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Extension > Environment > Trees and woodlands > Sampling and measuring timber in the private woodland

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Sampling and measuring timber in the private woodland

Charles R. Blinn
Thomas E. Burk

Copyright © 2002 Regents of the University of Minnesota. All rights reserved.

Introduction

Knowledge of the quantity and quality of timber on a private woodland can be extremely useful to the landowner. Timber measurements provide information on the status of the tract and are an integral part of any forest management plan. They also are useful for prescribing management treatments, such as thinning, that improve the overall health and vigor of the forest and its quality for recreation, wildlife habitat, and other uses.

This folder presents guidelines for sampling and measuring trees in a woodland. It is not meant as a substitute for assistance from a professional forester. It will, however, help you understand and implement proper procedures for sampling and measuring trees.

Before you begin

If you have little or no experience in woodland management or timber measurements, seek the assistance of a professional forester.

The Minnesota Department of Natural Resources (DNR) Division of Forestry provides "on-the-ground" forestry advice and assistance to owners with holdings of less than 1,000 acres. Upon written request, a trained forester will, at no charge, examine your woodland, prepare a management plan to help you achieve your ownership goals, estimate timber value, prepare timber sale bids and contracts, and provide market advice and assistance. For a nominal charge, a DNR forester will mark or designate timber to be harvested, measure and scale products (veneer and sawlogs, pulpwood, firewood, poles, posts, etc.) to be harvested, and designate timber sale boundaries. DNR foresters will not examine or appraise timber strictly for sale purposes, for settlement of an estate, or for settlement or evidence in litigation.

You may contact a DNR forester by calling the appropriate telephone number listed for the Natural Resources Department under the "Minnesota state offices" listing in your telephone book or by writing to: Private Forest Management Supervisor, Minnesota Department of Natural Resources Division of Forestry, Box 44, 500 Lafayette Rd., St. Paul, MN 55146.

Many of the larger forest industries in the state also provide forest management, timber measurement, and marketing assistance to private woodland owners. In addition, there are a number of consulting foresters who, for a fee, provide a wide range of land management services, including timber harvesting and marketing assistance. Fees are usually a percentage of the gross receipts of the sale. You can obtain an updated list of consulting and industrial foresters through your local DNR forestry office.2

Estimating volume of standing trees

To determine the volume of wood in a timber tract, you must first estimate the tree diameter, merchantable height, and defect percent of a representative number of trees by sampling ("cruising") the tract. Using appropriate tree volume tables and expansion factors, you can then calculate the volume for each species in the tract. Each of the factors required for estimating the volume of standing trees is discussed below.

You'll need an accurate measurement of the size of your woodlot to obtain reliable volume estimates. Stand acreage may be computed from information obtained during a traverse or land survey. Woodlot acreage may also be available through a local governmental office, such as a tax assessor.

A word of caution: timber cruisers need experience in several different timber tracts to produce consistently reliable results, so you may want to get professional help to obtain accurate volume estimates.

Sampling your woodland

To determine the volume of wood in your tract, you may evaluate each tree individually, or you can set up sample plots and use these to estimate overall volume.

Measure and evaluate each tree individually in tracts of five acres or less, or that contain highly valuable material such as black walnut veneer. To estimate merchantable tree volume, measure tree diameter, merchantable height, and percent defect for every tree in the tract using methods described later in this publication, and then add the individual tree volumes.

The simplest method of estimating the volume of timber in a larger area is to estimate the volume on a sample of the forest and then apply an expansion factor based on the area of the tract and the area sampled.

An accurate volume determination may be obtained by taking tree height, diameter, and percent defect measurements on a sufficient number of 1/20th-acre (radius 26.33 feet) circular sample plots. Sample one plot per acre in tracts of 30 acres or less. In larger tracts, sample 24 plots plus an additional plot for each 5 acres in the woodlot. (As an example, in a 37-acre tract, you would sample 32 [= 24 + (37/5)] plots.) This sampling intensity will usually provide estimates within 10 to 20 percent of the actual woodlot volume.

Sample plots should be well distributed throughout the tract to obtain a representative estimate of volume. Plots should not be located in what appears to be an "average" part of the tract. Broad distribution of plots is especially critical in tracts that are not homogeneous (i.e., tree size, species composition, and tract density vary). One way to obtain a broad distribution of plots is to locate plots at equally spaced distances along parallel compass lines. Make sure the lines run perpendicular to any major geographic, physiographic, or cultural features in the woodland.

The following formula can be used to determine the distance, in feet, between lines and plots: Forumla for distance between liens and plots

Where:

D = Distance (feet) between lines and sample plots on a line
A = Total number of acres in the tract
n = Number of sample plots

As an example, if there are 37 acres in the tract and 32 plots are sampled, the distance between lines and sample points along a line would be 224 feet.

An example of plot layout for the 32 sample plots in a 37-acre rectangular woodlot is shown in Figure 1.


Figure 1. Sample plot layout for 32 plots in a 37-acre woodlot.

Establish a scheme for laying out sample plots before you begin to cruise the woodland. When laying out a large area to sample, start from a known point, such as a property comer. With a compass, pace or measure with a tape to the first sample plot center. Starting at a natural landmark such as a dead or unusual tree, proceed in a clockwise direction, recording measurements for all merchantable trees within 26.33 feet of the plot center. If possible, you may wish to have one person measure trees while another individual records information. Repeat this process for each sample plot.

Professional foresters frequently utilize aerial photographs to assist them in laying out sample plots. Through the use of aerial photographs, the woodlot is broken down into homogeneous (uniform) forest types. Fewer sample plots are then necessary in each forest type to arrive at an estimate of volume within a desired degree of accuracy.

Estimating tree diameter

Tree diameter is the most important measurement of standing trees. Diameter is measured at 4 1/2 feet above the ground (foresters refer to this as the diameter at breast height, or DBH) on the main stem. DBH's are usually measured to the nearest 1/10-inch and recorded by one-inch classes. As an example, trees with a DBH between 5.6 inches and 6.5 inches would be assigned a value of 6 inches when information is recorded.

Source: Stott, C.B. 1953. Techniques for using unpainted steel diameter tapes. USDA For. Serv., Upper Darby, PA. 8 pp.

Figure 2. Correct method of holding diameter tape for measuring tree DBH.

A steel diameter tape, calibrated to permit direct tree diameter readings, is frequently used to measure tree DBH. Wrap the tape around the tree at breast height perpendicular to the lean of the tree (Figure 2), standing on the uphill side of the tree. If there are branches or other projections at DBH, place the tape above them. If a fork appears below breast height, consider the tree to be two separate trees, recording separate measurements for each tree.

If a diameter tape is not available, measure tree circumference by wrapping a cloth tape or string around the tree at breast height. Divide the resulting circumference by 3.14 to determine DBH.

Estimating merchantable tree height

Merchantable tree height, the usable length of the tree, is the distance between stump height (generally one foot) and the limit of utilization in the tree. The limit of utilization may occur at a main fork, at a serious defect such as a hollow or a point of decay, where excess limbs or forks occur, or at a minimum usable top diameter. Merchantable tree height is measured in eight-foot lengths, called half-logs or bolts. For pulpwood, bolts are often referred to as sticks.

Figure 3. Major products that can be produced from a tree include veneer logs, sawlogs, pulpwood, and firewood.

Merchantable product height depends on the product to be made from the tree (Figure 3). Local market opportunities may limit production of all products; for example, in northern Minnesota most timber is marketed as pulpwood.3

The usual minimum top diameter (inside bark) for sawlogs and pulpwood is the larger of 50 percent of tree DBH or eight inches for sawlogs and four inches for pulpwood. Veneer trees must have a top diameter (inside bark) of at least 10 inches. When estimating top diameters, it is helpful to picture the bottom of a pail as representing the approximate top diameter for sawlogs and the bottom of an oil can as the approximate top diameter for pulpwood.

Figure 4. Estimating merchantable tree height with a graduated stick.

You can estimate merchantable heights using a stick or piece of lath with markings at four-inch intervals (Figure 4). Each mark represents one eight-foot bolt when the stick is used as follows:

  1. Stand 50 feet from the tree center such that the tree does not lean toward or away from you.

  2. With the stick in hand, extend your arm out 25 inches from your eye. Hold the stick in line with the trunk of the tree being measured.

  3. Determine the point where utilization is limited in the tree.

  4. Sight along the bottom of the stick to stump height (i.e., one foot). Move your eyes, not your head, and read the stick measurement that corresponds to the limit of utilization point. Record the number of bolts between the stump and the merchantable height.

If you are unable to get a good sight on the tree from a distance of 50 feet, stand 25 feet from the tree and divide the resulting height measurement in half.

Sample forms for recording DBH and merchantable height for pulpwood and sawlog trees are contained in Tables 1 and 2.

Table 1. Sample tally form for recording number of pulpwood trees by DBH and number of eight-foot bolts per tree.

Tract location: __________________________________________
Tract name: ____________________________________________
Date: __________________ Species:________________________
Plot number: ____________________

DBH (inches) Number of eight-foot bolts
1 2 3 4 5 6 7 Totals
5
6
7
8
9
10
 
 
 
 
 
Totals

Table 2. Sample tally form for recording number of sawtimber trees by DBH and number of eight-foot bolts per tree.

Tract location: __________________________________________
Tract name: ____________________________________________
>Date: __________________ Species:________________________
Plot number: ____________________

DBH (inches) Number of eight-foot bolts
1 2 3 4 5 6 7 Totals
10
11
12
13
14
15
 
 
 
 
 
Totals

The dot-dash tally method is a convenient way to record timber cruise data. Each dot or dash represents one tally tree with a specified DBH and merchantable height according to the following convention:

dot-dash tally method

As an example, a dot indicating one sample tree would be placed in the upper left-hand comer of Table 1 for a tree with a DBH of five inches and one eight-foot bolt of merchantable height. Separate tally forms are frequently used for each species and sample plot.

Estimating tree defects

Many trees contain defects-branches, crook, decay, bird pecks, etc.-that may make them unusable, depending upon the type of product. It is important to consider defects when measuring timber.

If the entire tree is unsuitable because of an excessive amount of defect, do not measure it. For usable trees containing some defects, note the percent of defect by product. This percent is then deducted from the overall estimate of tree volume. Because tree diameter decreases with increasing height in the tree, a defect occurring near the top of the tree will require a smaller percentage deduction than a similarly occurring defect near the base. Percentage deductions for tree defects are best estimated by a professional forester and recorded either directly in an extra space in the table or along the margins.

Estimating individual tree volumes

Tree volume tables are used to determine volume from DBH and merchantable height measurements. It is important that you be aware of the specifications (stump height, top diameter, bolt length, etc.) used in constructing the table to ensure that you use the table appropriately.

Volume tables generally are specific for each type of wood product. Volume estimation for various products is discussed below.

Sawtimber and veneer trees

Trees that are large enough and of high enough quality to produce logs that can be sawed into lumber are referred to as sawtimber. To qualify as sawtimber, trees should have at least one eight-foot bolt, should be at least 10 inches DBH, and should have a top diameter inside bark that is the larger of either eight inches or 50 percent of tree DBH. Minimum sawlog top diameter inside bark for a tree with a DBH of 20 inches is therefore 10 inches. (These diameter specifications are typical; individual buyers may have different specifications.) Sawtimber trees must not contain too many defects such as excessive branchiness, decay, scars, bulges, bark distortions, holes, branch stubs, and crookedness.

Individual trees of many species (black walnut, sugar maple, elm, red oak, white oak) that are of exceptional quality, have at least one eight-foot bolt, are at least 16 inches DBH, and contain bolts that have a top diameter inside bark of at least 10 inches often can be sold as veneer trees. Logs harvested from these trees will be sliced or peeled to produce veneer. Such trees are more valuable than sawtimber.

The basic unit for estimating volume for both sawtimber and veneer trees is the board foot. A board foot is a piece of wood of any shape that contains 144 cubic inches of wood (12 inches X 12 inches X 1 inch, 6 inches X 6 inches X 4 inches, etc.).

The Scribner log rule is one method for estimating sawlog volume in Minnesota. Table 3 describes tree volumes obtained by applying the Scribner rule to the logs in trees of various sizes. As an example, Table 3 indicates that a tree with a DBH of 22 inches and 32 feet (four bolts) of merchantable height will yield approximately 290 board feet. If there were 10 percent defect in the tree, total tree volume would be reduced to 261 board feet. A reasonable range for board foot volume estimates per acre is 1,000 to 15,000 board feet.

Table 3. Tree board foot volume (Scribner rule) to a variable top diameter based on the number of eight-foot bolts in the tree.*

DBH (inches) Number of eight-foot bolts
1 2 3 4 5 6 7 8 9 10
10 20 30 50
11 20 40 60 80
12 40 60 90 110 140
13 50 70 100 130 160
14 80 110 140 180 210
15 90 120 160 200 230 270
16 100 140 170 220 260 300
17 110 150 190 240 290 330
18 140 180 230 290 340 400
19 150 200 250 310 380 440 470
20 170 230 300 360 440 510 550
21 250 320 390 480 550 600 630
22 290 370 450 540 630 680 720
23 310 390 480 590 680 730 780
24 350 450 550 660 760 820 870
25 380 480 570 710 820 880 940
26 420 530 660 790 910 980 1040
27 440 560 690 840 970 1040 1110
28 490 630 770 930 1070 1150 1220
29 520 660 810 980 1130 1220 1300
30 570 730 900 1080 1240 1340 1420
8From Burk, T. E., T. D. Droessler, and A. R. Ek. 1986. Taper equations for the Lake States composite volume tables and their application. University of Minnesota, College of Forestry, Department of Forest Resources Staff Paper Series.

Board foot volume is assumed to be the gross scale above a one-foot stump to a top diameter inside bark that is the larger of either eight inches or 50 percent of the tree DBH. The standard bolt is assumed to be eight feet in length. Volumes outside the tabulated range may be estimated by applying the following formula to each eight-foot bolt and then summing values to provide tree volume estimates:

Bolt volume (board feet) = 0.395 d2-d-2
Where d = small end inside bark diameter of the bolt, in inches

There are several other log rules used in different parts of the United States. Some purchasers of veneer quality trees (especially buyers outside Minnesota) may buy material on the Doyle rule. These buyers may either remeasure each tree to be sold using the Doyle rule or apply a conversion factor to change Scribner volumes to Doyle volumes.

Pulpwood

Trees that are too small or too poor in quality to be sold for sawlogs are often sold for pulpwood. Ultimately, these trees are chipped or ground up to manufacture products such as paper, hardboard, and various types of structural board. A large number of tree species growing in Minnesota can be sold for pulpwood. However, in some areas of the state, it is difficult to find buyers for certain species because of the long distance to the nearest mill that uses the species.

Minimum DBH for pulpwood trees is 5 inches. Minimum top diameter inside bark is the larger of either 4 inches or 50 percent of tree DBH. Minimum pulpwood top diameter inside bark for a tree with a DBH of 12 inches is therefore 6 inches.

Figure 5. A standard cord measures eight feet long, four feet high, and four feet wide and contains 128 cubic feet of wood, bark, and air.

The basic unit for estimating pulpwood volume in trees is the cord. A standard cord is the equivalent of a stack of wood eight feet long, four feet high, and four feet wide (128 cubic feet) (Figure 5). The wood content (excluding bark and air space between stacked pieces) of a cord varies from about 65 to 95 cubic feet depending on the diameter, roughness, and crookedness of the pieces. An accepted average value in the Lake States is 79 cubic feet of wood per cord.

Table 4 provides tree volume estimates for pulpwood. According to the table, a tree with a DBH of 10 inches and 24 feet (3 bolts) of merchantable height will yield approximately 0.10 cords. If the tree contained 10 percent defect, tree volume would be reduced to 0.09 cords. A reasonable range of cord estimates per acre is 5 to 45 cords.

Table 4. Tree volume in rough cords to a variable top diameter based on the number of eigh-foot bolts in the tree.*

DBH (inches) Number of eight-foot bolts
1 2 3 4 5 6 7 8 9 10
cords
5 .01 .02  
6 .02 .03 .04 .05
7 .02 .04 .05 .06 .08  
8 .05 .06 .08 .10 .11 .14
9 .06 .08 .10 .12 .14 .16 .19
10 .07 .10 .12 .15 .18 .21 .24
11 .11 .14 .18 .21 .25 .28 .
12 .14 .18 .22 .26 .30 .35
13 .16 .20 .25 .29 .34 .35
14 .19 .24 .29 .35 .41 .47  
15 .21 .27 .33 .39 .46 .53  
16 .25 .31 .38 .46 .54 .62 .69  
17 .28 .35 .42 .50 .60 .68 .76  
18 .31 .40 .49 .58 .69 .78 .88  
19 .35 .43 .53 .63 .75 .85 .95  
20 .39 .49 .60 .72 .85 .96 1.08  
21 .53 .65 .77 .91 1.04 1.16 1.29
22 .59 .73 .87 1.02 1.16 1.31 1.45
23 .64 .78 .93 1.10 1.25 1.40 1.56
24 .70 .87 1.03 1.22 1.38 1.56 1.73
25 .75 .93 1.10 1.30 1.48 1.66 1.84
26   .83 1.02 1.21 1.43 1.62 1.83 2.03
27 .88 1.08 1.28 1.52 1.72 1.93 2.15
28 .96 1.18 1.41 1.66 1.88 2.12 2.36
29 1.02 1.35 1.48 1.75 1.99 2.23 2.49
30 1.10 1.35 1.62 1.90 2.16 2.43 2.71
*From Burk, T. E., T. D. Droessler, and A. R. Ek. 1986. Taper equations for the Lake States composite volume tables and their application. University of Minnesota, College of Forestry, Department of Forest Resources Staff Paper Series.

Volume is standard unpeeled cords and includes the stem wood above a one-foot stump to a top diameter inside bark that is the larger of either 4 inches or 50 percent of the tree DBH. Careful piling of harvested bolts is assumed, equivalent to 79 cubic feet of wood or 92 cubic feet of wood and bark per cord. The standard bolt is assumed to be eight feet in length. Volumes outside the tabulated range may be estimated by applying the following formula to each 8-foot bolt and then summing values to provide tree volume estimates:

Bolt volume = 0.0003(d2 + D2)
Where:
d = small end inside bark diameter of the bolt in inches
D = large end inside bark diameter of the bolt in inches

Estimating total woodlot volume

After estimating individual tree volumes, you can calculate the total volume of your woodlot. Use the following formula to estimate total tract volume from 1/20th-acre sample plots:

Where:
V = Estimated tract volumne
v = Sum of individual tree volumes
A = Total number of acres in the tract
n = Number of sample plots

As an example, in a 37-acre tract in which you sampled 32 plots, you would multiply the sum of the individual tree volumes by 23.125 [ = 20 X (37/32)] to estimate the total tract volume. If the sum of tree volumes from the 32 sample plots was 35 cords, the estimate of the total tract volume would be 809 cords (35 X 23.125).

Glossary

Board foot–A piece of wood of any shape that contains 144 cubic inches of wood (12 inches X 12 inches X 1 inch, 6 inches X 6 inches X 4 inches, etc.).

Bolt–A timber measurement term for a tree section that is 8 feet in length, also known as a half-log.

Circular plot–A sample plot with a known radius inside which all trees are measured.

Cord–A standard cord contains 128 cubic feet of wood, air, and bark. It is the equivalent of a stack of wood eight feet long, four feet high, and four feet wide.

DBH–Diameter of a standing tree at breast height; measured at 4 1/2 feet above the ground on the uphill side of the tree.

Merchantable height–Usable length of the tree measured as the distance between stump height (generally one foot) and the limit of utilization in the tree. The limiting point of utilization may occur at a main fork, at a serious defect such as a hollow or a point of decay, where excess limbs or a fork occurs, or at a minimum top diameter.

Merchantable tree–Tree that is at least five inches DBH and has a diameter (inside bark) at the top of the bolt that is the larger of either four inches or 50 percent of tree DBH.

Pulpwood–Wood used to produce pulp. Pulpwood is usually wood that is too small, of inferior quality, or of a species that is not used in the manufacture of lumber. Pulpwood trees should have at least a five-inch DBH and a diameter (inside bark) at the top of the bolt that is the larger of either four inches or 50 percent of tree DBH.

Sawtimber–Trees that are large enough and of high enough quality to produce logs that can be sawed into lumber. Sawtimber trees should have at least a 10-inch DBH with a diameter (inside bark) at the top of the log that is the larger of either eight inches or 50 percent of tree DBH.

Top diameter–Small end diameter of a stick, bolt, or log.

Veneer tree–Tree that is large enough and of high enough quality to produce logs that can be sliced or peeled to produce veneer. Veneer trees should have at least a 16-inch DBH and a diameter (inside bark) at the top of the log of at least 10 inches.

Approximate conversion factors

1 cord = 500 board feet
1 cord = 79 cubic feet of solid wood
1 acre = 43,560 square feet

1 The authors are, respectively, extension specialist/assistant professor and assistant professor, Department of Forest Resources, College of Forestry, University of Minnesota, St. Paul, MN 55108. The authors wish to acknowledge the assistance of T. D. Droessler and A. R. Ek in developing the timber volume tables used in this folder.

2 A listing of consulting and industrial foresters and DNR forestry offices appears in the Minnesota Forest Products Directory, a cooperative project of the Minnesota Extension Service and the Minnesota Department of Natural Resources, Division of Forestry. Copies may be purchased from: Extension Store, University of Minnesota Extension Service, 20 Coffey Hall, 1420 Eckles Ave., University of Minnesota, St. Paul, MN 55108.

3 For more information on marketing timber and estimating its value, obtain: Marketing Timber From the Private Woodland (FO-2723). Copies are available from Extension Store, University of Minnesota Extension Service, 20 Coffey Hall, 1420 Eckles Ave., University of Minnesota, St. Paul, MN 55108. Specific questions about the properties and uses of native woods can be addressed to: Extension Forest Products, Department of Forest Products, College of Forestry, University of Minnesota, 2004 Folwell Ave., St. Paul, MN 55108.

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