LESSON TWO: THE SHELTERWOOD HARVESTING SYSTEM
GENERAL
Shelterwood harvesting is an even-aged system used to establish and develop desirable natural regeneration. By removing a mature stand in two to three cuts over five to 20 years, more sunlight reaches the forest floor to stimulate seedling development. The overstory provides seed and shelter for seedlings that can grow under partial shade.
Increased sunlight and the protective overstory give existing or new regeneration a better chance to grow and develop. The shelter encourages water to remain in the organic layer, reducing the risk of seedlings drying out.
The stand is thinned, removing poor quality species and trees as a seed source. This provides space for the remaining better quality trees to increase crown and seed production and their growth rate. Ideally, the first two cuts should be done when the trees have lots of seed. Table 4 gives the minimum and optimum ages for seed production and the time between good seed crops for all commercial species occurring in Nova Scotia.
The shelterwood system is recommended for long-lived species that can grow in partial shade like red spruce, eastern hemlock, white pine, yellow birch, or, white ash, sugar maple. However, it also works in poorer quality stands that are made up of least 30 percent of these species.
Table 3 lists stand characteristics which lend themselves to the shelterwood method. Although primarily used in even-aged stands, it may also be used in some uneven-aged stands when even-aged regeneration is desired.
Shelterwood harvesting is usually the most difficult and costly during the initial cut when the poorest quality trees are removed. However, revenue produced by the wood extracted during the release and final cuts will reduce the financial impact and should result in a positive financial gain for the land owner.
Shelterwood cuts are considered more attractive than clearcuts. By removing a stand gradually, the area always has either some mature forest cover and/or a new, developing forest on the site. This approach also reduces the impact on resident wildlife.
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| Figure 4: Stages of uniform shelterwood method. |
The Initial Cut
The main idea behind this cut is to allow sunlight to reach the forest floor. It should also encourage tree crown enlargement and an increase in seed production for the remaining trees. Twenty-five to forty percent of the total basal area (similar to wood volume) is removed.
Sometimes this cut is not necessary if regeneration is already present. This may occur where previous thinnings left some openings or where trees have died, creating natural openings.
As with all cutting, creating a good seed bed is important. Therefore, where possible it is best to drag or skid the wood to mix the organic and soil layers. If this is not possible, some other site preparation might be needed.
Lay out wood extraction trails through the stand during the initial cut. The trails will then be in place for the release and final cuts and may continue to provide access into the stand for many uses. Lay out trails to minimize any damage to the stems or roots of standing trees.
To avoid confusion, we will discuss commercial thinning, a treatment done the same way as the initial cut. Unlike a shelterwood harvest, this stand tending treatment is generally applied in stands less than 50 years old. The objectives of commercial thinning are to increase yields by harvesting trees that would normally die and to promote the growth and quality of crop trees. It does not try to promote regeneration.
The Release Cut
When adequate regeneration is present, thirty to forty percent of the remaining volume is removed, again leaving desirable species such as red spruce and yellow birch. The idea of this cut is to release the young trees by giving them more light, water and nutrients. This encourages them to push their roots through the organic layer and into soil.
Since damage to some of the established regeneration is often unavoidable during the final cut, remove as many overstory trees as possible while still maintaining adequate seedling protection, and a seed source if necessary. Uncut trees should preferably be left uniformly scattered throughout the stand, but can be left in clumps.
If regeneration is not present five to ten years following the release cut, consider site preparation to better prepare the seed bed. In some cases, planting may be necessary. On the other hand, if the roots of the regeneration pushed into the mineral soil following the initial cut, you may skip the release cut and harvest the remaining standing timber in one cut.
The Final Cut
Once the seedlings are firmly rooted in mineral soil and have attained a height of approximately 20 cm (8 inches) all the remaining trees can be removed.
Although some damage will likely occur during this harvest, it can be minimized by carrying the wood and harvesting in winter. Winter cuts reduce seedling damage because often the regeneration is protected by a blanket of snow and ice.
The final cut can be done in stages. Some areas may need release sooner than others. In some cases, residual trees may be left longer to increase their value, or the removal of remaining trees may be gradual to reduce the shock to the regeneration.
THE STRIP SHELTERWOOD METHOD
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| Figure 5: A comparison of uniform and strip shelterwood methods |
The strip shelterwood method consists of cutting narrow strips and uniformly thinning the uncut strips between the harvested sections (Figure 5). Cut strip widths can vary between six to eight metres (20-25 feet) while residuals can vary between seven and twenty metres (22-60 feet). A maximum width for cut strips of eight metres is recommended to maintain the shelterwood effect.
As with strip clearcutting there are three things to consider when laying out strips:
- Orient the strips northwest to southeast so that the shade from standing crop trees protects any new seedlings from the hot sun during the summer.
- Locate the long side of the cut at right angles to the prevailing winds to promote even seed distribution.
- Accessibility and layout in relation to roads. There is no orientation that is good for all situations.
The stages are the same as for the uniform system.
THE SEED TREE METHOD
An open shelterwood is called seed tree cutting (Figure 6). The seed tree cut leaves only a few standing trees, individually or in groups with the goal to obtain regeneration. It can be a cost effective way to obtain quality regeneration. Selected seed trees provide a seed source and are usually (although not always) harvested after the crop of new young trees has become established. They can also be left to develop into cavity trees for wildlife.
Unfortunately, seed trees will work only with some species and site conditions. Seed tree cuts are currently recommended for stands of white pine, yellow and white birch and sugar maple.
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| Figure 6: Seed tree cutting. Skidded dragging logs will help mix the organic and mineral soil. |
They are not generally recommended for shallow-rooted species such as spruce and eastern hemlock which are prone to blowdown. However, recent observations indicate that groups of deep-crowned red spruce trees can work on sheltered sites. These groups are less likely to be blown over by the wind, and might survive long enough to provide adequate seed for the next stand.
The seed tree method should not be used for red maple or poplar/aspen. Trees from stumps and roots of cut maples and aspen will "out compete" and smother the new crop of regenerating seedlings (if not controlled).
Regardless of species, seed trees should be healthy, windfirm, quality stems, and at a good seed producing age (Table 4). Leave enough seed trees to ensure adequate seed is spread over the treated area. Each species requires a certain spacing according to its ability to spread seeds; for white pine leave 20-30 trees/hectare (eight to ten per acre) and for birches and maples leave 15 trees/hectare (six per acre).
As with other natural regeneration techniques, the harvesting operation should mix organic and mineral soil to help new seedlings become established. Table 5 presents the advantages and disadvantages of the shelterwood methods. Many disadvantages can be overcome by proper training.