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Overview
Single tree selection is the predominant silvicultural practice in the northern hardwood forests of eastern North America, and creates closed canopy environments that favor shade-tolerant species and suppress shade mid-tolerant and intolerant tree species. Reduced regeneration of mid/intolerant tree species has reduced overall diversity and availability of mast in these forest ecosystems. In particular, mast-producing yellow birch (Betula alleghaniensis Britt), black cherry (Prunus serotina Ehrh.), and red oak (Quercus rubra L.) are underrepresented in the Great Lakes St. Lawrence forests relative to their historical abundance. These tree species also produce mast that is an important source of food for wildlife, and represent potentially valuable substitutes for American beech, which produces high-protein mast but is rapidly declining in central Ontario due to beech bark disease.
Group openings are one harvest treatment that is thought to promote the regeneration of mid/intolerant tree species. These are created around a seed tree of the desired species, and all trees within a certain radius (usually 15-30 m) are removed. Higher light conditions in group openings are also thought to reduce beech recruitment which is desirable due to the species’ foreseeable decline in value and its tendency to produce root suckers that suppress more desirable regeneration. Group openings can either be conducted as a primary harvest system, but are more typically interspersed throughout a stand that is primarily managed using single tree selection, based on the availability of seed trees and desired future species composition. While the Ontario Silviculture Guide recommends using group opening to promote regeneration of certain tree species, there are few studies that assess how well these actually work in northern hardwood forests of central Ontario. One relevant study used data from Algonquin park to show that group openings are effective at regenerating yellow birch, and season of harvest and soil disturbance are important factors affecting their success (Shabaga et al., 2019). However, no other species were investigated.
In this field trial, we investigated the effects of group opening treatments on the regeneration of yellow birch, black cherry, and red oak in Haliburton Forest and Wild Life Reserve, 2-3 years after implementation of the group openings. There were three specific objectives:
- To assess whether group opening treatments targeting yellow birch, black cherry, and red oak, conducted during the summer, are successful at regenerating the target species, and mid/intolerant species in general.
- To examine the effects of different seasons of harvest on group openings targeting oak regeneration.
- To assess whether group openings are successful at suppressing the recruitment of beech regeneration.
In most cases, group opening treatments increased the regeneration of mid/intolerant tree species compared to control (single tree selection without group openings). Among target species, only yellow birch regeneration significantly increased in group opening treatments. Beech regeneration was not reduced by group opening treatments. For red oak there was no effect of group openings on regeneration, but regeneration was greater in winter-harvested stands.
We conclude that group opening treatments have the potential to increase the regeneration of some, but not all, mid/intolerant tree species. It is particularly successful for yellow birch but limited for red oak and black cherry. Group openings were also not found to be an effective tool to reduce beech regeneration. Longer term monitoring of the sites is required for more conclusive results.
Silviculture Objectives
The stands included in this experiment were managed using single tree selection, but with group openings scattered throughout based on the availability of seed trees.
The objectives of single tree selection are to regenerate desirable tolerant hardwood tree species while providing periodic harvests and maintaining an uneven age structure.
The objectives of the group opening treatments are to regenerate desirable shade intolerant/mid-tolerant tree species (Table 1) as available, by increasing the availability of light (and in some cases creating additional soil disturbance).
Table 1: Classification of tree species as shade tolerant, mid-tolerant, and intolerant, as reported in Burns & Honkala (1990), and whether they are desirable for timber production.
| Species | Common Name | Shade Tolerance | Desirability |
| Prunus serotina | Black cherry | Intolerant | Desirable |
| Betula alleghaniensis | Yellow birch | Mid-tolerant | Desirable |
| Quercus rubra | Red oak | Mid-tolerant | Desirable |
| Acer saccharum | Sugar maple | Tolerant | Desirable |
| Fagus grandifolia | American beech | Tolerant | Desirable |
| Ostrya virginiana | Ironwood | Tolerant | Undesirable |
| Acer rubrum | Red maple | Mid-tolerant | Desirable |
| Fraxinus americana | White ash | Mid-tolerant | Desirable |
| Tsuga canadensis | Eastern hemlock | Tolerant | Desirable |
| Populus tremuloides | Trembling aspen | Intolerant | Undesirable |
| Abies balsamea | Balsam fir | Tolerant | Undesirable |
| Picea spp. | Black and white spruce | Tolerant | Desirable |
| Picea rubens | Red spruce | Mid-tolerant | Desirable |
| Acer pensylvanicum | Striped maple | Tolerant | Undesirable |
| Prunus spp. | Other cherry | Intolerant | Undesirable |
Study objectives
The study was conceptualized and conducted 2-3 years after the harvest treatments had occurred.
The objectives of this study were:
- To assess whether group opening treatments targeting yellow birch, black cherry, and red oak, conducted during the summer, are successful at regenerating the target species, and other desirable mid/intolerant species.
- To examine the effects of different seasons of harvest on group openings targeting oak regeneration.
- To assess whether group openings are successful at suppressing the recruitment of beech regeneration.
Pre-treatment stand description and condition
Stand establishment and management history
The forests of the Haliburton highlands were severely high-graded during the late 1800’s but, since the inception of Haliburton Forest and Wild Life Reserve, the forests of the area have been managed according to sustainable silviculture best-practices. The area is dominated by sugar maple and American beech, with smaller components of other hardwoods and softwoods.
The group opening sites assessed in this study were implemented during harvests in 2019 and 2020 and exist within a matrix of forest managed using single tree selection. All stands had been harvested every 20-30 years prior using single tree selection since the 1980s. Prior to that, diameter limit cutting was practiced. There were three sites: one had a significant component of yellow birch, another of red oak, and a third of black cherry.
Pre-treatment species composition
Local stands are primarily composed of shade-tolerant species such as sugar maple, American beech, and Eastern hemlock, but mid-tolerant yellow birch, red oak, white pine (Pinus strobus L.), and shade intolerant black cherry also exist as structurally and ecologically important constituents. Group openings were implemented in areas with a viable seed tree, a lack of desirable regeneration, and a lack of other desirable crop trees. There were three sites - one had a significant component of yellow birch, another of red oak, and a third of black cherry. Pre-harvest inventory data is not available.
Pre-treatment forest health issues
Stands across HFWR have been affected by the invasive beech bark disease, which arrived in the area in the early 2000s. At the time of harvest in 2019/2020 the forest was already in the aftermath stages of the disease. It is a silvicultural priority to suppress beech regeneration in favor of ecologically and commercially viable crop species.
Beech bark disease-induced beech loss necessitates the accelerated promotion of replacement species to maintain stand diversity and productivity. The seriousness of this issue is further aggravated by the fact that beech occupies an important niche as a high-protein mast producing species. During the fall, beech nuts are a major dietary component of many mammalian species as they prepare for hibernation and/or winter scarcity. There are also few substitute mast species in the predominant maple-beech forest type at Haliburton Forest’s latitude. In conjunction, these conditions threaten to create cascading ecological disruptions that will reverberate throughout the forest’s complex food web. Group openings are therefore viewed as a relevant silvicultural countermeasure since, in theory, they create the conditions needed to recruit some of the only climatically viable mast alternatives: oak, yellow birch, and black cherry while also potentially suppressing the rhizomatic regeneration behavior of beech
Landowner objectives/situation
Haliburton Forest is a privately owned, multi-use forest which is managed for both timber production and tourism/recreation.
Silviculture Prescription
The prescription was single tree selection with group openings as appropriate. In practice, this meant that tree markers identified potential group opening sites during routine tree marking operations. The decision to create a group opening was based on ecosite, presence of viable seed tree, a lack of desirable regeneration, as well as a lack of other desirable crop trees. The target seed trees were marked for retention in blue with “GO” and a number indicating the radius of the opening. These points were added to a stand map provided to contractors and harvest supervisors. All over-story and mid-story trees and shrubs were removed around the seed tree. For areas harvested during snow-free seasons, there was a target of 50% soil disturbance and slash alignment, implemented by cable skidder blade. Once a field supervisor had determined that a group opening had been successfully implemented, the group opening was assigned a code and updated in the GIS system so that it could be monitored in the future.
What actually happened during the treatment
Data was collected in the summer of 2022, 2-3 years post-harvest. Five sites were selected, each containing 5-6 replicate group openings. Sites 1-3 contained red oak group openings, with one site harvested in the summer (2020), one in the fall (2019), and one during winter (2020). Site 4 contained black cherry group openings harvested during the summer (2020), and site 5 contained yellow birch group openings harvested during the summer (2020). The group openings were all created following OMNRF silvicultural guidelines (OMNR, 2004), and included a seed tree(s) of the target species, with most other stems removed from within a 15-m radius of the seed tree. Good seed years for yellow birch and red oak occurred in the area in 2019 and 2021.
Each group opening represented a treatment plot. Five control plots were also established in each site within the surrounding single tree selection harvested area. Control plots were established around a central seed tree of the target species, but where a group opening had not been created.
In each plot, three subplots, each 1.13 m in radius, were placed 7.5 m from the seed tree at directions of 0°, 120°, 240° (Figure 1). A fourth subplot, 2 m in radius, was also established in the center of the group opening, around the seed tree. In each subplot, every tree seedling was counted, the percent-cover of all plant species was assessed, and the ‘leading species’ and ‘secondary leading species’ were recorded. The basal area of each plot was also recorded, and a visual estimate of canopy openness. The primary tree species in the perimeter of each group opening (conspecifics) was also recorded.
The data was analyzed by calculating the stems/ha of regeneration of all tree species in each group opening and control plot. Tree species were also categorized into intolerant, mid-tolerant, and tolerant species. We then used ANOVAs to assess whether there were significant differences in the amount of regeneration of target species, tolerant/mid-tolerant/intolerant species, and of American beech, between group openings and control plots. We also assessed whether there was a significant effect of harvest season on red oak regeneration in group openings and control plots. Finally, we used regression to assess whether there were trends between the residual basal area and regeneration of target species in all of the plots (control and group opening).
Figure 1: Layout and dimensions of regeneration subplots for sampling group opening treatment and control plots; r = radius, x = distance from plot center.
Post-treatment assessment
Effect of groups openings on regeneration of shade tolerant, mid- and intolerant tree species
Result: There was an increase in the recruitment of desirable shade intolerant and mid-tolerant tree species in the group openings compared to the controls, and a decrease in the recruitment of shade tolerant tree species (Figure 2).
Management Implication: Group openings can be used to increase the diversity and proportion of mid- and intolerant trees.
Figure 2: Number of desirable regenerating intolerant, mid-tolerant, and tolerant trees <1.3 m in height in group opening plots (all combined) compared to control plots. Means are plotted ± standard error. See Table 1 for a list of tree species and their shade tolerance and desirability.
Effect of group openings on regeneration of target tree species
Result: Only yellow birch group openings successfully increased regeneration of the target tree species - there was approximately 13 times more yellow birch regeneration in yellow birch group openings compared to control plots (Figure 3). Black cherry regeneration was relatively unaffected by group opening treatments, even when black cherry was both the seed tree and the dominant species in the group opening boundary. See the following section for further red oak results.
Management Implication: Group openings are effective at regenerating yellow birch. When specifically trying to regenerate black cherry, group openings appear ineffective.
Note: It is important to remember that group openings are used to initiate regeneration, not to release existing regeneration. A potential explanation for poor black cherry regeneration in group openings is that the process of creating the group openings damaged the existing black cherry regeneration and produced a more hostile germination environment for future regeneration. In closed canopy environments with low levels of light, large numbers of black cherry seedlings successfully germinate in the vicinity of seed trees. In the absence of disturbance, most of these survive less than ten years under the dense shade cast by the overstory (Burns and Honkala, 1990). Regardless, the population of black cherry trees in the understory remains relatively stable because seedling germination generally replaces sapling mortality in highly shaded stands. The process of creating a group opening, however, damages existing regeneration, often through deliberate scarification and the movement of harvesting machinery, and may also reduce subsequent germination rates due to soil compacting and drying (Burns & Honkala, 1990).
Figure 3: Stem density of target species <1.3 m in height, between control and group openings. Means are plotted ± standard error.
Effect of having mature conspecifics in the perimeter of group openings on target species regeneration
Result: Presence of mature target species in the perimeter of the group opening influenced red oak regeneration, but not for the other tree species. When all the red oak group openings were considered, there was not a significant increase in red oak regeneration compared to control (Figure 3). However, for the group openings where red oaks were also the dominant species in the opening’s perimeter, there was an increase in red oak regeneration (Figure 4). However, absolute recruitment of red oak was still low.
Management Implication: Group openings for red oak are more effective where red oak is the dominant species in the surrounding canopy.
Figure 4: Stem density of target species <1.3 m in height, by dominance of mature conspecifics in plot boundary, for group opening plots only (controls not included). Means are plotted ± standard error.
Effect of harvest season and group openings on regeneration of red oak
Result: Red oak regeneration was more successful in the winter harvest compared to summer and fall (Figure 5).
Management Implication: While this result suggests that harvesting red oak in the winter should be prioritized, it could also have been a site-specific effect. A larger sample size is required, with replication across sites and season, to validate it.
Figure 5: Density of red oak stems <1.3 m in height by season of harvest and treatment type. Means are plotted ± standard error.
Effect of residual basal area on regeneration of target species
Result: For yellow birch and red oak, regeneration of the target species increased as the residual basal area decreased. There was not a strong relationship between residual basal area and black cherry regeneration (Figure 6a-c).
Management Implication: Lower residual basal areas should increase the regeneration of red oak and yellow birch but may not increase regeneration of black cherry.
Figures 6a-c: Density of stems <1.3 m in height for (a) red oak, (b) yellow birch, and (c) black cherry, based on residual basal area. Group opening and control data combined.
Effect of group openings on American beech regeneration
Result: There was a notable decrease of American beech regeneration in yellow birch group openings compared to control, although there was a large amount of variability in the data. There was a slight increase in American beech regeneration in the group openings targeting red oak and black cherry (Figure 7).
Management Implication: American beech abundance was approximately three times lower in the group openings compared to control, and yellow birch abundance was approximately seven times greater than that of American beech in the group openings. This suggests that group openings may be a useful tool in managing beech regeneration in the presence of yellow birch. We did not record the pre-harvest conditions, and so cannot say whether the yellow birch outcompeted American beech establishment, or whether it was outcompeting existing regeneration from before the harvest. Further research should also be done to investigate this.
Figure 7: Density of American beech stems <1.3 m in height between group openings and control, and for each target species. Means are plotted ± standard error.
Plans for future treatments
All group openings described in this case study were recorded in Haliburton Forest’s GIS database. This allows for future assessment of regeneration trends, though no specific follow-up is planned at this time. Haliburton Forest continues to implement opportunistic group openings. Since reviewing these findings, Haliburton Forest has maintained or slightly increased the use of group openings for yellow birch, and has de-emphasized the implementation of group openings for most other species: both red oak and black cherry, but also white pine.
Costs and economic considerations
Haliburton Forest currently pays loggers $100 per group opening, on top of production-based pay. This is designed to cover the cost of mid-story removal, slash alignment, and blade scarification, depending on the target species needs. A group opening covers 706 m2. To have group openings on just 10% of the area of a 100-ha cut block, ~142 group openings would be required, at a total cost of $14,200 ($142 per ha for the entire block, or $1,420/ha for the area directly treated).
Based on this math, group openings should be seen as a ‘boutique’ and slightly inefficient form of silviculture - they can make a difference to regeneration trends on a small scale where there are appropriate opportunities but quickly become financially unviable when implemented on a larger scale. If the goal is to regenerate a meaningful amount of the target tree species, a switch to more extensive treatment options, such as some form variant of shelterwood with midstory cleaning and site preparation as would be more cost effective and have a greater effect on the landscape-level abundance of target species.
Summary / lessons learned / additional thoughts
Summary
Group openings can be used to increase the diversity and proportion of mid-tolerant trees. Our results indicate that, when attempting to regenerate yellow birch, group openings are an effective treatment. When specifically trying to regenerate black cherry, group openings appear ineffective. Group openings may be effective for red oak, but more information is required to understand under what conditions. Group openings may be effective at giving yellow birch a competitive edge against beech regeneration, but further research is also required to validate this. Group openings should be seen as a ‘boutique’ form of silviculture - they can make a small difference on a small scale, but quickly become financially unviable when implemented on a larger scale. If the goal is to regenerate a meaningful amount of the target tree species, a switch to irregular shelterwood, or regular shelterwood, would be more cost effective.
Additional Considerations
- This study was not able to capture the impact that seed year strength has on regeneration, and all three target species involved in this study have mast years and years of low seed production. The group openings were created during 2020, except for the fall red oak group openings which were created during 2019. Both 2019 and 2021 were deemed good seed years for yellow birch and red oak with 2020 being a rest year. Black cherry seeding was deemed adequate in 2019, the year preceding the harvesting of the black cherry-targeted group openings sampled in this study.
- Observed stem density at 2-3 years post harvest may not translate into sufficient free-to-grow stocking of the target species. Re-censusing of the sampled plots 5 and 10 years post-harvest will help to discern the long-term success of the group openings and better understand how the regeneration community changes over time. In some situations, we think that larger gaps may have been more successful than the sizes prescribed.
- The tree marking guide suggests that when group openings are used in a matrix of single-tree selection (STS), the residual basal area (BA) of STS should be increased to account for the decreased BA in group openings, making the average BA across the stand match with normal STS prescriptions. This is not logical – the target BA for STS is based on what will optimize tree growth. If we increase the BA in STS stands that have group openings in them, then the STS area will not be growing at its full potential. Therefore, if group openings are incorporated into a matrix of STS, the STS prescription should remain the same.
Supplemental content
Citations:
Burns, R M., & Honkala, B. (1990). Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654. U.S. Department of Agriculture, Forest Service, Washington, DC. Vol.2, 877 p.
OMNR. 2004. Ontario Tree Marking Guide, Version 1.1. Ontario Ministry of Natural Resources. Queen’s Printer for Ontario. Toronto. 252 p.
Shabaga, J. A., Jones, T. A., Elliott, K. A. 2019. Group-selection silviculture conditionally enhances recruitment of Yellow Birch in a shade-tolerant hardwood forest. Forest Ecology & Management. 444. 244–255.