A Look at Thinning

Here in North Florida, in the middle of one of the world's biggest concentrations of paper mills, pine pulpwood plantations make good financial sense. Prepare the land, plant, and once the trees are big enough to sell, harvest and start again. Most of our pine plantations can be clearcut for pulpwood 20-25 years after planting. It's a good way to go, but on many sites it's not the only way. By including one thinning you can grow most of your trees to about age 30, harvest some higher value wood as well as pulpwood, and come out looking about as good--sometimes better--financially. Somewhat older, thinned plantations have appeal to landowners who are interested in improved wildlife habitat, recreation, aesthetics, or in putting more that one product class of wood on the market.

As far as timber production goes, thinning allows the remaining trees (residual stand) to keep up their growth rate, rather than slow down as they would without thinning. A well-timed thinning means you get larger trees--and higher value products--sooner. If you are going to thin, do it before the trees have grown so crowded that only the very top of the tree has live branches. As a rule of thumb the "live crown", that portion of the tree with living branches, should not be less than 35% of total tree height when the trees are thinned. Following a thinning, each remaining tree receives more sunlight. However, a tree with a very small crown cannot take full advantage of the extra light. Keep an eye on the percent live crown of trees in the interior of the stand; trees on the edge--often next to some sort of clearing or stand of younger, shorter trees--will often mislead you with larger crowns.

Let's compare a pulpwood rotation with no thinnings to a longer rotation with one thinning. For this example we use a slash pine plantation on a cutover flatwoods site of average productivity. Planting density is 700-800 seedlings per acre, with a typical survival of 600-700 young trees after one year. We assume that site preparation and vegetation management practices appropriate to the site are used. In the shorter rotation, the stand is clearcut at age 20-25, yielding a total harvest of 30 or more cords per acre, almost all pulpwood. The exact age at harvest time would depend on the landowner's cash flow situation and the attractiveness of pine pulpwood stumpage prices. In the longer rotation, a thinning would be done when the trees are about 17-19 years old, yielding around 8-10 cords of pulpwood per acre. The final harvest would occur about ten years later (age 25-30), and would yield 27 or more cords per acre, including about 10 cords of "chip-'n-saw", which lately has been bringing the landowner at least 60% more value per cord than pulpwood.

Please take note: the above example applies only to certain type of stand on a certain type of site. If you are dealing with old field plantations, upland or sandhill sites, other species of pine, or poorer or better (fertilized) flatwoods sites, the yields and the timing of thinning and final harvest will probably differ from this example. In particular, old field plantations are likely to grow faster, permitting a first thinning for present income and optimum future stand growth around age 14-16. Also, especially if you're growing loblolly or longleaf pines, you may want to consider rotations longer than 30 years and more than one thinning.

Remember that compared to final harvests, thinnings usually involve smaller average tree size, lower total volume for sale, and less volume to be cut per acre. For those reasons, thinnings often involve higher logging costs per cord and that means lower stumpage prices. As a rule of thumb (for any sale), if total sale volume is under 500 cords and/or if per acre volume is much below 10 cords, you will probably have to settle for a lower than average price for your timber.

A common and easy way to thin is to take out every third row of trees. Each remaining tree then gets additional sunlight on the side next to where a row of trees was cut. Sometimes selected trees--mainly diseased, poorly formed, and smaller ones--are also cut from the other rows. Another common method is to cut every fifth row and use the space left by each cut row as a path to enter the stand and harvest selected trees from the other rows. (Figure Below)

This method is better for future growth of the stand, but requires loggers who are skilled and experienced at thinning and willing to be careful not to damage standing trees. Either way, 30-40% of the wood volume is removed on the first thinning. In Florida, as a rule of thumb, for the best payoff after a thinning, a stand will need to grow about ten more years.

If you have fully-stocked young plantations or natural stands that you'd like to grow past the age of about 25 years--for wildlife, aesthetics, recreation, biodiversity, or wood product diversification--you are probably going to need to thin them at least once. If your goal is maximum timber profits, a short rotation pulpwood-growing operation (20-25 years and no thinning) may be your best bet. But, don't take it for granted. For most sites it is worth taking time to compare the following four options:

  1. no thinning or fertilization; clearcut at about age 20-25 (mostly pulpwood);
  2. fertilization at age 8-12 but no thinning; clearcut at about age 20-25 (mostly pulpwood and some chip-n-saw );
  3. thinning but no fertilization; thin at age 14-19 (all pulpwood); clearcut at age 27-30; (up to 40% chip-n-saw and perhaps some sawtimber);
  4. thinning and fertilization; thin at age 14-19 (almost all pulpwood); clearcut at age 25-30 (pulpwood, c-n-s, probably some sawtimber/plylogs).

Many landowners will find that well-timed thinning or a combination of fertilization and thinning is an attractive option for at least some of their stands.

On a holding with one very large young stand, the landowner might find it advantageous to thin one portion, clearcut the other portion a few years later, and several years after that, clearcut the thinned area. Instead of one big clearcut and one large sum of money, the owner would receive timber income at three separate times and increase habitat diversity for wildlife.

A possible option for a holding with a large continuous area of pine plantations of about the same age is to clearcut most of the timber for pulpwood while leaving 30-50 yard-wide bands of uncut trees as wildlife corridors. Thinning within those strips of trees would further benefit wildlife and enhance future timber values. The strips of trees could be harvested once the new stands planted on the clearcut areas were well-established and many of the trees in the strips were large enough for higher-value products.

Overcrowding is not the only thing that makes tree growth slow down. If crowding is not the problem, then thinning is not the (whole) solution. On nutrient-poor sites, particularly in the flatwoods, trees stop growing because certain key nutrients are no longer available. They need fertilizer and/or weed control (to free up nutrients that had been taken up by understory vegetation). Once the nutrient problem is remedied, a thinning might eventually be worthwhile. Also, there are quite a few cases of slash pine being planted on soils that are too well-drained for that species. When those stands practically stop growing, thinning won't help. There may be nothing you can do except hope the trees manage to get big enough to sell before you have to cut them down. Then, start over with a species that is more suitable to that kind of site (longleaf or sand pine).

In a thinned stand, the freshly cut stumps and wounds left on standing trees can attract pests that endanger the remaining trees. In a high risk area for Southern Pine Beetle and other bark beetle attacks, avoid thinning in the summer, when the beetles can spread most rapidly. In areas where the root rot fungus, Fomes annosus , is a problem, avoid thinning in mid-winter, when weather conditions are right for this fungus to spread and grow. If both of these pests are a problem in your area, thin in winter and immediately apply borax to the fresh stumps to keep Fomes annosus from growing on them and later attacking the roots of live trees.

If you are considering a thinning, please contact a forestry consultant or your county forester for assistance.