Forest Solutions Technical Note

Dr. Tom Baribault,Forest Solutions Inc., Hawaii, 2015-05-28

1. Summary

1.1. Stand development
Under plantation conditions, Acacia koa exhibits poor natural pruning. Without supplemental pruning, managed koa stands will produce low yields of clear wood free of knots. In many areas across Hawaii, koa may be successfully pruned to dramatically improve potential yield of high quality timber. The objective of this protocol is to explain techniques of green pruning, or the removal of live branches, at particular times early in a koa rotation. Although dry pruning, or removal of dead branches, can be implemented later in a rotation with some success, it is less cost effective and yields lower volumes of clear wood.

1.2. Pruning objectives
Koa may be managed for habitat, production of merchantable logs, or specially tended for production of canoe logs. Although habitat stands will benefit from pruning, the cost of pruning can be justified chiefly in production settings. This protocol focuses on production of intermediate-length saw timber, although the techniques presented here may be repeated in order to yield canoe logs.
The greatest proportion of timber value in a tree occurs in the first log, typically at a length less than 16’. Pruning to a merchantable length of 8’ to 10’ can be as much as three times cheaper than pruning to 18’, so from a financial perspective, intermediate length logs are lucrative. Conventional pruning recommendations limit the treatment to high quality sites. In koa, however, we have found that tree performance at low quality sites without pruning yields stands with essentially zero financial value.

1.3. Growth tradeoffs
Cutting living branches will reduce the amount of photosynthetic area only temporarily. New branches rapidly replace the lost leaf area, and the growth penalty is eclipsed by improved wood quality. One third to two thirds of the live crown can be removed without affecting long-term diameter growth. Heavy pruning (>50% live crown removal) has had, in our experience, only temporary effects on diameter growth at high index sites.
In stands where growth rates are extremely slow—coincidentally where poor form is more common—koa may be pruned using a technique called “singling” in which only competing leaders are removed to favor a single stem. Although this method will have a lower impact on growth rate, the resulting form will be unpredictable.

1.4. Profitability of pruning
To our knowledge, there are no examples of intensively managed planted koa that have completed a full rotation. Consequently, it remains unknown whether pruning is ultimately a profitable treatment. Our preliminary assessments, however, suggest that unpruned stands have very limited capacity to produce merchantable timber, whereas pruned stands at least have the potential to produce more than 100 crop trees on a productive acre. In our estimation, the decision to prune a koa plantation makes the difference between a failed investment and a profitable one.

2. Sequential Pruning

Maximizing clear wood for the length of the merchantable log is a process that takes multiple years and depends on tree growth rates at the site. Sequential pruning across several years will yield merchantable logs with greater volume than unpruned trees; each pruning entry (or “lift”) increases the clear-wood length by some appreciable fraction of the current live crown depth, depending on site index. A greater proportion of the live crown may be removed during the first lift, up to 60% on a top quality site. At the second lift, up to 40% live crown may be removed to yield clear wood to 12’.
Koa trees growing on quality sites (Site Index I) can tolerate significant pruning, up to 60% crown removal at first lift, and 40% removal at second lift. In contrast, koa growing on tough sites (Site Index III) should be pruned less aggressively, or 30% reduction at both first and second lifts.
Merchantable logs ranging in length from 9’ to 12’ should be the objective of sequential koa pruning. Final log length will depend on tree growth rate, since two lifts on a slowly growing tree will produce a shorter log than two lifts on a rapidly growing tree. Longer logs, e.g. for canoes, can be produced by implementing third or fourth lifts as necessary.

3. Techniques

3.1. Branch removal
To prune koa trees, standard branch removal practices should be implemented. Cuts should be made as close as possible to the branch collar, but not so close as to increase wound size unnecessarily. Branches should be pruned before they reach a diameter of 1”, and ideally smaller than ½”. The angle at which branches are cut does not seem to influence wound closure.

3.2. Selecting crop trees
Only a fraction of the trees in a plantation will ultimately become crop trees. With an original planting density of 400 trees per acre (tpa), crop trees may number as few as 120 tpa. Ideally, only crop trees would be pruned in order to reduce costs. In practice, it is often necessary to prune more than 80% of the young trees in a stand. At these early stages, the very poor quality trees can be identified by fundamental deformities, but crop trees may be difficult to distinguish from those that will ultimately be thinned. Crop trees may be identified within a two years after the final lift. After pruning, koa forest managers should implement an aggressive thinning program to promote only those trees with the best form that will eventually yield highest timber value.

4. Diseases and Pests

Forest Solutions has implemented the preceding pruning techniques at elevations from 1,800’ above sea level (asl) to more than 5,000’ asl, with good results. We have not, however, applied these methods to koa growing in lowland areas. Lowland stands may be subject to higher pressures from fungal disease and insect pests. Pruning in these conditions may cause unacceptable mortality or tissue damage. Alternatives to this aggressive sequential pruning include singling (discussed above), tip pruning, or a combination thereof. Rather than pruning close to the branch collar, cuts should be made at 2/3rd the length of each branch that the forester wishes to eventually remove. These branches will die off over the course of several years, and can be dry pruned when they readily fall off the tree. Although this option should reduce disease, the volume of clear wood will be reduced.

5. Pruning services available

Do you have questions about pruning koa on your property? Forest Solutions is a consulting forestry company with 18 years of experience in Hawaii, and we would be happy to discuss forest management options for your parcel.

Phone: +1 (808) 776-9900

Nyland, R. 2007. Silviculture: Concepts and Applications. Ch. 20 P. 469 – 482.
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