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