LESSON THREE: THE SELECTION SYSTEM
GENERAL
If the landowner wants a more or less continuous harvest from a particular stand or location in the woodlot, the selection system should be considered. Selection cutting allows a regular harvest of trees from several age classes without ever removing the entire canopy. It is often favoured by woodlot owners who do not want to clearcut.
Selection cutting removes poor quality, mature and immature trees and provides space and seedbed conditions for the establishment and development of new trees. As in a shelterwood the best trees are left for seed and shelter.
Stand tending and harvesting are done at the same time. It encourages the development and maintenance of several age classes in the stand. Stands suited to selection cutting are presented in Table 6.
Also, even-aged stands containing young, healthy, good quality, long-lived species (ie. red spruce) are more suited to a commercial thinning ( see page 10) than a selection harvest. However, these stands may become more suitable as they mature. Although it is an uneven-aged system selection cutting can also be used to make an even-aged stand uneven-aged.
If not properly done, selection harvesting can turn into "high-grading" or selective cutting. As discussed in the introduction, high grading takes only the biggest and best trees and leaves the less desirable trees. An expression used in Nova Scotia sums it up as "... take the best and leave the rest! ". This problem is compounded because new trees are regenerated by an inferior seed source.
To ensure continual harvesting of a stand, wood should not be cut faster than it can be grown. For example, if a stand increases in volume by 5.5 cubic metres per hectare per year (one cord per acre per year), you may choose either to harvest this amount every year or to cut ten cords per acre every ten years from that stand.
If wood is cut faster than it is grown, the mature usable timber will eventually be used up. If, however, the stand contains a high proportion of over mature trees, more wood can be cut during some cuts to use all mature timber and gradually develop the younger age classes.
On the other hand, if the growth rate is undercut, eventually there will be an excess of mature to over mature trees in the stand and trees will die. Periodically examining and cutting accordingly, should ensure cutting balances growth.
In theory, an uneven-aged stand has the potential to produce more wood fibre than an even-aged stand because the irregular canopy occupies more growing space and wood is salvaged on a regular basis. This is not fully proven.
Landowners unfamiliar with the selection system will find it easier to mark trees prior to harvesting. This will allow them to visualize what the stand will look like after the harvest.
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| Figure 7: Group and single tree selection methods. |
It can take several harvests before a stand that started out in poor condition will consistently yield good quality
timber with each harvest. The most common approaches to selection harvesting are single tree selection and group selection (Figure 7). Often, operations are a combination of both.
Advantages and disadvantages of the selection methods are presented in Table 8.
SINGLE TREE SELECTION
With single tree selection, trees are removed individually and each tree is judged on its own merit. Table 7 lists characteristics of trees to leave or cut.
Choosing trees to remove also depends upon the age class distribution of trees in the stand (Figure 8). Trees larger than a certain maximum diameter and a percentage of total stand volume are removed. The most important thing is that the best trees are left with room to grow and that trees are removed from all age classes.
Because only small openings are created, this method favours shade tolerant species that can thrive in low light conditions.
Considerations for selecting trees to cut or leave while practicing single tree selection. All these things should be considered before making a decision. If a tree meets any one of the conditions on the right it should be cut, unless it creates a large opening.
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| Figure 8: Example of a stand with several age classes showing trees to cut |
GROUP SELECTION
Group selection is the harvest of small groups or clumps of trees rather than single trees. The openings created allow the regeneration of some species that cannot grow in the shade.
Although similar to small patch cuts, these openings do not remove the effect of the surrounding trees from most of the harvested area. In addition, the openings are too small to be managed as even-aged stands. As with patch cutting, group selection is suited to many of the patchy stands created by cutting habits of many Nova Scotia landowners.
Group selection has several advantages over single tree selection. It is less expensive because the harvested trees are concentrated in patches. Tree marking and felling take less time. Assessing the regeneration quality is easier because regeneration is concentrated in clumps. Finally, early thinnings are easier to define and carry out.
As in the single tree method, thinning and harvesting should be done at the same time. The new trees regenerating in the clearings are considered part of the larger stand that contains trees of many ages.