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The Maryland Envirothon
Forestry 2010 Resource Site

This is an online study guide for advisors and students participating in the Maryland State Envirothon competition. It is not inclusive of all content you may need to know for the state competition. Web sites provided are for reference only and may not be exactly what students are tested on.

Forestry - Measurements

Like many other disciplines, forestry is a science based on measurements. While participating in the Envirothon program, you will learn to use the same instruments and collect the same data that professional foresters use to learn about and manage our forest resources. Many students enjoy the forestry section of Envirothon because it is very “hands on”. Becoming proficient with basic forest measurements is very important, because many of the more complex measurements require accurate forest data collection.

Learning Objectives:

At the end of this section, you should:

  • Understand why measurements are important in forestry and understand which tools are used to obtain specific measurements.
  • Demonstrate proficiency in “pacing” to measure distances and determine how many paces you have in a chain (66 feet or 19.8 meters).
  • Demonstrate proficiency in the use of the following forestry tools:
  • Diameter Tape
    - Biltmore Stick/Merritt Hypsometer
    - Clinometer
    - Wedge Prism
  • Conduct a sample plot as part of a forest inventory using forestry instruments
  • Apply data to specific charts and tables to determine forest growth conditions.
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Tree diameter

Tree height

Determining the number of logs or sticks in a tree

Determining the volume of a tree

Determining tree age

Determining Site Index

Determining the firewood volume of standing trees

Determining the basal area

Determining the stocking level

Determining stocking level using a table

About sample points


The most basic forest measurement is pacing or counting your number of steps to determine how far you’ve traveled in the woods. A compass will help you determine which direction you are walking, but pacing allows you to determine distance.

In forestry, distance measurements are based on a chain, which equals 66 feet. Many years ago surveyors literally dragged a 66-foot-long chain around with them to measure properties, which were measured in chains and links.

Today, foresters measure chains by knowing how many steps they take in 66 feet (19.8 meters). To determine your pace, measure out 66 feet (19.8 meters) using a 100-foot (30 meter) measuring tape, and count every other step (for example, every time your right foot hits the ground). Most people have between 12 and 15 paces per chain.

  • 80 chains = 1 mile (1.61km)
  • 10 square chains = 1 acre (very helpful in determining the size of wildfires!)
  • Several forestry tools are calibrated to be accurate at one chain.

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Tree Diameter:

Diameter Tape Tree diameter is an important measure of tree growth, especially when combined with additional measurements such as the height and age of a tree (such as determining the volume or site index of a tree). Diameter is always measured on the uphill side of a tree at 4.5 feet (1.3 meters) up the trunk or Diameter Breast Height (or DBH for short) using a diameter tape (or d-tape for short).

The first thing you’ll notice about the d-tape is that the inches look a lot longer than usual (3.14 inches or 7.97cm to be exact). This is because the d-tape is calibrated in “diameter equivalents of circumference by inches and tenths of inches” saving you the trouble of converting circumference into diameter. Remember that diameter measurements should be expressed to the nearest tenth of an inch (for example 11.7” or 29.7 cm). The reverse side of the tape shows feet calibrated in inches and tenths of inches.

Measure Tree at DBH Measure to nearest of an inch

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Tree Height:

Fig.1: Top right - Tree height; Top left - % of slope; left - Cross-Hair extended by optical illusion. Tree height is measured using the principle of triangulation with a clinometer. Of all the forestry tools you will use, the clinometer requires the most practice and skill. Assuming that the tree grows at a right angle to the ground (even on a slope), we use the clinometer at 1 chain (66 feet or 19.8 meters) away from the tree using the following steps:

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Determining the number of logs or sticks in a tree:

Trees are renewable resources that are used for thousands of products we use every day. To determine the volume of a tree, we must first know how many logs or sticks are in the tree. A log is a unit of measurement equaling 16 feet (4.8 m). For sawtimber (trees large enough to be cut into lumber at a sawmill), logs are measured from the stump of the tree to an 8” (17.9 cm) diameter top (about the size of a paint can). For pulpwood (trees that will be ground up into chips at the mill to produce paper pulp) we measure the number of 8-foot (2.4 m) sticks from the stump to a 4” (10.1 cm) diameter top.

The instrument used to measure logs and sticks is the Merritt Hypsometer, which can be found on one side of the wooden Biltmore Stick. The Biltmore Stick, or forester’s yardstick, is named after the first forestry school at the Biltmore Estate near Ashville, North Carolina.

The Merritt Hypsometer works much like the clinometer, but it’s much easier to use: Standing one chain away from the tree.

Merritt Hypsometer

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Determining the volume of a tree:

The board foot. Once we know the diameter, height, and the number of logs or stick a tree contains, we can determine the volume of the tree. This is an important number, especially when determining the value of the tree if it is sold for forest products. Just like in geometry, we are essentially determining the volume of a cylinder (tree trunk). In forestry, volume is expressed in board feet, which is an imaginary chunk of wood 12” (30.4 cm) X 12” X 1” (2.5 cm) thick.

To make measurements easier, foresters often use volume tables to quickly compute volume. For eastern forests we use the International ¼ inch rule volume table, because it provides the most accurate volume measurements for the size classes of trees we have in Maryland. Some Biltmore Sticks have volume tables on them; because you are measuring a standing tree, make sure to use the tree scale side of the stick, not the log scale side. For the Envirothon competition, a volume table or scale stick will be provided.

Volume Table

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Determining tree age:

Another important measurement is tree age. Not only is counting the number of growth rings important to determine the tree’s age, but interpreting the spacing of growth rings can tell us much about its life. Examining the growth of a tree can tell us many things about the growing conditions about the site, or environment the tree is growing in.

Growth rings on pine tree crosscut.

A tree's age, when compared to its diameter or height, can be an excellent indicator of how productive a specific site is for growing a particular species of tree, or can be an indicator of stocking (or how crowded the trees are).

Because trees are many times older than we are, growth rings serve as a “history book” of the tree and its surrounding community. Droughts, wet seasons, injuries, and even forest fires can be reflected in tree growth rings.

On standing trees, age is determined by using an increment borer, which is a threaded hollow drill that is turned by hand into a tree’s trunk until the borer reaches the center of the tree. The increment core, a pencil-thin sample of wood, is extracted from the trunk, showing the tree’s growth rings and ring spacing. Although this boring does not usually harm forest trees, it is not recommended for urban or other “specimen” trees. During the Envirothon competition, a sample increment core or the tree’s known age will usually be provided.

Increment borer.

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Determining Site Index:

Determine Site Index. The productivity of a site, or forest community depends on many factors. Soils, climate, aspect (compass direction the slope faces), and other physiographic factors influence the tree’s ability to thrive and produce wood. In North America, site index is the most common expression of site quality, and is based on tree growth patterns at a specific age, which for eastern forests is 50 years of age.

Because each species of tree has different growth requirements, each species has its own site index. Many decades of researching species-specific growth patterns of trees has yielded site index tables, or curves for each major species of tree.

For example, using the site index table, if you measure a dominant (typical of the largest in the stand) yellow poplar and find it to be 30 years old and 58 feet high, the site index is 70. By age 50, dominant yellow poplars grown in uncrowded conditions on that site could be expected to grow 70 feet high. A relatively low height of 58 feet may suggest that other species found on the site should be favorably considered in thinning, harvesting, or future reforestation.

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Determining the firewood volume of standing trees:

The cord. Many of us heat our homes with firewood because it is an inexpensive and renewable resource. Because we want to wisely use our forest resources, poorly formed trees that may not be usable as lumber, and large branches left after harvesting may be used as firewood. The volume of firewood is expressed in a cord, a stack of firewood measuring 4 feet (1.2 m ) wide X 4 feet high X 8 feet ( 2.4 m) long. Firewood is sold by the cord, and in Maryland can only be sold by the cord or half-cord. Firewood sold by the stack, pick-up load, bundle is illegal and regulated by the Maryland Department of Agriculture.

To determine the firewood volume of standing trees, foresters use a cord volume table.

Foresters Use a cord volumetable to determine the firewood colume of standing trees.

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Determining basal area:

Using the wedge prism is very important to remember that the instrument must always be held directly above the 'plot center' stick for accuracy. Once we’ve learned how to take measurements on individual trees, we will now look at the characteristics of the forest community. One important measurement is determining the basal area, or level of tree stocking on a particular site. Basal area is a measurement of the cross-sectional area of a given tree stem (or trunk) expressed in square feet at DBH (4.5 feet or 1.3 m). The basal area of a forest stand is the sum of the basal areas of individual trees, and is expressed in square feet per acre.

Determining the basal area of an individual tree requires a d-tape and a calculator. The formula is:


The wedge prism. Foresters use a surprisingly simple, yet innovative instrument to determine the basal area on a specific site or sample point. The wedge prism is a small piece of glass that has been ground to refract light rays at a specific offset angle, which creates an “optical illusion”. Technically speaking, in most eastern forests, we use a 3.03 diopter prism ground to an angle of 104.8 minutes. Most importantly, this means that tree that is measured or tallied is equal to 10 square feet of basal area, so we are using a wedge prism that has a basal area factor of 10 (BAF = 10).

When using the wedge prism, it is very important to remember that the instrument must always be held directly above the “plot center” stick for accuracy.

The “optical illusion” the wedge prism creates (as a result of the angle it is ground) appears to “offset” a portion of the tree’s stem or trunk when viewed, preferably at DBH. If the offset portion viewed through the prism appears to connect with the main stem of the tree, you will “tally” that tree as “in” or “countable”. If the offset portion appears completely removed from the main stem of the tree, do not “tally” that tree as it is “out” or “not countable”. For trees that appear to be “borderline” or on the edge, simply tally every other tree. The following diagram illustrates this measurement:

The borderline.

After determining the number of “in” or countable trees, as well as any every other “borderline” tree, simply multiply that number by 10 to determine your basal area (because you are using a 10 BAF prism). For example, if you have 8 “in” trees, your basal area is 80, or you have “80 square feet of basal area per acre”. If you think about it, an acre (.4 hectares) contains 43, 560 square feet, while in this example, only 80 square feet is actually occupied by tree stems, or trunks. The branches and crowns of the trees, as well as other smaller vegetation occupy the remainder of a forested acre.

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Determining the stocking level:

In addition to basal area, another important piece of data is the stocking level, often expressed in “trees per acre”. Nobody has time to measure off an entire acre (.4 hectare) and count every tree, so we again depend on a sample point to give us representative number of the stand or property we are collecting forest data on.

To complete this measurement, we begin at our “plot center” and measure out 26 feet (7.8 m) in each of the cardinal directions (north, south, east, and west) and flag the circle boundaries. This circle is equal to 1/20th acre. We then count all of the trees within this circle that are greater than 2” (5.08 cm). We then calculate the trees per acre by multiplying that number of trees by 20. In the example below, 13 trees within the circle multiplied by 20 indicates there are 260 trees per acre.

120th Acre Circular Plot.

An important tip: Although each measurement is sometimes conducted on the same sample plot, don’t confuse basal area with the 1/20th acre tree count. When using the wedge prism above “plot center”, measure all of the trees as far as you can see (there may be a huge tree in the distance that is “in”). When counting trees in your 1/20th acre plot, count only those within the radius of your circle. During the Envirothon competition, each of these measurements would be conducted on a separate plot to avoid confusion…

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Determining stocking level using a table:

After we determine the trees per acre by conducting a 1/20th sample acre plot, we can combine this data with the basal area of the site to determine whether the forest is understocked, fully stocked, or overstocked. This information is very important to the forester when making forest management recommendations and deciding which silvicultural practices to implement based on the landowner’s objectives (remember that nearly 76% of Maryland’s forest land is owned by private landowners). A chart like the one shown below is used to determine stocking level.

Determine stocking level.

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About sample points:

Foresters often conduct a stocking level measurement on the same sample point where they collect basal area information, site index, volume measurements, tree age, height, and species composition data. This information, when combined with numerous other sample points (one sample point may be conducted for every five acres) gives a clear, statistically accurate picture of the forest community being studied. Sample points are designated based on a grid pattern (for example: 7 chains x 7 chains if you wanted to complete a sample plot every 5 acres) while the property is being mapped in the office. This ensures statistical accuracy and eliminates bias (and sometimes causes you to have to collect data in wetlands, on steep slopes, and thick brush!). Conducting sample points involves bringing all of your forestry skills (compass reading, pacing, tree identification, and proficiency in using instruments) together!

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