LESSON TWO - PRE COMMERCIAL THINNING (PCT)
WHY DO PRE-COMMERCIAL THINNING?
Pre-commercial thinning will concentrate future growth on the best trees and the best species.
Some tree species of the Acadian forest are prolific seed producers. Many naturally regenerated stands in Nova Scotia have too many trees for the site to grow to maturity. Seed bearing balsam fir can produce more than five million seeds per hectare in a single cone crop. Even with high seed losses to animals and diseases, there will be considerable numbers left viable in the soil. Seedling densities in softwood and mixedwood stands range from 4,000 to 36,000 stems per hectare.
Pre-commercial thinning (PCT) is the removal of selected trees from young stands to allow more space for crop trees. There is no immediate economic benefit from a PCT because no saleable wood is removed.
Some stands may have too few trees or seedlings. Thick organic or duff layers may reduce seedling survival. Seedlings must get their roots into mineral soil to ensure a reliable supply of water. This must occur before summer conditions begin to dry out the organic layer.
Exercise 2: Use a 2 m string to measure the number of stems in a young stand on your woodlot. Use a tree or a stick as the plot centre and attach the rope. Swing the rope in an arc around a circle and count every tree that is within the circle. Do this in several locations in the stand and average the numbers together (eg. counts of 10, 16, 19 averages 45/3 = 15). Multiply this number by 800 to give the number of stems per hectare. The example above gives 15 x 800 = 12,000 stems/ha.
|Average||= total count/number of plots|
|Number of stems/ha = average x 800|
Long-term benefits of a PCT include:
- Diameter growth of remaining trees will increase due to reduced competition. This focuses available energy on the site into the growth of fewer trees. The result is larger, faster growing, and healthier crop trees.
Growth lost in the removal of some stems will be redirected into remaining trees. Some growth acceleration may be due to the addition of organic material left from the treatment.
- Trees are ready for harvest sooner, shortening the rotation of the stand.
- Harvest costs will be reduced. Fewer stems, similar sizes and uniformly spaced trees will reduce the costs of harvesting. Usable or merchantable volume will be increased at harvest time. Unmanaged stands lose a portion of their volume in stems that are too small to be commercially processed. A PCT puts more growth in merchantable size ranges.
- Reduction of short-lived, less desirable species will improve stand composition. Leaving too many fast growing species, such as white birch and balsam fir, will reduce choices at commercial thinning and harvest times. At an early age these trees will out-compete more valuable species.
- Larger crowns and root systems produce more wind firm trees that are better suited for commercial thinning and selection cuts later on. This is important as climatologists now predict that Nova Scotia will see more major weather events like windstorms.
- Trees will suffer much less from the physical damage caused by wind whipping. This encourages development of stronger stems and branches. Young trees will better adapt to conditions around them increasing their durability to physical damage.
This thicket of crowded trees will benefit from a thinning.
WHEN AND WHERE
PCTs are done on dense young softwood stands that are less than 20 years old and between 2 and 6 metres in height. In these stands, growth will have slowed due to competition among trees. Stands older than 20 years may have already begun thinning themselves and the dominant trees may be taking over. Although stand quality may be improved from a thinning, growth benefits would be reduced.
A PCT should be done as soon as side branches begin to die. This will help trees respond faster to the thinning. However, PCTs are not commonly done on stands less than 2 metres in height because live branches will likely be found on the stem near the ground, making thinning difficult. Better trees may not yet exhibit dominant characteristics and some species may be subjected to insect damage (eg. white pine weevil damage).
Doing your PCT early will help avoid volume losses of crop trees.
- Shade foliage adapted to growing just under the canopy in a thicket will now be in full sunlight. Needles with cell arrangements suited for shade conditions are now in full sun. Many leaves fall off reducing the growth capacity of the tree.
- If the stand is left too long before doing a PCT, winds can sway the stems more than they have grown accustomed, which can damage feeder roots. The pulling action physically tears off the fine feeder roots. Reduction of the nutrient gathering capacity of the tree reduces growth until roots grow back.
- Snow and ice storms can bend and break tall, small diameter trees with short crowns.
- The cost of thinning larger trees is greater. Larger trees require more time to cut and fell. Selecting crop trees is also more difficult due to crowded working conditions and higher crowns. It is easier to remove preferred species in this situation.
- The effects on certain species of wildlife can be minimized. Snowshoe hare may be drastically affected by removing the cover found in the thicket. Early, frequent thinning avoids drastic changes in habitat.
- those that are dense and have obvious interference of branches between the crowns.
- those with an average tree height between 2 and 6 metres (6 and 20 ft).
- those with Stocking greater than 60 per cent or 60 per cent of the site is occupied with commercial or desired tree species.
- those with a site capability of at least 4. This means the site is capable of producing at least 4 cubic metres per hectare per year. This is known as an LC 4 site (land capability). Land capability is determined by measuring the height and age of dominant trees in a given area or stand. Better sites produce larger trees in a shorter time. The maximum capability found in Nova Scotia is LC 13. Sites with an LC of less than 4 are generally too poor for commercial forestry. Sites with a higher LC will respond better to thinning.
Generally, any species is suitable for a PCT with the exception of poplar and other intolerant trees.
DIAMETER GROWTH INCREASES BUT NOT HEIGHT
Reduced competition increases diameter growth. Young stands that have just reached crown closure will respond the fastest. Increased growth can be seen by viewing the cross section of a crop tree stem two years after a PCT treatment. Count two years, or rings in, after the PCT and look at the width of the annual ring.
The rings of most softwood species are easy to distinguish because springwood is white and generally the wider portion of a ring. Summerwood is the darker, narrower portion of the annual ring. Both are laid down in one growing season.
Older PCTs will not increase their growth as fast as younger stands. The response is delayed because a longer thicket stage left trees with fewer leaves and roots prepared for the sunlight and space after the PCT. Response will occur but take longer.
Studies have shown that more widely spaced trees grow faster in diameter.
Despite better growth responses, earlier PCT may result in more branches. These will result in more knots in the finished product.
However, increases in the width of annual growth rings may be delayed. Some trees go into shock for a year before they get the full benefit from a thinning.
When a commercial thinning (CT) is planned after a PCT, space according to what product you want to harvest during the CT.
RULE OF THUMB
Softwoods: Average Diameter in Inches at Full Stocking = Spacing in Feet
Hardwoods: Average Diameter in Inches at Full Stocking = Spacing in Feet – 2
Diameter is at breast height in inches and full stocking represents the time when the crown is fully closed. This is when the trees would be in full competition to the point of suppression.
For example, consider a red spruce stand that is pre-commercially thinned to a spacing of 6 X 6 feet. When the stand eventually becomes overcrowded and competition begins to result in reduced growth, the average diameter would be six inches. For a similar sugar maple stand, the average diameter would be four inches.
At what spacing would the average diameter of a white pine stand be 8 inches at full stocking?
This rule of thumb will tell you at what diameter your stand will be too crowded. This would reduce the immediate benefits of the CT if not done at that time. If the trees are healthy and still growing, you will harvest saleable wood from your CT. If the trees are beginning to compete strongly, some may be too small to be merchantable and other trees may be dead already. To avoid competition that results in tree loss or reduced growth, it is best to modify the rule. Therefore, if you want a 6 inch (15 cm) diameter product from a future CT treatment, it would be preferable to space your red spruce at 7 X 7 ft (2.1 m) or your sugar maple at 9 X 9 ft (2.7 m). This will ensure that your desired product size will be reached before you start to lose the benefit of the PCT (eg. lose fast growth). Remember, spacing hardwoods too wide can result in too many epicormic branches.
If you want to commercially thin the white pine stand when it is 6 inches (15 cm) in diameter, what spacing should it be spaced to?
Some considerations should be made when planning PCTs in hardwood stands.
Crop trees selected for hardwood saw logs or veneer logs should be a high value species such as red oak, yellow birch, white birch, or sugar maple. At the PCT stage they should show good form - straight stems, no forks, right angle branching and no signs of insect or disease. Choose trees that are fast growing, usually specimens that are dominant or co-dominant.
Trees selected for wildlife should be long-living, mass producers such as red oak or beech.
Thinning for maple sugar production requires healthy, vigorous trees, but not necessarily saw log quality.
Spaced hardwood stands can provide high value products in the future.
PCT in hardwoods is done later than softwoods, preferably when they reach a height of 6 to 9 metres (20 to 30 ft). This additional time allows the stems to straighten and self-prune lower branches off valuable lower bolts. This will also help prevent the formation of epicormic branches, which develop from dormant buds under the bark. They form on the main stem and reduce the value of the finished product.
Spacing is generally wider than a softwood site since hardwoods grow fewer stems per hectare. However, wider spacing may encourage epicormic branching. A good compromise would be spacing to 8 feet (2.4m), which would discourage branching while producing a 6 inch stem at commercial thinning time.
When selecting hardwoods to thin, preference should always be given to individuals that have grown from seed. When this is not possible, trees from sprouts may be utilized.
Spacing hardwood clumps to one or two stems will result in valuable stems later.
Sprouts, common in maples and birches, are caused by growth of dormant buds near the ground on remaining stumps. This occurs immediately after cutting, but is more noticeable after the felling of mature trees. These sprouts grow very fast as they develop from an established root system.
Release one or two widely spaced stems per stump. Selected stems should be positioned on the stump as close to the ground as possible. This will encourage sprouts to develop their own root system as the stump begins to decompose. Sprouts that leave the stump in a J-shape are preferable to those of a V-shape. The V-shape is more likely to collect debris and introduce rot to the developing clump. J-shaped sprouts are also more likely to grow their own root systems.
HOW TO DO A PRE-COMMERCIAL THINNING
Decide on Spacing
Strive for uniform spacing of suitable crop trees. Crop trees will be determined by desired product and available species on the site. Where no crop trees are present, any healthy tree should be left. Spacing is determined to some extent by the capability of the soil to grow trees (land capability).
(stems per ha)
|hard woods||soft woods|
|4 to 7||2.4||2.1 to 2.4||2300 to 1700|
|8 to 9||2.4||2.1 to 3.0||2300 to 1100|
Uniform or consistent spacing is most desirable, but there will likely be some open areas or holes. Trees around these openings should be left tighter than the desired spacing. The trees will be compensated for the closer spacing by increased light, moisture and nutrients on the open side.
Mark Crop Trees to Leave
Choose straight, well-pruned stems or boles. Give preference to trees with larger diameter boles and well developed crowns in a dominant or co-dominate class. These will most likely be the healthiest trees.
Remove trees with damaged or double tops. This includes damage from porcupines as well as mechanical damage from other trees or machinery. Diseased trees should also be removed. If nearby trees appear infected, have a knowledgeable person determine the nature of the disease. Some diseases can be transported from one tree to another by saws or other cutting devices. Delay treatment until you know you're not spreading a disease. Dead trees can be cut or may be left standing. If they aren't likely to cause mechanical damage to crop trees or create a safety hazard, then nothing is gained by the felling. They may also be useful for wildlife.
If a choice must be made between two double-topped trees, always leave the U over the V. Look at where the tree splits on the main stem. The shape created by this joint will be preferable if it is in the shape of a U. This will allow debris and moisture to fall from the joint more easily. Moisture and debris can create decay that weakens the joint and may eventually lead to structural failure.
If selection is being left up to the contractor, which is quite often the case, make sure it is clear what future crop you want. The contractor should also be informed of any special leave trees, such as boundary or nest trees.
The species to favour will depend on the objectives. Where a choice of species must be made, the following rule may be used.
R U L E O F T H U M B
Softwoods: white pine, red spruce, red pine, hemlock, black spruce, white spruce, tamarack, fir
Hardwoods: sugar maple, yellow birch, white ash, red oak, white birch, poplar
Actual spacing of the treatment should be based on site quality and species. Remember to consider future treatments such as commercial thinning. Wider spacing is generally preferred on better sites; growth is more rapid because of greater nutrient availability. Trees take less time to replace the volume that was removed and crowns close faster.
Mark Strips and Begin Cutting
The most efficient work method is to divide the area into strips 40 to 50 m wide. If possible, strips should be run at right angles from a road and across slope. Using a compass or sighting a landmark on the other side of the treatment area will assist you in keeping lines straight and parallel. Mark strip edges with flagging tape or string to keep you or the workers within the strips. If you don't plan to collect the string or tape, after completing the work, use a biodegradable product.
When working on steep slopes start at the bottom of the strip and proceed up. Your footing will be better and you can cut trees much easier.
Avoid nicks to crop trees that will slow growth and leave a wound that could allow disease or insects to enter. If you damage a crop tree during the treatment, remove it and choose another. Depending on the severity of the damage, it may be better to leave a less desirable species rather than leaving a severely damaged crop tree.
It can be difficult to cut trees below low growing branches which can turn up and form new trees.
Always leave trails and roads open. Trails will give you access to the back of the stand when doing the thinning.
Knock the cut trees to the ground to speed decomposition of this material into nutrients for the remaining trees and to minimize damage from falling trees after the treatment is complete. It also makes for a better looking job.
Many people take the time to cut the branches off all cut trees. This reduces the overall bulk of the brush and slash. This is an aesthetics only procedure. If your stand has been properly managed, there should be ample organic material from the harvest operation to produce a suitable nutrient supply for many years to come. If the PCT brush is up off the ground because branches have not been removed, it will only delay the breakdown of this debris. The delay will not be long because the snow and ice will push it to the ground in a year or two. Debris left on the site is slow release fertilizer for your remaining crop trees.
Check Your Spacing
It is a good idea to do an easy check to ensure that your spacing is adequate as you do the thinning operation. This is a good check to perform, especially if you're new at thinning; you're working in unfamiliar surroundings; or it's the first time you've worked with a specific type of tree. Use a 4 metre string fixed to a centre point in a thinned area. Walk away from the centre until the string is taught. Count all crop trees within the circle as you move completely around the centre point. It's like drawing a circle with a protractor. Once you are sure you have all crop trees accurately counted, multiple your answer by 200. This will give you the number of trees per hectare. If the number of crop trees equalled nine, for example, you will be leaving:
9 crop trees in your circle X 200 = 1800 trees per hectare
This would indicate that your spacing would be about 2.4 X 2.4 metres. This is determined by taking the square root of the number of trees divided into the number of square metres in a hectare.
|10, 000 / 1800|
Go to a mature stand of trees and estimate density in the same way. The results may surprise you.
It is not essential that your spacing always be the same. However, uniform spacing will ensure growth is more predictable.
METHODS OR TOOLS
Manual methods are dangerous, slow, and physically demanding. These methods are seldom used.
However, they do provide exercise, cardiovascular, strength and endurance. They do not burn any fossil fuels and are quiet compared to motorized methods. A good alternative to the axe is the lopping shears. An improved blade and lighter handle materials have made this a better tool than it was in the past. Many of them are capable of cutting 5 cm (2 inch) diameter hardwoods with ease. If you have a small area to do or need to touch up, don't overlook this tool.
Axe, brush axe, brush hook, and long handled anvil pruners are very labour intensive but might be make sense for some landowners.
Chemical thinning is becoming less popular. Biodiversity of plant complexes can be drastically affected. Early thinning or cleaning can be completed by aerial application using herbicides. Extremely large areas are completed quickly, but this method offers very little choice in selection of leave trees. Most hardwoods will be removed, which is not preferred in current thinning practices.
Most conifers tend to be resistant to damage, especially if applied in the fall. This is advantageous if you're growing conifer crop trees. The hardwoods, shrubs and herbs can all be thinned out with very little effect on the conifers.
Monsanto easy-ject herbicide injector
A more practical method for using herbicides in PCTs is with injection systems or basal treatments. These are useful tools, especially when thinning hardwoods. Remember, hardwoods can re-sprout quickly causing undesirable growth in the understory of your PCT. These sprouts can rob nutrients from crop trees even in an understory position.Injection or basal application of herbicides can control both the top growth of the tree and the complete root system.
Always seek professional advice when using herbicides. You must have a Pesticide Applicators Certificate to use forestry herbicides in Nova Scotia, even on your own woodlot.
Spacing With Machines
Machine spacing is the use of a mowing device attached to a tractor, skidder, or other machine. They are commonly used for controlling woody growth on the shoulders of highways. When used for PCTs, the device is driven across the stand in parallel lines, mowing down approximately one half of the area to be treated. The leave strips can then be lightly thinned or left to slowly encroach into the cut strips. This method does not lend itself to uniform spacing and crop tree selection is limited. Other problems include damage to the edge trees in the leave strip and high stumps that encourage re-establishment of the cut trees. This method was designed as an economical method for extremely large areas most often managed by large forest companies.
The chain saw has been used for PCTs, but is not preferred. It is physically tiring and can lead to lower back problems if improperly used. The chain saw was not designed to cut small trees close to the ground. High cuts can often miss lower conifer branches that will quickly grow a new tree. Low cuts will often result in a damaged chain, which makes safe use more difficult. Chain saws require skill and safety when used for their intended purpose and more caution must be used for PCTs. The chain saw may be more productive than manual methods, but it falls far short of the clearing saw. Its use is based primarily on its availability over the clearing saw; many woodlot owners already have one.
Clearing or Spacing Saw
This is by far the most common and preferred method for PCT. The danger zone of the blade is out of the operators way, so even if it strikes a tree improperly the chances of injury are very low. The biggest risk is when two people are working too close together. This is easily rectified by keeping workers a safe distance apart.
Properly balanced spacing saws are easy to use and cause far less fatigue than the chain saw or manual methods. It is designed to cut trees low to the ground and provides good visibility to avoid injury to crop trees. Although most clearing saws can cut trees up to 10 cm (3 inches), smaller diameters are more easily cut. In smaller diameter PCTs this method is very fast.
The clearing saw has become a standard in both hardwood and softwood PCT. The disadvantage for small woodlot owners is its specialized nature and cost. Fortunately, spacing saws can be rented when you're working on your PCT. Proper use of the clearing saw can be learned easily from an experienced operator.
Safe use of both the chain saw and clearing saw is described in the Professional Forest Worker Safety Manual. This manual can be obtained from the N.S. Dept. of Environment and Labour. See Appendix 1 for contact information.