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When and How Much to Thin

Many landowners are unsure about when to thin their timber and which methods to use to estimate how much timber should be removed. The techniques for determining when to thin and how much to remove are not difficult to use and can be learned quickly.

When to thin often depends on the owner's interests. If your objective is to grow pulpwood only, then thinning may not be the appropriate step to take. On the other hand, if you are growing a stand for multiple products or for sawtimber, thinning may be the fastest way to obtain a marketable timber crop. Thinning should pay for itself and, in most cases, provide the owner some revenue for this effort.

One way of estimating when to thin is by keeping track of the ``live crown ratio'' of trees in the stand, defined as the height of the live crown (the part of the tree with live branches) divided by the total height of the tree. When the average live crown ratio falls below 35 to 40 percent, the stand should be thinned. For example, if the average height of your stand is 40 feet and the average height of the live crown is 14 feet, then the live crown ratio is 35 percent and the stand would be a candidate for thinning soon.

Another way you can determine if your stand needs thinning is to measure the stand density by estimating the basal area per acre. Basal area is simply the area in square feet taken up by an individual tree trunk at DBH (diameter at breast height); basal area per acre is the sum of these individual values for all the trees growing in 1 acre.

Figure 1: Basal area of a tree.

To avoid measuring a large number of trees over several acres, foresters use a tool called the wedge prism to provide an accurate estimate of the basal area per acre in a stand. You can construct a similar tool to estimate basal area per acre by following the steps outlined in Figure 2.

Figure 2: Constructing a tool for estimating basal area per acre in stand of timber.

To use the tool, go to a part of the tract that represents the average conditions in the stand. Place the tip of the tool against the bridge of your nose and sight in on the first tree that is plainly visible to you. As you sight in on the tree, check to see if the trunk (at 4.5 feet up the tree) is larger or smaller than the square on the end of your stick (See Figure 2). If it is smaller, turn until you sight in on the next tree and follow the same steps. If it is larger, count that tree ``in'' and turn to sight in on the next tree.

As you sample the trees, be sure to stay in the same spot. Simply turn around in a tight circle, keeping your body in the center. Turn around the spot completely until you have sighted in on every tree you can clearly see from that spot. Then, count up the number of trees that were ``in'' (larger than the wedge on the stick) and record the total number of ``in'' trees you counted at the measurement points.

After you have counted all the ``in'' trees at one point, move to another spot in the stand at least 50 steps away and follow the same procedure at that spot. Take measurements at 10 spots in the stand and add up the number of ``in'' trees you counted at the measurement points.

Figure 3: Using a basal area calculator to estimate the basal area per acre in a stand of timber.

If you counted 120 trees or more as being ``in'' (that means that the basal area per acre is at least 120), you should thin the stand. If the number is less than 120 but more than 100, you will want to thin the stand only if the trees are large enough to make the thinning profitable. If you counted fewer than 100 trees, you will want to wait a while to thin the stand.

If the stand needs to be thinned, you must decide how many trees to remove. A common approach is to thin the stand down to some specified basal area per acre, usually between 70 and 90 square feet per acre. Table 1 details the number of trees to leave per acre for three stand density levels and several different stand diameters. Highly productive sites support more trees and could be thinned to a basal area of 90, while poorer sites are often thinned to a basal area of 70.

Table 1: Number of trees per acre required to reach specific stand density at a given average stand diameter.
Mean Stand Diameter (inches outside bark @ DBH) Desired Number of Trees per Acre for Basal Area of:
  70 80 90
6 360 410 460
7 260 300 335
8 200 230 360
9 160 180 205
10 130 145 165
11 105 120 135
12 90 100 115

Table 2 gives the average cord volume in a single loblolly pine based on its total height and DBH. You can use this information with that provided in Table 1 to estimate how much volume per acre will be taken out during the thinning.

Table 2: Merchantable rough cord volume and green wight per tree for loblolly pine planted in Piedmont. W.M. Harrison and B.E Borders, 1996. Yield prediction and growth projection for site-prepared loblolly pine plantations in the Carolinas, Georgia, Alabama and Florida. PMRC Tech. Rep. 1996-1. Warnell School of Forest Resources, The University of Georgia, Athens. Green weight is to a 3-inch top and includes bark. One cord equals 5400 pounds or 2.7 tons.
Diameter (inches at DBH) Total Tree Height (feet) Units
  20 30 40 50 60 70  
6 0.01546 0.02385 0.03250 0.04134 0.05033 0.05946 cords
  83.507 128.816 175.496 223.221 271.794 321.082 pounds
7 0.02145 0.03314 0.04519 0.05749 0.07000 0.08269 cords
  115.811 148.983 244.003 310.431 378.001 446.537 pounds
8 0.02820 0.04363 0.05950 0.0751 0.09220 0.10891 cords
  152.307 235.619 321.324 408.852 497.861 588.119 pounds
9 0.05534 0.05534 0.07548 0.09605 0.11696 0.13817 cords
  193.054 298.827 407.606 518.675 631.603 746.100 pounds
10 0.04409 0.06827 0.09313 0.11851 0.14432 0.17048 cords
  238.078 368.651 502.910 639.978 779.924 920.595 pounds

For example, if you had a stand containing approximately 650 trees per acre, with an average DBH of 6 inches and average height of 40 feet, you would want to leave about 357 trees per acre to reach a stand density of 70 square feet of basal area per acre. You would harvest about 290 $(650 - 357 = 293)$ trees per acre at an average volume of 0.03250 cords per tree (from Table 2). The total volume removed will average 9.5225 cords per acre $(293 \times 0.03250 = 9.5225)$, enough to allow a profitable thinning harvest. If in your state the timber sale transaction must be based on weight, not volume, the conversion would equal 25.7108 tons per acre $(9.5225 \times 2.7 = 25.71075)$.

By knowing the number of trees per acre that will be removed and the average size of those trees, you can estimate the volume or weight of wood that will be taken out. Remember that you will probably have to thin out five to six cords per acre (at least 13.5 tons) to make the thinning profitable.

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Warnell School of Forest Resources