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Group Selection: Managing for Mid-Tolerant Tree Species
Single-Tree Selection System
Selection silviculture is designed to mimic small-scale disturbances to maintain wildlife habitat and forest aesthetics, while still allowing timber harvesting.
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Evidence suggests that mid-sized gaps in the forest canopy that allow more light to reach the forest floor were common components of historic and old growth forests. |
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Single-tree selection and diameter-limits leave the forest canopy primarily intact and allow very little light to reach the forest floor. Although we recognize that single-tree selection has been successfully used to manage for a multitude of values, its long-term use appears to be affecting the regeneration success of a number of mid-tolerant and intolerant tree species. Oak and hickory and other Carolinian species in the south, along with yellow birch, black cherry and red oak in central Ontario, are now declining in forests where single-tree selection has been followed.
Evidence suggests that mid-sized gaps in the forest canopy that allow more light to reach the forest floor were common components of historic and old-growth forests due to blow-down, ice storms and disease. These gaps are lacking in forests managed through single-tree selection.
What is Group Selection?
Group selection is a modification of the single-tree selection system. It involves the removal of small groups of trees, rather than individual trees, and emphasizes creating openings in the forest canopy of sufficient size to encourage the development of mid-tolerant tree species.
Why study Group Selection?
The structure and composition of the upland hardwood stands that dominate the remaining forests of southern and central Ontario have evolved under a history of small-scale disturbances that cause canopy gaps ranging from a single tree to small groups of trees. However, there is little documented information that demonstrates the best way to use group selection as a component of our forest management activities. The group selection system needs further study in order to advance our knowledge of how to successfully use this harvesting system in the regeneration of mid-tolerant hardwood species.
Details of Group Selection Prescriptions
Several factors need to be looked at when you are managing for mid-tolerant species using group selection. They include:
1. Species priority
2. Opening size
3. Location of openings
4. Shape of openings
5. Harvest
6. Other considerations
1. Species Priority
Group selection is typically used in tolerant hardwood stands, such as maple, that have a high component (about 1/3 of the basal area) of mid-tolerant species. The quality of the mid-tolerant species present, especially those trees located where the gaps will be created, should be of equal or better quality than the surrounding tolerant species.
2. Size of Openings
The size of the canopy openings needed to successfully regenerate mid-tolerant trees is based primarily on the light requirements of specific species (refer to Table 1). If the gap created is too small, sufficient light for mid-tolerant species will not reach the forest floor. Instead, an environment more favourable to tolerant tree species will be created. However, if the canopy opening is too large, conditions will be more suited to the establishment of intolerant species such as poplar and white birch and possibly undesirables such as invasive shrubs, trees and vines.
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The gaps created by group selection allow more direct sunlight to reach the forest floor. As a result, mid-tolerant tree species that would normally be lost to suppression in the light-deficient understorey can be established.
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| Table 1: Recommended Gaps for Regeneration (Source: Ontario Tree Marking Guide) |
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Species
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Size of Gap
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Other Considerations
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| Sugar maple and beech |
- 0.01 to 0.03 ha
- less than 20 m in diameter or 1/2 of the canopy height
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- very shade tolerant and may germinate and survive under dense canopy; however, requires a gap to enter into the canopy
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| Yellow birch |
- normally 0.04 to 0.05 ha (not to exceed 0.2 ha)
- 22 to 55 m in diameter
- 1 to 2 times canopy height
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- retain 5-12 seed trees/ha within stand and near canopy gaps
- time site preparation with seed year
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| Black cherry |
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- must be released soon after germination in order to have a chance of surviving and growth
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| Red oak |
- minimum gap 0.1 ha, or 36 m in diameter
- at least 1.5 times canopy height
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- canopy openings 1-2 times tree height have been the most successful in terms of species, tree numbers and growth
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| Basswood |
- gap diameter at least 1.5 times height of adjacent trees (see red oak
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- regeneration from seed is unreliable; however, basswood is prolific stump sprouter
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3. Location of Openings
When creating gaps in your woodlot to manage for mid-tolerant species, give consideration to the following guidelines:
Advanced desirable regeneration – you will have better results if advanced regeneration of desirable mid-tolerant trees is already present. Such regeneration is often lost if single-tree selection harvesting has been used.
Worst first – look for groups of mature trees, off-site species, or poor quality trees that exhibit relatively low potential growth or financial return. Gaps can be located in areas where the acceptable growing stock1 (AGS) drops to below 4–6 m2/ha.
Mature trees on gap edge – ensure that the gap will receive quality seed from mature trees located right beside the opening. Also, keep any uncommon species found nearby and the gap will provide an open area for their seedlings.
Ideal micro sites – look for favourable site indicators: dry ridges with a root restriction layer provide a good competitive advantage for species such as red oak and white pine; lower slopes with higher water tables are more suited to yellow birch; and mid-slopes provide the best sites for fast-growing species such as basswood, white ash and black cherry.
Neighbouring gaps – it is recommended that you do not leave a small strip of mature trees between gaps. Such a strip can complicate future harvest operations, restrict gap growth and may even degrade the quality of the residual trees. Consider expanding older gaps by removing some of the perimeter trees.
Wildlife, ecological and aesthetic considerations – creating gaps along edges can increase edge effect. As well, consider windthrow risk on windward edges and within old growth interiors. Match the size and location of openings to gap-dependent wildlife species. Be aware of aesthetics (views from houses, roads, lakes, trails, etc.). Gaps are much more obvious and long-lasting disturbance indicators.
4. Shape of Openings
Circular or blocky shaped gaps are recommended as they provide more light. Try to match the shape of the gaps to the trees and the landscape. Avoid creating long, narrow openings.
5. Harvest
When creating your gaps, harvest all trees 2 cm in diameter and greater, even if they are better quality trees. If it appears that too many of the trees being removed are of high quality, then perhaps the gap is located in the wrong location. Remember try to locate gaps in the “worst first” locations as noted above.
6. Other Considerations
Remember, a group selection harvest is intended to manage mid-tolerant species. This system is all about sustainable forestry practices, not just cutting more trees.
The amount of wood to be harvested needs to be regulated. This can be done by area control, basal area control or volume control. For example, volume control should be used to determine the amount of wood for harvest when the openings in one cutting cycle will constitute more than 10% of the stand area. As a general rule, the volume removed (within the opening and between the gaps) should be equal or less than growth during the cutting cycle.
When planning your harvest you should consider establishing fewer openings until successful regeneration of pole wood size trees (10–24 cm diameter) can be demonstrated.
A Typical Group Selection Harvest
When managing a stand for shade tolerant species (e.g., maple, beech) using a single-tree selection system, there may be opportunities to assist the release and regeneration of mid-tolerant species by establishing a number of gaps within the stand.
The best way to regulate how much is harvested in such stands is through slightly modifying the residual basal area target (i.e., slight increase in the overall basal area) to accommodate the complete removal of trees within the gap openings.
Fully Regulated Group Selection Harvest
Now that group selection is better understood, you may want to consider experimenting with variations of the technique for other objectives or ecological reasons. The intensive or fully regulated application of group selection silviculture is designed to have all of the wood harvested from within a grid of group selection gaps (usually 2–5 gaps per ha) evenly distributed across the site. During each harvest interval, all the wood extracted comes from the gaps, while the inter-gap areas remain unthinned and relatively undisturbed.
At this level of intensity, putting the gaps on a grid pattern makes the most operational sense. After harvesting, you are left with a checkerboard pattern of gaps in the unharvested matrix of closed canopy forest.
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Right) Post-harvest gap. (Left) Three years post harvest.
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Monitoring and Follow-up
A commitment to undertake follow-up tending at the outset will assist in successful regeneration. For example, advanced sugar maple regeneration will flourish on most sites and will require vegetation management if other species are desired.
After the harvest, the gap openings must be monitored. Watch for the invasion of non-target species (e.g., red maple) that will interfere with the establishment and growth of desirable mid-tolerant species. In addition, well-established tolerant regeneration in the stand may become a competition problem in future gaps.
Some thinning between gap openings in the remainder of the stand is acceptable if it is for stand improvement. However, be careful to avoid over-cutting. If significant improvement thinning is needed, then it is not the correct time for group selection.
Group Selection Research
As noted in the introduction, MNR is evaluating the group selection silvicultural system and its effects on the regeneration of mid-tolerant tree species and associated plants and wildlife. This evaluation work has been undertaken in partnership with the local forest industry in southwestern Ontario (Middlesex and Norfolk counties) and in Algonquin Park.
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Figure 1: Average single year height growth for pinned (i.e., sampled) seedlings (2006). The data in the graph above shows that mid-sized gaps result in better height growth.
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What have we learned so far?
Here is a brief summary of what has been learned to date about managing mid-tolerant tree species using group selection.
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- Keep seed trees close to the gap openings. The closer the seed tree is to the opening, the better the chance of successful regeneration.
- Mid-tolerant species like gaps but so do their competitors (e.g., maple, raspberry, sedges).
- Tending works! Tending (the removal of competitors) has a dramatic effect on reducing the number of shrubs and competing trees and provides mid-tolerant species with a competitive advantage.
- Does more light equal more growth? No, not necessarily. Generally, you will get what you had before cutting unless you provide a competitive advantage to the mid-tolerant species through tending. If left untended, maple overtopping oak in the understorey will simply result in larger maple trees still overtopping oak trees.
- Medium-sized gaps might be better (i.e., gap size of 1 x canopy height). It’s believed that if just enough light is provided to maintain growth of tended mid-tolerant species, there is too little light to allow resprouting competitors to outgrow the mid-tolerant species.
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Results for Other Forest Values
Here is a brief summary of research findings about the impact that gap selection can have on other values found in your woodlot.
Spring Plants – the harvests had negligible effect on spring plants. In fact, unharvested plots lost significantly more plant species than harvested sites. No species declined in abundance in response to harvesting and overall cover increased proportionally more in single-tree selection plots than for in-group selection or reference plots.
Habitat (Cavity Dependent Wildlife) – the reference sites had significantly higher densities of declining, dead and cavity trees. Intensive group selection retains significantly more snag trees than typical (i.e., 13 vs. 7 per ha).
Densities (Cavity Dependent Wildlife) – in years 2–3 after harvest, the primary cavity-nesting bird species were significantly lower in the group selection sites in the Carolinian study area. However, they were not significantly lower after 1 year in the Algonquin study area. Secondary cavity-nesters were not affected by harvesting.
Wood thrush (Carolinian sites) – group selection harvest may contribute to the localized declines in wood thrush populations through decreased stand occupancy and nest survival. However, population densities and nesting success responded positively to single-tree selection.
Hooded Warbler – nest success was equal across all treatments and they are now seeing more usage of gaps and area surrounding gaps. In group selection openings, the fledging success increases in proximity to areas with more cover in the taller understorey layers.
Species at Risk – all species (hooded warblers, Acadian flycatchers, cerulean warblers, red-bellied woodpeckers) continued to use the sites that were being monitored.
Salamanders – continue to show a preference for single-tree selection sites over all other treatment types.
Conclusions
Mid-tolerant tree species are successfully dominating all the Carolinian group selection openings and medium-sized gap openings seem to provide the best compromise in terms of tending effort and growth response.
Black cherry, white and green ash, sassafras and red maple have responded the best to group selection. Oak is also doing very well; however, their slow growth suggests further tending is required 5-7 years after establishment.
Response from wildlife is mixed and species specific. Careful planning, using group selection as a component of your forest management activities, makes sense; however, it shouldn’t be the main silvicultural system used.
Recommendations
Don’t be afraid to use group selection as a silviculture tool to assist in the regeneration of mid-tolerant tree species. Keep in mind that to be successful, site selection and a commitment to tending are critical. Pre-harvest site and gap selection, with understorey tending 2 to 3 years prior to the harvest of the overstorey, could reduce tending efforts.
Additional Reading
Ontario Tree Marking Guide, pp. 151-158 <http://www.mnr.gov.on.ca/en/Business/Forests/Publication/MNR_E000526P.html>
A Silvicultural Guide to Managing Southern Ontario Forests, pp. 175-188
Acknowledgements
This article is based in part on a presentation made by Ken Elliott, Forestry Specialist, Ontario Ministry of Natural Resources, at the Ontario Woodlot Association’s annual general meeting in Acton on March 28, 2009.
Ken has been studying the use of group selection and its effects on the regeneration of mid-tolerant tree species and associated plants and wildlife. The wildlife focus has been on the breeding success of forest birds and the impact on salamander populations.
Footnotes
1 Acceptable growing stock (AGS) are those trees that exhibit form and appearance that suggest they can reasonably be expected to maintain and/or improve their quality and can be expected to contribute significantly to future crops in the form of vigorous, high quality stems. Ontario Tree Marking Guide.
© Ontario Woodlot Association, 2010
This article was featured in the Summer/Fall 2009 edition of the S&W Report the newsletter of the Ontario Woodlot Association.
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