The two main tree-tending operations (pruning & thinning) are usually carried out in the first 4-10 years of the trees life:
Pruning
Pruning removes the lower branches from the tree. Branches form knots in timber. If the wood in the trunk grows around the branches when they are living, the knots formed are tight and will not necessarily weaken the timber. If the branches die on the tree, the new wood grows around the dead branches. When the timber is milled from the tree, these knots formed from the dead branches are called loose knots or bark encased knots and are likely to fall out and leave a hole in the timber.
It is desirable that there are no knots in high quality timber. Removal of all branches from a tree allows knot-free clearwood to grow over the cut branch stubs. Clearwood is cut for higher-quality end uses such as furniture, or sliced off for veneer or plywood. In addition to improving the quality and value of the wood. Pruning also:
• Improves disease control
• Gives improved access to the forest
• Helps fire control
• In agroforestry, allows more light to reach the forest floor for
pasture growth.
Once a tree is pruned, the new wood in the trunk grows over the stubs of the removed branches. The diameter of the pruned tree trunk is called Diameter Over Stubs (DOS). The ‘defect core’ contains these branch stubs plus occlusion scars created as the new wood grows over them. It can be regarded as a central core running the length of the log produced from the pruned butt of the tree.
In good pruning operations, the object of the operation is to remove the whorls of branches as close to the tree stem as possible without leaving protruding branch stems or ‘coat hangers’. ‘Coat hangers’ result in encased knots and bark pockets which reduce timber value.
Pruning Methods
Various techniques have been used; anything from jack saws to long-handled pole saws for high branches, and pruners (long-handled secateurs) for the lower branches. Hand saws, still used for the occasional large branch, are:
• Slower;
• Require greater effort, for a longer time;
• Difficult to manage amongst a large number of tight
branches.
The pruners used by the contract pruning teams are much like the home-pruner, but with a few design modifications made by the pruners themselves. They are:
• Quicker
• Require less effort, and
• Can be manoeuvred through and around tight branches
easily.
Specially adapted chainsaws have been used. They are faster than hand pruning but their speed is outweighed by:
• The extra weight in the chainsaw and equipment
• Cost in time, due to fuel stops, overhauls
• Noise
• Extra cost for fuel and oil changes
• Risk of more serious injury.
Pruning Height
It is common practice in New Zealand to prune tree stems to a height of 5.5 m or 6.0 m. This gives a clearwood butt log of 5–6 m. These measurements correspond to traditional sawn log lengths for transport and sawmilling equipment, which has been manufactured to cope with these lengths.
In the past, all trees were pruned to the same height regardless of their size. That caused problems such as over-pruning small trees and under-pruning of large trees. The result was an undesirably large variation in the defect core size.
Variable Lift Pruning
Variable lift pruning is a technique by which the height each tree is pruned is assessed relative to its overall height and/or diameter. This avoids the under and over-pruning practices of the past, and in doing so creates an effective means of controlling the size and variation of the defect core.
Pruning aims to leave 3–4 m height of green crown foliage above the pruned stem on each tree. The crown length has been determined by balances between the removal of too much crown (which slows the tree growth down) and the removal of too little crown (which promotes the growth of branches leaving too much defect core). In practice, it is difficult to judge how much pruning is needed for a tree to leave a 3 to 4 m crown.
The stand must be pruned when the average stem diameter at the lowest whorl reaches the target DOS. Stem calipers are used by contract pruners to help make this decision. The calipers are set to a pre-determined size, which has been decided by previously measuring the average diameter of trees at the height at which they have 3–4m of crown above this measurement point. Newer technology includes computer models which predict height and diameter relationships, and estimate DOS size in relation to pruning height.
Three pruning lifts per tree are usual, but four or five lifts may be required for stands on more fertile land. Typical practice is to prune the tree up to 2 m high when it is about age 4–5 (5–6 m tall), 2–4 m when about age 6 (7–8 m tall), and 4–6 m at about age 7–8 (10–12 m tall).
Thinning
It is wasteful and uneconomic to prune defective trees. If possible, any defective trees should be removed as soon as they are identified, so leaving more growing space for the better trees.
Thinning is the removal of some trees from a stand of trees. The trees culled (removed) are usually the poorer quality trees. This means that the remaining trees are subjected to less competition for available resources and the best trees will be nurtured to their full potential. Thinning enables foresters to:
• Select larger and straighter trees
• Select healthy trees
• Select trees with small branching habit
• Allow for the potential growth of the best looking trees by
producing more space between them
• Increase tree size, and decrease the growth time to reach a
marketable size.
When to Thin
The first thinning is best carried out during the first pruning stage. The second and subsequent thinnings may be carried out at the same time as medium and high pruning. Both operations (thin and prune) are usually carried out by the one contractor. The final thinning can be delayed if:
• The ‘cull’ trees are required to control branch sizes;
• The ‘cull’ trees are to be harvested for posts, poles,
pulpwood, etc (production thinned);
• Extreme weed problems are present.
The ground area available to each tree influences its growth. Thinning regimes are calculated on a trees-per-hectare basis. Thinning may not result in even distances between trees, however trees should not be too close (2.5 m), otherwise competition will restrict growth. On the other hand, tree spaces greater than 12 m apart are likely to under-use the available area and waste valuable land.
Crop Tree Selection
The quality of a crop tree is assessed by the characteristics of the butt log and the second log. The butt log provides the highest financial return from the tree, especially if it is pruned. Between them, the bottom two logs contain practically all of the tree’s commercial value.
To determine which of the trees in a forest will be thinned out, the potential quality of the timber that a tree will ultimately produce must be assessed. The quality of the butt log (the log cut from the lowest part of the tree trunk), can be confidently assessed when the tree is about 6 m tall. The quality of the second log, the log cut directly above the butt log, can be assessed when the tree is approximately 12 m tall.
Not all defective young trees will be thinned out. There are three types of defects which will determine the value of a crop tree:
1. Defects which will definitely reduce the value of the crop
tree.
2. Defects which might, or might not, correct themselves.
3. Defects which can be corrected by subsequent pruning.
Defects which will definitely reduce the value of the crop trees are shape changes, such as kink or sweep (as in the curve of a banana).
Also, even straight trees that have developed a lean, greater than 5 degrees from the vertical, will be thinned. Generally the tree will correct itself and end up forming a sweep malformation. This results in compression wood which is hard to mill and reduces the value of the log. The extent of the lean can be checked by placing a 1.4 m straight edge vertically against a tree at its base. If the top of the straight edge is greater than 120 mm from the tree, the tree will be rejected and thinned.
Defects which might, or might not, correct themselves include:
• Dead leading shoots
• Broken leading shoots
• Double and multiple leading shoots.
If the leader is damaged, another leader will often take over. In many such cases the tree will completely overcome the defect. In cases where the leader has been damaged in a very young tree, a kinked stem may develop and the tree will be rejected.
Defects which can be corrected by subsequent pruning include:
• Multiple stems
• Multiple leaders
• Ramicorn branches.
Trees with these defects are often left at the first thinning as pruning may later overcome these problems. These trees are further evaluated for thinning in subsequent treatments in that stand.