High pruning of conifer and broadleaf trees
High pruning is the removal of all branches from the bottom 6 metres of tree stems to encourage the formation of knot-free high quality timber.
High pruning differs from shaping. Shaping is usually only carried out on broadleaf trees to ensure a tree has a single stem. Shaping starts when trees are one to two metres in height whereas high pruning normally occurs around the time of, or after first thinning of both conifers and broadleaves.
Why high prune?
High pruning produces trees with higher quality timber with more potential high value end uses. It results in stronger timber which is more easily worked and finished. Timber from trees that have been high pruned may attract a price premium. Unpruned timber has more knots resulting in weaker structural timber and a potentially lower market value.
Forests approaching -or at- first thinning stage may be suitable for high pruning. Only healthy, productive and stable forests should be considered for pruning.
Conifer species such as Sitka spruce, Norway spruce and Douglas fir with high value end uses are most suitable for pruning.
Ideally, the diameter of selected trees should not exceed 18 centimetres DBH (Diameter at Breast Height, measured at 1.3 metres above ground level using a special measuring tape) and should have the potential to increase their diameter 2.5 times before clearfell.
How to high prune conifers
Select 500-600 trees per hectare as the proposed final crop. These should be the straightest and most vigorous trees. Ensure that selected trees are evenly distributed throughout the forest. Inspection paths should be cut or “brashed” every 80-100 metres in unthinned forests to facilitate high pruning. Otherwise it can be carried out after first thinning where it is easier to pick out and access superior trees.
High pruning is normally carried out in two phases called “lifts”. The first lift involves pruning to a height of 3.5 metres. These trees are pruned to 6 metres at the second lift. A period of between two to four years must be left between the first and the second lift.
Cuts should be clean and at a right angle to the branch. Cutting flush with or into the stem should be avoided.
Broadleaf trees that have good form, vigour and that are disease free can also be suitable for pruning. The aim of high pruning in broadleaves is similar to that in conifers, i.e. to encourage the formation of knot-free high quality timber. All ‘Potential Crop Trees’ can be pruned (at least 300 per ha depending on species).
When removing branches on broadleaves; it is important to identify two key features on the tree stem (see image):
- The Branch Collar - usually a swelling around the base of the branch where it joins the main stem;
- The Branch Bark Ridge (BBR) - identified as a raised ridge of rougher bark that develops in the branch crotch and runs downward at an angle to the main stem.
These are part of the tree’s defence mechanism. It is important to avoid injury or damage that can lead to disease. Avoid “flush cuts” that remove the branch collar. Also avoid leaving “stubs or pegs”.
Unlike conifers, high pruning broadleaves can sometimes involve cutting large branches (over 5cm in diameter). Proper care must be taken when cutting these branches to avoid causing damage to the main stem. Stem damage can delay the healing process allowing a longer window for disease to infect.
There are three steps in standard large branch pruning:
- The first step is to make a deep cut underneath the branch at about 30 cm up the branch
- The second step is to move up the branch and make a second cut completely through the branch from above. This prevents snapping and tearing of the stem when the branch falls.
- The third step is the final pruning cut which should be at a slight angle away from the main stem just outside the collar and the BBR (see image). Care should be taken to ensure that the bark does not tear when the remainder of the branch comes away from the main stem.
Equipment such as secateurs, loppers and handsaws can be used to prune branches up to approximately three metres. There is a wide range of telescopic pruning equipment available for pruning stems up to six metres such as special hand-saws with curved blades, small power saws and loppers. It is important to ensure that all pruning equipment is sharp and properly adjusted to give a clean cut.
Cleaning pruning equipment
To minimise risk of disease infection when carrying out pruning operations, it is essential that all pruning equipment is frequently disinfected with a 70% alcohol solution or by using household anti-bacterial cleaner.
When cleaning pruning equipment, ensure that the blades are free from debris and dirt so that the disinfectant can reach all of the cutting surfaces. It is important to wipe off excess disinfectant from the surfaces to prevent damaging the trees.
A longer soaking in disinfectant may be required for pruning equipment that no longer has smooth cutting surfaces. The older the equipment is, the more likely that the surfaces have become pitted and can harbour disease-causing microbes that can remain unaffected by quick sterilization. This is especially true of canker-causing organisms.
Appropriate personal protection equipment is essential when high pruning.
High pruning is a rewarding job that improves the overall tree quality of a forest and also makes trees more saleable in a competitive market.