ANR-396, New Aug 1983. Frank A. Roth II, former Forester--Woodland Management Demonstrations

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Thinning Pine Stands for Top Returns

It has long been a practice of many landowners to "selectively cut" their pine stands every 5 to 10 years as they need a little income. This is often accomplished by cutting the pine timber to a diameter limit (everything larger than the limiting diameter is harvested). Unfortunately, this practice usually results in lowering the quality of the timber stand, reducing its economic potential, and eventually eliminating the pines in favor of competing hardwoods. Properly done, thinning results in increased quality and economic potential.

WHAT IS THINNING?

It generally takes from 30 to 40 years to grow a stand of pine sawtimber to economic maturity. However, it is usually necessary to cut some of the trees before the stand reaches maturity. Cuttings made in immature stands to stimulate the growth of the remaining trees and thereby improve the yield of the stand are called thinnings.

In any timber stand the trees compete with each other for light, soil moisture and nutrients. The more crowded the trees are, the more intense the competition. In a very crowded stand the growth rate is reduced, and eventually the weaker trees die. However, the volume of wood produced by a timber stand of a certain age on a particular site is about the same over a wide range of stand densities. This means that if the number of trees in a stand is reduced, the same volume of wood can be produced with fewer trees, while maintaining a good rate of growth. The main objectives of thinnings are (1) to redistribute the growth potential of the stand to the well formed, high quality trees, (2) to maintain the growth rate of the stand, and (3) to utilize to financial advantage all merchantable timber produced by the stand.

PINE TREES GROW QUICKLY

Most pine stands are even-aged. That is, all of the trees are within a few years of being the same age. If all trees are about the same age, then the larger trees must have been growing at a faster rate. Improper thinning operations, such as diameter limit cuts, remove the larger, faster growing trees for immediate income and leave the smaller, slower growing trees for future growth.

On good sites pine trees will grow more than 10 percent each year up to age 35, nearly doubling in volume and value every 7 years (see Table 1). Trees in the 6- to 8-inch diameter range are usually sold as pulpwood, but when they reach the 10-inch diameter size or larger they can be sold as sawlogs at 3 to 4 times the pulpwood value.

Table 1. Pine Trees Increase Rapidly in Volume and Value.

Age	14	21	28	35
Diameter at 4-1/2 ft.	7"	10"	12"	14"
Total Height	42 '	54 '	64 '	72 '
Pulpwood Volume Per Tree (Cords)	0.06	0.14	0.23	0.34
Sawlog Volume Per Tree (Bd ft Scribner scale)	--	44	85	146
Pulpwood Value Per Tree (at $15 per cord)	$0.90	$2.10	$3.45	$5.10
Sawlog Value Per Tree (at $150 per 1000 bd ft)	--	$6.60	$12.75	$21.90

ALWAYS LEAVE THE BEST TREES FOR FUTURE GROWTH

The result of any thinning operation should be to provide more growing space for the well formed, fast growing trees; while harvesting the trees which are diseased, damaged or poorly formed, and those that will not live until the next scheduled harvest. The following types of trees should be harvested during thinning: (1) overtopped trees, (2 ) forked trees, (3 ) trees with broken tops, (4) trees with one-sided crowns, (5) trees with crooked stems, (6) trees with fusiform cankers, and (7) trees damaged by insects, fire or weather (see Figure 1).

Figure 1. The trees above removed by thinning are (2) one-sided corwn, (3) fusiform canker, (5) forked stem, (6) overtopped tree, (8) crooked stem, and (11) small tree too close to larger neighbors.

WHEN TO THIN

The first thinning in a pine stand is usually made when the trees reach pulpwood size, about 6 inches in diameter. The trees will normally be between 12 and 15 years old when they reach this size.

Within a few years after the weak, diseased or damaged trees are removed by thinning, the branches and roots of the remaining trees will grow to fill in the gaps, and the same crowded condition that existed before thinning redevelops. Another thinning needs to be made before the trees get too crowded and the growth rate slows down.

A good way to determine when to thin is by looking at the live crown ratio, which is the percentage of the length of the stem which has live branches. When the live crown ratio drops to near 30 percent it is time for another thinning. If the live crown ratio drops below 30 percent the growth of the trees will be substantially reduced, and even after a thinning trees with very small crowns will not resume rapid growth until the size of the crown increases (see Figure 2).

Figure 2. When the live crown ration drops to near 30 percent, it is time for thinning. [Total Height 75 ft., Length of Live Crown 25 ft., 25 / 75 X 100 = 33 percent]

HOW TO THIN

In natural pine stands thinnings should be accomplished by individual tree selection. This provides the maximum opportunity to select and favor potential crop trees. Each tree to be cut should be marked and the logger should be penalized for cutting or damaging unmarked trees. This method will not bring the top price for the wood sold, but remember, your objective is to favor the best trees for future growth, not to produce maximum income from the thinning operation.

In pine plantations row thinning has become popular because it is quick, easy and economical. But since row thinning is nonselective, it does not improve the quality of the stand.

A better method is a modified row thinning in which every fourth row is removed to provide access to the stand and intermediate rows are thinned by individual tree selection. In a subsequent thinning the middle row of the three remaining may be removed by row thinning.

In plantations with a high level of fusiform rust disease it may be desirable to remove every sixth row and thin the intermediate rows more heavily.

A point to consider when thinning plantations is planted pines are susceptible to Annosus root rot disease after a thinning operation. Thinning in these stands should be done between May and September, or the freshly cut stumps should be treated with borax.

KEY POINTS TO REMEMBER

1. Thinnings are cuttings made in immature stands to stimulate the growth of the remaining trees and improve the yield of the stand.
2. Trees compete for light, moisture and nutrients, and if they become too crowded, growth slows and they may eventually die.
3. Pines grow rapidly, and trees grown for sawlogs are worth far more than trees grown for pulpwood.
4. The result of a thinning operation should be to provide more growing space for the best trees, while harvesting diseased, damaged or dying trees.
5. The first thinning in a pine stand is usually made between ages 12 and 15, when the trees reach pulpwood size.
6. Subsequent thinnings should be made before the live crown ratio drops below 30 percent.
7. In natural stands thinning is best accomplished by individual tree selection where each tree to be cut is marked.
8. In pine plantations a modified row thinning is best. Every fourth or sixth row should be removed and intermediate rows thinned by individual selection.