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By Bill Emmingham and Stephen Fitzgerald
This article appeared in the Northwest Woodlands magazine, Winter 2000 - Published quarterly by the World Forestry Center for the Oregon Small Woodlands Association, Washington Farm Forestry
Association, Idaho Forest Owners Association and Montana Forest Owners Association.
Why Should You Prune?
Pruning removes the lower branches of young trees and allows the early formation of clear, knot-free wood.
Pruning is done to increase tree and stand value.
Lumber manufacturing studies show that the value of lumber cut from pruned logs can more than pay for the cost of labor and interest on the money invested in pruning. The value increase depends upon a sawmill's willingness to pay premium prices for pruned logs–a trend that seems likely. However, like all forestry investments, pruning must be considered a calculated risk because future markets are unknown. Best estimates indicate that pruning 100 trees per acre for one 16.5-foot log at age 20 will produce an added $6,000-$10,000 per acre when harvested 40-80 years later.
Pruning Objectives
- Although pruning is done primarily to enhance clear wood production and tree value, it can fulfill other objectives such as increasing fire resistance by removing fuel ladders and improving access into dense stands. Many owners simply like the looks of a well-pruned stand.
- Pruning can also increase the amount of light beneath the tree canopy, which enhances grass and forage production. This is important in intensive agroforestry operations where livestock and high-value trees are grown and managed together for added income.
- Pruning can be used to lessen the impact of blister rust in young white pine stands. Pruning can also reduce dwarf mistletoe infections if the mistletoe is confined to the lower branches, and the overall infection level in the stand is moderate to low.
Pruning is intensive management
Because pruning is a technique for adding value to trees growing in forest stands, you can think of it as the woodland manager's "value-added" effort. It is, however, a long-term investment. For example, even under favorable tree growing conditions, it takes 20 years to add a five-inch layer of clear wood on a Douglas-fir tree. Pruning is also a laborious and costly operation. For example, pruning 100 selected trees per acre likely would take you several days of hard work, or it could cost $150-$250 if you hired a contractor to complete the work.
Pruning Management Options
- Choose stands carefully. Since pruning is a large investment and the payoff is many years away, evaluate potential stands carefully. Not every stand can be pruned profitably, so you should prioritize stands to be pruned.
- Select stands on your more productive sites: e.g., site class three or better. They should be accessible for workers and preferably on gentle topography, making multiple thinning entries easy. Stands should be healthy and vigorous.
- Do not prune stands on exposed, wind-throw-prone areas such as ridges, or sites with a high water table. Avoid pruning stands with high incidence of root rot or other diseases.
- What species can be pruned? Douglas-fir and ponderosa pine are good candidates for pruning. Studies have shown that pruning these species can be profitable if mills are willing to pay for the value in pruned logs.
- The market value of other species is less certain, but species such as western larch, western hemlock, western white pine and noble fir are considered good candidates for pruning. Most species of conifers can be pruned, although some, such as hemlock, true firs and spruce, are more prone to invasion by wood-decaying fungi.
- Deciding on a pruning strategy. Pruning is part of an intensive management operation and should be planned and carried out in stands you intend to manage intensively.
- The basic objective of pruning is to minimize the knotty core of wood in the valuable butt log of the tree. First, target a maximum knotty core size, and prune when the tree reaches that size. Second, decide what length of log you wish to market. Third, coordinate the pruning regime with your thinning regime for the stand, because density management is the main tool for keeping the tree's diameter growth at a desired rate.
- Core size. The knotty core is measured as the diameter outside stub (DOS), that is, to the point where the branch is cut (see figure 1). Mills commonly peel logs down to a four-inch core, and smaller cores of about two inches are now possible.
- Strategies for keeping the DOS small include pruning in multiple lifts. How high you prune depends on how willing you are to gamble your work or labor costs on the uncertain future log market.
- Logs are sold in a variety of lengths to different end users. Conifer mills that peel or slice clear boles into veneer use short logs of 17 or 25 feet. Mills manufacturing lumber look for longer logs.
- When marketing pruned logs, always negotiate a special price based on the DOS, growth rates and other quality characteristics of the logs from your stand. Remember that log grades used today were not developed with pruned logs in mind.
Basics of tree pruning
In pruning trees, you may remove both live and dead branches. Removing dead branches doesn't affect the tree's photosynthesis "factory" or tree growth. Removing live branches does affect the factory, so be careful to leave the tree with an adequate crown. A general rule of thumb that applies to all tree sizes and species is to leave 50 percent of the total tree height in live crown (see figure 2). Assume your objective is to prune half the trees in a young stand to 18.5 feet. You could prune to that height all at once, but to maintain 50 percent live crown, the trees would need to be at least 37 feet tall. However, by the time a ponderosa pine tree in eastern Oregon is 37 feet tall, it usually is 8-12 inches dbh. If your objective is to maintain a knotty core of four inches, it makes sense to prune the same tree several times, each time pruning to a higher level, or "lift." Intensive pruning operations use two or three lifts, depending on target pruning height, pruning tools used and size of operation.
Combine pruning with other operations
Pruning and thinning operations should be coordinated. Use early thinning to maintain vigorous and steady diameter growth that will heal pruning wounds quickly and maximize clear wood growth. A good time to start pruning is right after the first early thinning, which is usually precommercial. In a well-stocked stand, limbs are small and easy to remove. Small knots heal over quickly. Carefully select trees to prune at this time because they need to survive and grow well until harvest 20-50 years later. Prune only the best future crop trees in the stand. When trees are well spaced, nitrogen fertilizer can often increase diameter growth beyond that from thinning, producing added clear wood and value. However, before investing in large-scale fertilization, check to see if your site and stand are good candidates.
Pruning Mechanics
Identifying trees to prune. Trees to be pruned may be identified ahead of time or at the time of pruning. It is a good idea to permanently mark pruned trees so that you, other forest workers and potential log buyers can identify them easily, even 20-40 years later. Plastic or metal tags or tree marking paint are the most common methods for long-term tree identification.
Optimum pruning diameter. Conifers up to eight inches in diameter are excellent candidates for pruning; small trees are even better. The upper limit for pruning depends on how long the stand will be held and how it is managed.
Delaying pruning and/or prematurely harvesting pruned trees decreases clear wood and increases the relative volume of knotty wood in the center core.
How many trees to prune. Remember the first basic rule of pruning: don't prune too many trees. You will reap little benefit from pruning every tree in a dense young stand because some will die or be thinned out before they produce very much clear wood. Typically, only 50 to 100 trees per acre are pruned.
Where and when to prune. When cutting off branches it is important to avoid injury to the bole of the tree and the branch collar (see figure 3). The branch collar is the swollen area at the base of the branch. Cut branches outside any enlarged branch collar and never cut flush with the bole. By cutting only the branch, the tree will heal faster because the wound diameter is smaller.
Never treat pruning wounds with a wound dressing or paint. Studies have shown that paints and dressings trap moisture behind the dressing and promote the growth of wood-decaying fungi.
The best time of year to prune is during late summer, fall and early winter when the tree's cambium is not actively growing. This helps minimize bark injury and pitch flow.
Summary
Pruning selected trees in your forest stands can be a good investment. Many owners like the appearance of a pruned stand, but it really doesn't make good economic sense to prune all trees in a young stand. Instead, prune the final crop trees in stands you plan to manage intensively for several decades.
Prune in one to three lifts so that you can prune the bole before it grows over four to eight inches in diameter. Always leave at least 50 percent of the total height of the tree in live crown, which minimizes height and diameter growth loss. Now get out there and prune!
Bill Emmingham is an Extension silviculture specialist, and Stephen Fitzgerald is an Area Extension forester, Central Oregon, Oregon State University. This article was condensed from Extension Publication EC 1457, which is available through your local extension office.
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