Guidelines for Applying Group Selection Harvesting
NEIL I. LAMSON is a silviculturist with the USDA Forest Service, Northeastern Area, State and Private Forestry at Durham, New Hampshire.
WILLIAM B. LEAK is a research scientist with the USDA Forest Service, Northeastern Research Station at Durham, New Hampshire.
The authors gratefully acknowledge the helpful reviews of this material by David Kittredge, Thom Kyker-Snowman, and Gary Miller.
What is it?
Why do it?
For species dependent upon advance regeneration, such as red oak and white pine, the size of group to be cut is often determined by the aerial extent of the advance regeneration. The group is usually cut slightly larger than the area of advance regeneration, unless advance regeneration is abundant and uniformly distributed throughout the stand.
Technically, there is no upper limit on the size of groups, as long as the groups are not recognized, mapped, or retreated as individual stands. Keep in mind that the primary purpose for cutting groups is to establish regeneration. Group selection has been successfully applied on large ownerships by cutting groups as large as 2 acres in size.
How many groups?
Harvested volume can be used to control the number of groups (Miller and others 1995). After estimating the periodic volume growth of the stand, groups are established and the stand is thinned between the groups until the volume harvested is equal to the periodic volume growth. This requires a convenient way to compute volume, as a running total of harvested volume must be maintained. Group size is determined by light requirements of the desired species.
Where to locate the groups?
Laying out the required acreage and number of groups, and distributing these groups throughout the stand has been approached in several different ways. Usually a preliminary examination is conducted to establish the size and number of groups to be cut. It is best to consider the stand conditions to determine group location. Placing groups on a fixed grid is not recommended unless the stand is quite uniform. On the other hand, traversing the stand without any spacing control may lead to excess cutting in certain portions of the stand and minimal cutting in other harvestable areas. One possibility is to follow these steps: (1) use the estimated number of groups to determine a rough spacing, (2) pace the distance between grid points, and (3) at each grid point, establish a group at the nearest location that has the appropriate overstory/understory conditions.
When a new group is located near an old group, it is best to put the new group right against the old group without leaving a narrow strip of trees that cannot easily be accessed in future cuts. Another option is to enlarge some old groups rather than create all new ones. A strip wide enough to create desirable light conditions is cut around existing groups. This method has been used successfully in Europe, although it is a bit more difficult to compute the acreage of cut groups.
Should small trees be cut?
With other species, however, some small trees are left in the groups. This is a common situation with red oak, white pine, tolerant softwoods such as spruce, fir, or hemlock, and sometimes with sugar maple or ash. The procedure is to identify clumps of desired advance regeneration and then to remove the overtopping overstory trees. It is necessary to take appropriate precautions to protect the advance regeneration through snow cover and careful logging methods.
In northern New England, advance beech regeneration can interfere with more desirable species. A dense understory of beech often develops in stands that have been lightly cut or where the beech bark disease has weakened or killed overstory beech. In such cases, it is recommended to cut all the stems larger than 2 inches d.b.h. to control the beech regeneration. Failure to do so will predispose the regeneration within groups to be dominated by beech.
How to locate skid trails?
Another way is to locate and construct all the main skid trails before any groups are designated. During the first entry, all the main skid trails are constructed, as well as any connectors needed to access groups. The advantage of this method is that roads are not constructed through existing groups in future cuts. However, it does incur the cost of constructing all the main skid roads during the first entry.
Thinning between the groups or not?
It is generally recommended to leave a residual basal area between the groups of about 70 square feet/acre, excluding the new groups (Leak and others 1986). It is also recommended that 40–50 square feet/acre of the residual basal area be left in sawtimber to ensure that there will be adequate volume for future cuts. This also prevents diameter limit cutting, a practice that generally is not sustainable. It has been suggested that this thinning can follow single-tree selection guidelines in terms of residual density and stand structure. However, after several group selection cutting cycles, the stand will become a mosaic of groups, and applying single-tree selection guidelines will be quite challenging. In practice, marking between the groups usually resembles an improvement cut or thinning that favors the development of groups of overstory trees. After one or two group selection cutting cycles, some of the thinning between groups will be in the older groups where crop tree release, weeding, cleaning, or other timber stand improvement can be undertaken.
Concentrate cutting between groups along existing skid trails and around the edges of new groups. In this way, regeneration in older groups can be avoided and damage to residual trees can be minimized. Large trees that lean towards newly cut groups should be cut to prevent future damage to regeneration or young stands developing within the groups. There is some concern that trees along group borders will decline in quality since they are drastically exposed on one side. This has not been a significant problem, although it is well known that such exposure may trigger epicormic branching or the maintenance of live limbs. For that reason, it is recommended to leave border trees that will suffer little quality loss (e.g., trees with dominant crowns, wildlife trees, and small trees that will not compete with the trees developing in the new groups).
Thinning between the groups will increase the amount of advance tolerant regeneration. Where this advance generation is of a desirable species, clumps of regeneration can be released by cutting future groups. If the advance regeneration is undesirable (e.g., beech or sometimes fir), it can be removed when new groups are cut.
Group selection has only a moderate impact on breeding bird populations. One study showed that recent clearcuts, early group selection cuts, and mature stands were used by 46, 33, and 30 bird species, respectively (Costello 1995). Only a few bird species were found in the mature, unmanaged stands that were not found in the group selection areas. About 13 breeding bird species were found only in large clearcuts.
Beech is an important wildlife species, but throughout most of its natural range, beech bark disease has reduced the amount of beechnut production. Group selection can be used to increase the amount of beechnut production. First, using small groups (less than 1/4 acre) can successfully regenerate beech. Second, when thinning between the groups, individual beech trees that show resistance to the disease can be released to produce larger crowns and hence more beechnuts. And third, the best beech can be retained in the stand and never cut. This can be accomplished by locating newly cut groups so that desirable beech is left on the edge of the groups. This will promote crown development on the desirable beech, while encouraging other species of regeneration within the newly cut groups. When new groups are cut adjacent to existing groups, the same beech can be retained along the edges of newly cut groups. Thus, the most desirable beech will be retained and fully released while not interfering with the development of regeneration within groups.
Many private forestland owners appear to prefer group selection to larger clearcuts because small openings are more acceptable than large cuts. Newly cut groups can improve esthetics by opening up views of the sky and creating vistas.
Group selection is a good timber producing system. Our experience in New England forest types is that group selection harvesting produces enough high quality, large diameter logs at each harvest so that it is economical and readily accepted by loggers. Group selection is an uneven-aged system which produces a sustainable income every 15 to 20 years. This is an important consideration for many landowners who own less than 100 acres and want to have a regular income from timber harvests.
Group selection is a very flexible regeneration system. Group size can be varied to favor regeneration conditions for a variety of species. Wildlife habitats can be improved by varying the group sizes and varying the treatments between groups.
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