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Andreas Ehring

Andreas Ehring

Forest Research Institute of Baden-Württemberg (FVA)
Dept. of Forest Growth

Wonnhalde 4
79100 Freiburg
Phone: +49 761 4018 253

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Author(s): Andreas Ehring
Editorial office: FVA, Germany
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Pruning for quality improvement

Branchiness is a significant feature in assessing the quality of trunk wood. In 80-90% of cases branchiness is decisive in the sorting process with a corresponding impact on the sales price. Thus, it makes cost-effective sense to prune the future crop trees in good time.

Content

Pruning deciduous trees
Fig. 1: Pruning deciduous trees (Photo: Andreas Ehring).

Tree species and stands

Basically, all tree species can be pruned. It is worth pruning high quality stands with vigorous growth which promise diameters at harvesting age with a ratio of at least 2:1 of the branch-free mantle to the knotty core. Stands with increased risks e.g. from wind storm, red rot, snow, haze or debarking should not be pruned. It is of fundamental importance for quality wood production to prune healthy branches on tree species which retain dead branches. This applies in particular to widely-spaced adult trees.

Working techniques

The determining factor for the result is pruning at the correct time. The healthy branches should not be more than 3 cm thick. The length of the green crown after pruning must still be 40-50% of the tree height, in order to avoid the risk of water sprout formation and a social decline. For the same reason competitors for the pruned trees must be eliminated. The careful pruning of healthy branches should generally not present any problems with wood-destroying fungi, if the branches are not too thick (see below) and are pruned with a smooth cutting tool close to the bark (Fig. 2a) or in the case of swellings on branches are cut on the branch collar (Fig. 2b).

The important thing is to remove the entire branch (do not leave stumps!). Contour cutting is usually superfluous. In young stands with few trees, however, pruning forking or steep-angled branches may be prudent. Pruning techniques must be planned with care and carried out bearing forest conservation in mind. The first requirement for targeted implementation and control is a written work assignment.

The particular points to note are:

  • avoid injury to the branch collar and the trunk
  • leave no stumps
  • remove all the thin branches
  • restrict pruning on future crop trees
  • observe the pruning height

In order to prevent injury to the cambium, climbing aids (other than ladders) should be used only outside the sap-producing season.

Incision for pruning healthy branches

Fig. 2: Incision for pruning healthy branches: a) without swelling, b) with swelling.

Pruning heights

  • Conifers
  • Vertical reach pruning at approx. 2.5 m Only reasonable if it is feared that the healthy branches will exceed the thickness referred to above at the time of pruning at 5 m or if it can be harvested for your own purposes (ornamental wreaths). To be carried out at top heights of 5-6 m (DBH approx. 8-10 cm, for a maximum of 150% of the number of future crop trees).

  • Pruning at 5 m Normal case, requires selection of future crop trees. To be carried out at top heights from about 12 m (DBH approx. 15-18 cm), for Douglas fir somewhat earlier (from approx. 10 m), since otherwise the basic branch diameter and callousing periods may become too great.

  • Pruning at 10 m Because of the ratio required of the core of the branch to the branch-free mantle of 1:2 and a top diameter of at least 40 cm, this is only reasonable if a diameter at breast height of approx. 65 cm can be reached. As a rule this can be expected of the Douglas fir, in the case of other conifer species only on very productive sites. To be carried out at top heights of about 20 m (DBH approx. 25 cm).
  • Deciduous trees

    Walnut varieties and cherry are candidates for pruning healthy branches, as well as other deciduous species, if they grow widely-spaced. The pruning height should be 25% of the final height required. In the case of deciduous trees repeated pruning sessions within relatively short periods may be necessary.

Pruning period

Dry pruning may be undertaken all year round, pruning using a tree bicycle only outside the growing season. The period for pruning healthy branches is between the end of the winter and the start of the growing period or after longitudinal growth has ceased (June/July) to be the most favourable for wound reaction and callousing. It is best to prune deciduous tree species which tend to bleed, such as maple and walnut, between June and August. If healthy branches are pruned during the growing season a particularly careful pruning technique must be used, since the danger of bark and cambium injuries is very high.

Number of trees to be pruned

Only future crop trees are pruned, with the exception of reach pruning. To prune at 5 m and higher the following numbers of trees per ha are recommended in support of the types of forest development in Baden-Württemberg (Richtlinie landesweiter Waldentwicklungstypen [State Directive for Types of Forest Development], Landesforstverwaltung Baden-Württemberg 1999):

Tab. 1: Recommended number of trees to be pruned per species and hectare.
Tree species Trees for pruning / ha
Douglas fir 100–150
Oak 70–90
Pine 100–200
Ash/Maple 60–80
Europ. Larch/Jap. Larch 100–150

Cherry

60–80
Spruce 150–250

Walnut

60–80
Fir
150
Poplar 100

The figures apply to pure stands with average performance expectations. In mixed stands the number of trees to be pruned should be derived from the desired (and feasible in the stand) percentage proportions of tree species.

Lead time values

The lead time values derive only for rough planning of the work; they apply only to conifer species, average working conditions and services. There are no adequate empirical values for deciduous tree species. The lead time may diverge sharply depending on the tools used, the features of the tree and the site. It is not possible to derive standard times directly from them.

Tab. 2: Time requirement for different pruning heights and techniques.
Pruning stage (m) Time requirement (min/tree) Pruning tool
 up to 2,5   3–6 Hand/pole saw
 0–5,0  8–15 Pole saw/folding ladder
 5,0–10,0  15–22 Folding ladder
 >10  25–34 Tree bicycle

Occupational safety

The provisions of § 7 Accident Prevention Regulation "Forestry" and German Statutory Accident Insurance 0.1 § 16, German Statutory Accident Insurance 1.13 § 1, 7, German Statutory Accident Insurance 6.4 § 16/1, § 20 and German Statutory Accident Insurance 10.4 apply.

The following points in particular must be noted:

  • a clear work space
  • operationally reliable tools
  • hand and foot protection
  • head and eye protection

Only persons employed in the pruning procedure may remain in the area of falling tree branches. Work must cease in conditions of strong wind, rain, snow, mist etc. Trees may be climbed only with safe, properly maintained equipment without defects.

This includes:

  • light, closely-fitting clothing
  • sturdy shoes
  • safety harness with life line

Documentation

The stands to be pruned are determined by the Forestry Planning Department. Long-term reliable evidence is an indispensable requirement for the economic success of pruning for quality improvement. Therefore this must be entered on pruning cards and in the Forestry Planning Department system.

The details to be recorded are:

  • area
  • tree species pruned, number of trees and pruning height
  • year of pruning

Undocumented pruning in old stock may be detected on a uniform shaft length without branches on the predominant trees with short transition to the dead branch zone. In cases of doubt whorl cuts at the top area of the piece of stemwood should clarify the issue.

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