2 feet (27.80 m) measured at 4.5 feet (1.37 m) high. Old stories sometimes talk about trees with even bigger girths. But these measurements might have been taken at ground level, including the wide base, or from trees that were actually many trunks joined together.
Single-Trunk vs. Multi-Trunk Trees
When measuring trees for science or champion lists, it's important to tell the difference between a single-trunk tree and a multi-trunk tree. Two smaller trunks growing together can get a huge combined girth much faster than one single trunk.
A single-trunk tree has only one central core (called a pith) if you were to cut it at ground level. A multi-trunk tree would have two or more piths at ground level. It doesn't matter if the trunks have grown together or if they came from the same root system. If there's more than one pith at ground level, it's a multi-trunk tree. It's important to keep these measurements separate to have good information.
How to Measure Girth Directly
Measuring girth usually means wrapping a Measuring tape around the trunk at the correct height. Even though it sounds simple, mistakes can happen. The most common mistake is mixing up single-trunk and multi-trunk trees.
Sometimes, tree trunks have bumps, hollows, or low branches. If these are at the 4.5 feet (1.4 m) measurement height, they can make the girth seem bigger than it is. In these cases, the measurement should be taken at the narrowest spot below the bump or branch. The height where you took the measurement should always be written down.
If a tree is leaning, you measure the girth at 4+1⁄2 feet (1.37 m) along the trunk's axis. This means you measure along the side of the trunk from its center base. The tape should be at a 90-degree angle to the trunk.
For very large trees, like some sequoias, measuring at 4.5 feet (1.4 m) from the center base can be tricky on a slope. The tape might even go below ground on the lower side! In these cases, it's better to measure 4.5 feet (1.4 m) above the ground on the higher side of the tree. Always note this in your measurement description.
For very short trees, like those found on mountaintops, a 4.5 feet (1.4 m) measurement might not make sense. For these trees, measuring the girth at 1 foot (0.30 m) above the base might be better. The main idea is to measure at the standard height when possible. If it's not meaningful, take an extra measurement at a better spot and write down that height.
It's best to record the raw girth number. Converting it to a diameter (how wide the tree is) can make the tree seem bigger than it is. You can always convert it later if needed.
Measuring tree girth is a great way for students to learn about nature. It helps with understanding measurement, numbers, and simple math. You can even use simple techniques to guess a tree's age!
Measuring Girth from a Distance
Sometimes, tree girth can be measured without touching the tree. This is called "remote measurement." It can be done using special telescopes or even photographs. In these cases, you measure the tree's diameter (width) from a distance. Then, you multiply the diameter by pi (about 3.14) to get the girth.
A monocular w/reticle is like a small telescope with a built-in ruler. You can look through it to measure how wide a tree is from far away. You also need a laser rangefinder to know how far away the tree is. This method can be very accurate, especially if the monocular is on a tripod.
You can also use photographs to figure out a tree's girth. You need to know the distance from the camera to the tree and have something of a known size in the picture (like a ruler or a person). With this information, you can estimate the tree's diameter and then its girth. This method can even be used to estimate the girth of trees in old historical photos!
Trees with Many Trunks
Multi-trunk trees are very common. Often, these are separate trunks growing from one root system. This can happen if the main trunk was damaged, and new shoots grew from the original roots. Even if they are genetically the same, they should be measured differently from single-trunk trees.
These multiple trunks often grow together at the base but split higher up. If they are separate at breast height, you can measure each trunk individually. If they have grown together at breast height, you measure their combined girth. Always write down how many trunks are included in the measurement.
Trees with Unusual Shapes
Not all trees have simple, straight trunks. Some have very unusual shapes because of how they grew or because of their species. These can include trees that are actually many separate trees joined together, trees that have fallen over, or trees with large roots growing in the air.
Because these trees are so unique, it's often hard to measure them in the usual way. The best approach is to write a detailed description of the tree. You can take any measurements that make sense, like its height or the area covered by its trunks and crown. Taking GPS locations and lots of photographs is also very important. Photos help others understand what the tree looks like.
For example, a "clonal colony" like the Pando aspen is a huge group of genetically identical trees that can cover many acres. For these, you would measure the total area they cover and the size of the largest individual trunk. For "Banyan-like trees," which also have many trunks spread over a large area, you might measure the area covered by all the trunks and the tree's height. The main goal for these unusual trees is to document their special features.
Sometimes, a tree's base is so complex that a simple tape measure won't show its true size. For example, the Tule tree in Mexico has a huge, irregular base. While a tape measure showed a diameter of 38 feet 1.4 inches (1,161.8 cm), a more accurate measurement of its wood area showed an effective diameter of only 30 feet 9 inches (937 cm). Scientists have even used special mapping techniques to create 3D models of these complex tree bases.
How Girth Changes Over Time
To track how a tree's girth changes, you need to mark the exact spot where you measure it each time. This makes sure you are always measuring in the same place. Trees usually grow slower in diameter as they get older. However, they can still add a lot of wood volume. For example, some old white pine trees can add about 11.9 cubic feet (0.34 m3) of wood each year! Even very old trees continue to grow, adding to their height and girth.
