LESSON ONE : THE CLEARCUT SYSTEM
GENERAL
Clearcutting is the removal of all trees from an area in a single cut, with the expectation that a new even-aged forest will be established following the harvest (Figure 1). To be considered a clearcut, the cut must be sufficiently large to increase the air and soil temperature, rate of organic matter decomposition, and decrease surface soil moisture content from the majority of the harvested area. This forest influence will not be re-established until a new forest occupies the site. The surrounding forest does, however, maintain some forest influence for some distance into the cut.
Small cuts where the forest influence is not removed belong to the shelterwood or selection systems discussed in lessons two and three. Thus, clearcut size can range roughly between 0.2 hectares (0.5 acres) and 50 hectares (125 acres) or, rarely in Nova Scotia, more.
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| Figure 1. A clearcut with wildlife clumps. Notice the regeneration in the foreground. |
Some trees or clumps of trees should be left for wildlife, but if excessive amounts of poor quality trees are left on site then the cut becomes more of a high grade cut. These poor quality trees should be felled and cut to make them lie close to the ground. This will speed up decomposition. Cutting these residual trees will also minimize their interference with the seeding and growth of the new forest.
It is often thought that clearcutting removes wildlife habitat. While this is true for the habitats of species that require mature forests, clearcutting also improves habitat for some wildlife species. Species that satisfy all or part of their life requirements in low vegetation and brush with -little overhead shade (ie: small rodent species and ground nesting birds) generally are abundant following a clearcut. In addition, birds of prey and other predators which eat small mice and rodents will also frequent the clearcut areas to hunt. At any rate, the effects of cutting can be reduced by following the wildlife guidelines discussed in Lesson 4.
Clearcutting should not be done without considering how a site will grow back with new tree seedlings. If planting and/or removal of competition for the seedlings will be required, landowners should set some money aside from wood sales to help cover these costs. Following the advice in the next section will reduce the chances of this happening.
Generally, clearcutting should be practiced in mature or older stands that contain species that can regenerate and thrive following a major disturbance, and where it satisfies the landowner's objectives for that area. More details are given in Table 1.
The advantages and disadvantages of clearcutting are presented in Table 2.
| Table 1. CHARACTERISTICS OF STANDS SUITABLE FOR CLEARCUTTING |
| * there is a high percentage of dead and /or unhealthy trees (stand is overmature) |
| * there is a high percentage of trees that are diseased and /or being attacked by insects |
| * there has been extensive damage by fire or wind |
| * stand is exposed to strong winds and/or is poorly drained |
| * short lived (eg. balsam fir) species and/or species that are not able to grow in the shade (eg. trembling aspen) make up more than half the stand |
| * where adequate regeneration of desirable trees is established or is predicted to establish after clearcutting |
CLEARCUTTING WITH NATURAL REGENERATION
More than two-thirds of clearcut areas in Nova Scotia regenerate naturally, but the resulting regeneration may not have the same species mix as the forest that was removed. Thinning the stand to favour desirable species ten to fifteen years after harvest can often improve the mix and quality of the new forest. In fact, this early thinning is beneficial to most naturally regenerated stands.
But there are ways to improve the species mix that occurs following a clearcut that should be considered prior to cutting. Partial cutting systems are sometimes an option and are discussed in lessons two and three. This discussion will focus on clearcutting.
First, we need to understand where new trees come from following a cut. They can come from seeds already on the forest floor, seeds blown from trees along the cut edge, seeds from harvested trees, or from new growth from roots and stumps of harvested trees. Finally, regeneration established prior to cutting also helps to reforest some clearcut areas.
So natural regeneration depends on natural seeding before, during, or after the cut. When planning the cut, first evaluate any regeneration present. If regeneration is present, will it survive? During harvest, avoid damaging it as much as possible. Damage can be minimized by forwarding (carrying) the wood and by cutting in the winter when snow is present. However, some damage is unavoidable and is acceptable if the regeneration is very dense and spread over the whole site.
If no regeneration is present, you must rely on seed. Although it may already be present in the forest floor, seed more than a year old of most tree species is not capable of germinating.
Therefore, new regeneration is usually dependent on the spreading of new seed. The chances of spreading seed are better if there are desirable tree species along the proposed cut edge, and if the cut is done when these trees and the felled trees are full of ripe cones.
A good seed bed is also important for seedling establishment. Each species has seed bed requirements that improve its chances of survival.
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| Figure 2: If possible, strips should be laid out to take advantage of the prevailing winds (a) and/or Northwest to Southeast to reduce seeding exposure to sun (b). |
However, mixing the soil and upper organic layer is helpful for most species. It is essential for species such as yellow birch. Mixing can be done by skidding (dragging) wood to roadside during summer, spring, and fall. More specialized mechanical equipment can also be used after any harvest to improve the seed bed.
Seed coverage can be improved by limiting cut size, by taking advantage of prevailing winds, and by making irregular edges.
More seed is dropped along the leeward side of a cutover than along its windward side (Figure 2a). If practical, lay out any long edges at right angles to the prevailing winds. In Nova Scotia, the prevailing winds generally come from between Northwest and Southwest.
Also, more seed is dropped near an edge than towards the centre of a cut. Besides improving seed distribution, irregular edges usually look better and are better for wildlife (see Lesson 4).
Variations of clearcutting have been developed to take advantage of edge and prevailing winds. These strip and patch cuts involve removing a stand in several cuts, usually over a period of five to ten years.
Clearcutting in Alternate Strips
One method that takes advantage of edge and prevailing winds is the alternate strip method (Figure 3a). The area to be cut is first divided into several equal width strips by marking the strip boundaries with flagging tape. Then every other strip is cut.
The leave strips provide seed for the cut strips and protect them from too much sun and wind. If practical, they should be laid out at right angles to the prevailing winds and/or in a northwest to southeast direction to minimize exposure of the seedlings to the sun (Figure 2b).
A good rule of thumb is to keep the strips narrower than twice the height of the average seed-bearing trees. A common strip width is 20 metres (66 feet). The leave strip should be wide enough to be windfirm, and have enough desirable trees to provide adequate seed. If the cut strip is very narrow (less than half the average height of the stand), then the harvest might be called a strip shelterwood cut (see page ).
Before removing the leave strips, make sure the cut strips are regenerated. Then decide how to regenerate the leave strips. In windfirm stands, a light shelterwood cut (see Lesson 2) can be used to encourage the development of natural regeneration within the leave strips. In some cases fill planting may be required to re-establish a forest cover.
Clearcutting in Progressive Strips
Clearcutting in progressive strips is the removal of the stand in three successive cuts over a five to fifteen year period (Figure 3b). In contrast to alternate strip cuts, every third strip is cut. The advantage of this is that at least two thirds of the area is regenerated before the final strips are cut. Otherwise the two methods are very similar.
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| Figure 3: Strip cutting in a) alternate and b) progressive strips. |
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Clearcutting in Patches
Clearcutting in patches involves harvesting the stand in a series of small clearcuts. It is recommended where:
- aesthetics are very important
- uneven ground limits accessibility
- marketable timber occurs in small clumps scattered throughout the stand (common in Nova Scotia due to past cutting practices)
- there is a high risk of blowdown, preventing partial cutting
- there is a hot spot of insect attack (eg. hemlock looper)
Because the cuts are small they should receive good seed coverage and adequately regenerate in a few years. Following this, future harvests can enlarge the patches and/or create new ones. To keep the seedlings growing fast and to develop an even-aged forest, the entire area should be cut within 10 to 20 years of the first cut.
CLEARCUTTING WITH ARTIFICIAL REGENERATION
If natural processes are unable to adequately regenerate a harvested area, it must be regenerated artificially. Attempts to seed cuts artificially generally have not been successful in Nova Scotia. Therefore, planting is the most effective option.
Planting an entire area (full planting) can cost $1,000/hectare ($400/acre) or more due the costs of site preparation, seedling purchase, planting and weeding. This is why we said on page 2 that money should be set aside to help cover these expenses.
If parts of a cut regenerate naturally, only the areas where no trees are growing need to be planted. This fill planting technique ensures that the area is fully stocked with trees, but it saves on site preparation and seedling purchase costs.
Prior to a full planting, site preparation is usually necessary. This involves the rearrangement and/or crushing of brush and tops to make planting easier and to reduce the fire hazard. Sometimes driving a loaded forwarder over the site is sufficient to do this.
Planting can also be made easier by mixing the soil and upper organic layer. This can be accomplished to a limited degree by dragging (skidding) the wood out.
In addition, special site preparation equipment can be used to do the same things. Less often, special ploughs can be used to improve the drainage of the site.
Once the site is ready, it is desirable to plant soon. This would allow the planted seedlings to get a head start on competing plants and take advantage of nutrients being released from decaying branches and organic litter.
However, because of the threat of damage from the seedling debarker weevil, mixed wood and softwood sites should not be planted for at least two years from the date of harvest. Aging makes the stumps and slash less attractive to the weevil.
Container stock grown in plastic trays is most commonly used for planting in Nova Scotia. It is available for most commercial species. Bare root stock used in the past is no longer readily available.
In general, clearcuts are difficult environments for newly planted seedlings. Therefore, seedlings must be chosen and planted carefully. Species should be carefully matched to site (eg. white pine on dry, deep sandy site and black spruce on moist, shallow soils). These are discussed in more detail in Module Five in this series.