Thinning for increased forest health and profit

Thinning a forest stand can provide many benefits, but only if you know what you’re doing and why, and if you do it correctly.

Trees are removed from an immature stand for a number of reasons: to promote growth in the remaining trees; to enhance nontimber use of the forest like grazing, wildlife, and recreation; and for reasons of forest health. Thinning can be done for a profit (commercial thinning) or not (pre-commercial).

The rationale for thinning is based on how trees grow. Trees make energy for growth through photosynthesis, a process that uses sunlight, water, and nutrients to produce energy that is stored as carbohydrates (tree food). Other life processes of the tree—respiration, food transport, and metabolism—use up energy. The amount a tree can grow is dependant on how much energy is left over after its basic needs are met.

When trees grow closely together in a stand, they compete for a finite amount of sunlight, water, and soil nutrients. Crowded conditions mean that taller trees have more access to sunlight and can shade smaller ones, blocking out that vital sunlight. By thinning the stand, more water, nutrients, and light become available to the remaining trees. The resulting increase in growth is primarily exhibited in diameter gain which is a benefit to timber producers.

Of course not all trees react to thinning in the same way—the amount of growth varies by species, age of tree, and condition. As a rule, young trees respond to thinning more than older trees; those that are intolerant of shade will also respond more readily.

The effects of thinning on a stand can be described with the six crown classes (see diagram):

1. Dominant (D)—these trees extend above the crown canopy and receive full sunlight from above.
2. Codominant (C)—trees at the general level of the crown canopy; more crowded on the sides than the dominants.
3. Intermediate (I)—shorter trees, with small, crowded sides.
4. Suppressed (S)—overtopped by other trees; they receive no direct sunlight.
5. Wolf (W)—trees that develop in the open and have full crowns on all sides.
6. Mortality (M)—dead trees in the stand.

At first, all trees in an even-aged stand are dominant. But as they grow, some trees grow taller and begin to shade others. Those in the shade are less able to photosynthesize and their growth is decreased. As this process continues, the various crown classes develop.

Thinning can alter this dynamic. High thinning (thinning from above) removes some of the dominant and codominant trees to release a large number of the remaining trees. Low thinnings (thinning from below) removes suppressed and intermediate trees as well as some codominants and dominants, leaving uniformly-spaced dominant and codominant trees.

Thinning can also change the species composition of a stand depending on which species are selected for removal.

A thinning prescription for any stand must be site-specific. The approach to thinning an even-aged stand, as presented in this article, would not necessarily apply to an uneven-aged stand. Decisions about when and how to thin depend on site conditions, species, and, of course, landowner objectives. One rule of thumb is to consider thinning when the crowns begin to touch. It is best to avoid cutting trees during the spring and early summer when they are growing. Also, trees cut during this time may attract insects such as bark beetles. Fall and winter are the best times to thin, but remember that wet soils are prone to compaction and erosion so take precautions.

For advice on thinning, talk to your local CDF Forestry Assistance Specialist or call the California Forest Stewardship Helpline at 1-800-738-TREE.

—much of this information comes from “Thinning: An Important Timber Management Tool” by William H. Emmingham and Norman E. Elwood, PNW 184, (A Pacific Northwest Extension Publication published by Oregon State Univrsity, Corvalis, August 1983, reprinted February 1993), 50¢