How to Measure a Tree

Diagram of circumference, height and average crown spread

When traveling over the State of Maryland, you will find a great variety of tree species growing. The importance of trees, not only in our landscape but in our very way of life, is being recognized by more people every day. Residents of the state show a universal interest in “Big Trees”, and this interest was first recorded by State Forester Fred W. Besley in 1925 when he compiled the “Noted Tree List” for Maryland.

Fred Besley, our first state forester, aroused interest in big trees across the state, and over the years collected measurements and photographs of distinguished trees of the state. Each year brought new nominations, but in order to complete the list of all the native species, a more intensive search was necessary. In 1925, interest in large and unusual trees was stimulated by a Big Tree Contest. Rules were adopted and prizes were offered to secure as many entries as possible. Each tree species was classified separately so the small trees like dogwood would not have to compete with larger trees such as oaks and maples. At that time, there were no standard rules to measure trees. In order to ensure fair comparisons of trees, certain measuring procedures had to be decided upon. Mr. Besley devised the following standards that are still used today:

Circumference in inches + height in feet + ¼ of the average crown spread = total points

Illustration showing the steps for measuring a tree.The circumference, or girth, is obtained using a flexible tape measure. It is usually taken at a point on the trunk 4.5’ above the ground. However, if the tree has a smaller circumference below 4.5’, due to a burl or protrusion, or because of a fork in the tree, the circumference is taken below 4.5’ at the narrowest point on the trunk. This measurement is recorded in feet and inches, but then converted to total inches for the first part of the formula.

The height, or how tall the tree is, is obtained using a clinometer or laser hypsometer. This is the most difficult measurement to obtain, and the volunteers who measure trees for the Maryland Big Tree Program will usually take a number of measurements from different locations and average them. The average height is then recorded in a whole number and added to the formula.

The crown spread, or distance from branchtip to branchtip, can be done either with the flexible measuring tape or laser hypsometer. Two measurements are taken; the first measuring the greatest crown spread, and the second at 90 degrees from the first measurement. In an open area, this is an easy measurement to take, but in heavy brush, or in small yards when the tree branches hang over into the neighbor’s yard, it becomes more difficult and less accurate. The two measurements are added together and divided by 2 to obtain the average. The average is then recorded in the formula. However, when calculating the total points, the measurer must remember to divide the average crown spread by 4 to obtain ¼ of the average.

 

Example:

A white oak has a circumference of 17 feet, 6 inches. Multiple 17’ x 12” + 6” = 210”.
It has a height of 83 feet.
The longest crown spread is 120 feet. The crown spread at 90 degrees is 96 feet.
120’ + 96’ = 108’.

Formula: Circumference (120”) + Height (83’) + ¼ of average crown spread (108’ x ¼ = 27) = 320 points

In this example, the white oak would be considered an “average” big tree. The State Champion currently (2010) is at 392 points. We have over 30 white oaks in Maryland larger than 320 points.

While obtaining the height of the tree is the most difficult (and the most inaccurate) measurement, obtaining the circumference has the most special rules and exceptions. One particular rule to remember is that a tree with heavy vines growing up the trunk cannot be accurately measured until the vines are cut and/or removed. For obvious reasons, trees with poison ivy vines should not be attempted until those vines are removed.

The Maryland Big Tree Program, sponsored by the Maryland Association of Forest Conservancy District Boards, provides a free service to measure big trees in Maryland.