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Article

Effectiveness of Silvicultural Options in Renewal of Trembling Aspen–Jack Pine Mixedwood Stands, 21 Years After Treatment

by
Rongzhou Man
Ontario Forest Research Institute, Ontario Ministry of Natural Resources and Forestry, 1235 Queen Street East, Sault Ste. Marie, ON P6A 2E5, Canada
Forests 2025, 16(4), 683; https://doi.org/10.3390/f16040683
Submission received: 14 February 2025 / Revised: 8 April 2025 / Accepted: 10 April 2025 / Published: 15 April 2025
(This article belongs to the Special Issue Forest Growth and Regeneration Dynamics)
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Abstract

:
Regenerating conifers after harvest through planting and postharvest broadcast application of herbicide is effective in ensuring the survival and growth of seedlings, but faces challenges in meeting broad social and ecological objectives of forest management. This study reports the effectiveness of alternative options in regenerating jack pine (Pinus banksiana Lamb.), 21 years after harvest of trembling aspen (Populus tremuloides Michx.)-dominated boreal mixedwood stands. The treatment options included (i) preharvest spray—aerial broadcast spray prior to harvest, (ii) postharvest partial spray—ground herbicide application in strips, (iii) partial harvest in strips, (iv) postharvest aerial broadcast, and (v) uncut reference. Twenty-one years after treatments, the four harvest treatments were similar in overstory density (4000 stems/ha) and basal area (BA, 20 m2/ha), but differed in composition and structure. The preharvest spray had an intimate mixture of aspen and jack pine (22% and 57% by BA, respectively), compared to spatial mosaics of aspen and pine corridors in the partial spray (36% and 41%), and aspen and maple corridors in the partial cut (21% and 31%). While the postharvest broadcast was pine-dominated (74% by BA) as expected, uncut and partial cut were similar in pine composition (10% by BA), which is inadequate for aspen and pine mixedwood stands. The early positive effects of preharvest spray and partial harvest on understory species abundance and diversity became neutral 21 years postharvest. The implications of these findings are discussed with respect to stand conditions before harvest, postharvest regeneration dynamics, and treatment objectives for the renewal of trembling aspen and jack pine mixedwood stands after harvest.

1. Introduction

In North America boreal forests, trembling aspen (Populus tremuloides Michx.) is widely distributed, and grows either in single-species stands or in mixtures with balsam poplar (Populus balsamifera L.), white birch (Betula papyrifera Marsh), black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss), balsam fir (Abies balsamea [L.] Mill.), jack pine (Pinus banksiana Lamb.), and lodgepole pine (Pinus contorta Dougl. ex. Loud) [1,2]. Quick to regenerate in high abundance after disturbances, aspen quickly dominates sites [3]. Conifers can be established through postharvest planting or natural regeneration that can be highly variable in stocking and duration [4,5]. To maintain healthy and productive conifer components for timber production [6,7,8] or meet broader ecological objectives [2,9], the density/growth of aspen regeneration needs to be controlled, especially for shade-intolerant conifers such as jack pine that cannot be sustained under forest canopy [10].
While postharvest aerial application of herbicide is effective in suppressing aspen and promoting conifer survival and growth [11], the drawbacks of broadcast spraying clearcut sites are recognized. First, aspen trees that survive herbicide spraying may not meet wood quality standards [9,12,13], despite the fact that mixedwood stands of this origin may not be different from natural mixedwood stands in stand composition and structure [14]. Second, other than broadleaves and woody shrubs that may compete with conifers, many non-competitive plant species are also affected [15], potentially leading to a reduction in the diversity of some understory species [15] or non-timber products [16]. Alternatively, herbicide use can be reduced through restricted application (partial spray) [7,8] or suppression of broadleaves and competitive shrubs through residual partial forest canopy (partial harvest) [17]. While partial harvest is often used for shade-tolerant confers such as black spruce and white spruce [7,17], the successful use of this silvicultural system is limited for shade-intolerant conifers [8,10].
Relative to postharvest broadcast or partial spray, herbicide can also be applied over a mature aspen canopy prior to harvest [18]. Early regeneration assessments indicate that preharvest spray is effective in suppressing aspen sucker density and growth, and allows for reasonable survival and growth of planted conifers [8,19]. The density of aspen suckers was higher after preharvest than postharvest spraying, but much lower than that in the unsprayed clearcut [8,19]. The quality of postharvest regenerated aspen suckers was not negatively affected in terms of stem stain and rot and stem deformation, relative to trees in postharvest spray and clearcut without spray [9]. Reduced herbicide impacts are expected for all postharvest-recruited understory plants. Even with understory plants that exist prior to harvest, their exposure to herbicide would be substantially reduced due to interception of the full aspen canopy [8,9,18]. It is unclear, however, if early conifer performance can persist over time with stand development.
This study reports 21-year postharvest assessments and compares the results with early assessments of alternative silvicultural treatments targeting the renewal of intolerant trembling aspen and jack pine mixedwood stands after harvest. The early assessments and reporting include 6-year regeneration [8], 11-year survival and growth of planted jack pine [19], and 11-year understory community [20]. Based on treatment objectives and early observations, I expected a pure jack pine stand (≥80% in BA) with postharvest broadcast spray, a jack pine-dominated pine and aspen blended mixture (>50% and <80% in BA) with preharvest spray, a mosaic of 50:50 pine–aspen mixedwoods with postharvest partial spray and partial cut, and a higher species and structural diversity with preharvest spray and partial cut.

2. Methods

2.1. Site Condition and Treatments

Details of the site conditions and experimental design are provided by MacDonald et al. [18] and Man et al. [8]. Briefly, the study site is located about 15 km southwest of Gogama, Ontario, Canada (between 47°30′ N, 81°57′ W and 47°30′ N, 82°00′ W). Block 1 (in cutblock one) is about 3 km from blocks 2 and 3 (in cutblock two). The ecosite types are mostly ES 7m (trembling aspen–white birch–medium soil), with some ES 7c (trembling aspen–white birch–coarse soil) and ES 3 (white birch–trembling aspen–black spruce–coarse soil) [21]. The soil is stony-to-bouldery ablation till, and varies from silt to silt loam and silty sand. The preharvest basal area of the overstory canopy varied and averaged about 26 m2 ha−1, and was predominantly trembling aspen (51%), with minor components of white birch (13%), jack pine (12%), red maple (Acer rubrum L.) (11%), black spruce (6%), white spruce (5%), and balsam fir (2%).
Red maple was the most abundant tree in the understory (5217 stems·ha−1), followed by balsam fir (1929 stems·ha−1), trembling aspen (1413 stems·ha−1), black spruce (380 stems·ha−1), white spruce (27 stems·ha−1), and white birch (27 stems·ha−1). Woody shrub species were mainly mountain maple (Acer spicatum Lam.), beaked hazel (Corylus cornuta Marsh.), bush honeysuckle (Diervilla lonicera Mill.), and showy mountain ash (Sorbus decora [Sarg.] Schneid.).
Harvesting started in the winter of 2002–2003, and was designed to create five silvicultural treatments: (i) preharvest spray—aerial broadcast spray prior to harvest to produce blended mixtures of conifers and hardwoods; (ii) postharvest partial spray—ground herbicide application in strips to produce alternating 10 m sprayed and 10 m unsprayed strips (conifer and hardwood corridors); (iii) partial harvest—partial cutting by alternating 10 m harvested and 10 m unharvested strips (referred to as harvested and leave strips, respectively) to produce a mosaic of conifer and hardwood corridors; (iv) postharvest broadcast spray—conventional postharvest aerial herbicide application to produce uniform conifer plantations; and (v) uncut—an uncut reference stand [8,18]. The preharvest spray involved aerial broadcast spray with glyphosate in August (1.05 kg ha−1 a.e.), followed by winter harvesting, postharvest mechanical site preparation in 2003 (year 1 postharvest), and planting with jack pine in 2004 (year 2 postharvest, no additional post-planting tending). The postharvest spray (1.40 kg ha−1 a.e.) was applied in 2005 (year 3 postharvest and year 2 post-planting), either through air for broadcast spray or ground application in the corridors of planted jack pine for partial spray. The planting density was 1500 seedlings/ha for preharvest spray and conifer/harvest corridors of partial spray and partial cut, or 1800 seedlings/ha for broadcast spray. The treatment plots ranged from 16 to 20 ha, and were arranged in three replications (blocks), following a randomized complete block design [18]. The silvicultural objectives of these treatments were intended to control trembling aspen and encourage the growth of jack pine, either in blended mixtures or spatial mosaics, after harvesting aspen-dominated boreal mixedwood stands [8,18].

2.2. Twenty-One-Year Postharvest Re-Assessment

A total of 24 regeneration assessment points were established in a nominal 60 m × 60 m grid within each treatment plot. Overstory plots with a radius of 11.28 m were re-measured in 2023, four years after establishment in 2019 (17 years postharvest), at six assessment points that were uniformly distributed within each of the treatment plots, and between harvested and leave corridors for partial cut, and between conifer and hardwood corridors for partial spray. All trees (>4 m tall, not tagged) were included in the overstory, identified according to their species, and measured for their diameter at breast height (DBH, cm). Due to restrictions of field season, total height (m) was measured for first 50 trees within each overstory plot (about 30%).
The understory trees of planting and natural origin (regeneration layer, ≥0.10 and ≤4.0 m) were surveyed at all 24 regeneration assessment points. Each understory plot had a radius of 2.26 m, and consisted of four 4 m2 vegetation plots (quadrants). All trees were measured for total height. The percentage cover of vegetation groups (i.e., shrub, herb, grass, fern, and moss), coarse woody debris of intact (1–3, CWD1) and well-decayed (4–5, CWD2) classes [22], and individual woody species (trees and shrubs) were visually assessed in two quadrants in July and August at the six assessment points of overstory plots.

2.3. Data Analyses

The height of overstory trees that were not measured for total height was estimated using height–diameter relationships established with the data collected in this study or available from previous research. The Chapman–Richards model was chosen, as recommended by Peng [23], and fitted with the nls function, available in the nlme package [24]. The estimated parameters were significantly different from zero for all species, including ash (red ash (Fraxinus pennsylvanica Marsh) and black ash (Fraxinus nigra Marsh)), jack pine, white birch, trembling aspen, red maple, balsam fir, black spruce, white spruce, and tamarack (Larix laricina (Du Roi) K. Koch) (Table S1). The sample size for eastern white cedar (Thuja occidentalis L.) and white pine (Pinus strobus L.) (<10) was inadequate for robust parameter estimation, and either a jack pine model (this study) or Peng et al.’s model for Ontario white pine [23] was used. Some species were low in overstory composition, and were combined into species groups: poplar (trembling aspen), birch (white birch), maple (red maple, red ash, and black ash), pine (jack pine and white pine), spruce (black spruce and white spruce), and fir (balsam fir).
Overstory species and structural diversity and understory woody species diversity were calculated using the Shannon diversity index, available in the vegan package [25]. Overstory data were stem counts by species or grouped into 10 cm DBH classes, and understory data were species cover.
Data analysis followed a randomized complete block design using a linear mixed effects (LME) model, available in the nlme package [24]. Silvicultural treatments were treated as fixed effects and blocks as random effects. The values for overstory and understory plots were averaged to the treatment plot level prior to statistical analyses of overstory density, DBH, height, BA, and Shannon species and structural diversity, understory tree density and height, percentage cover of vegetation groups and coarse woody debris, and Shannon species diversity on understory woody species. Multiple contrasts of means were performed using the pair function through pairwise comparison of all treatment combinations, with p values adjusted with the Tukey method, available in the emmeans package [26]. All analyses were carried out using R version 4.3.0 [27].

3. Results

3.1. Regeneration Density and Height

The regeneration density (≥0.10 m and ≤4.0 m tall) peaked at year 2 (postharvest spray) or 3 (preharvest spray and partial cut) postharvest (Figure 1). Regeneration density declined thereafter, except for some fluctuations in the partial cut. Along with a decrease in regeneration density, there was a proportional increase in shade-tolerant species (maple, spruce, and fir), particularly after the formation of the overstory canopy at year 21 postharvest. While aspen and maple were negatively affected by pre- and postharvest spray in terms of density differences before and after herbicide use, birch was not affected.
At year 21 postharvest, the understory tree density in the regeneration layer was the highest in the partial cut and lowest in the preharvest spray, and did not differ significantly among treatments (Table 1). Maple trees were the most abundant, ranging from 60% in the preharvest spray and partial cut to >80% in the postharvest broadcast spray, and were mostly under 50 cm in height (>60%) (Figure 2). Poplar trees were more abundant in the uncut, while other species, except for pine, appeared to have higher densities in the partial cut. Averaged among all species, understory trees were taller in the preharvest spray (Table 1 and Figure 2).
Between treatment corridors in the partial spray, birch trees were more abundant in the sprayed corridors, while maple trees were more abundant in the unsprayed corridors (Figure 3). In the partial cut, birch, maple, and spruce trees were more abundant in the harvested corridors, while fir trees were more abundant in the leave corridors.

3.2. Understory Vegetation

Shrubs appeared to be more abundant in the preharvest spray, herbs in the partial spray, grasses in the partial cut, and ferns and mosses in the uncut, although none of the differences reached the level of significance (p < 0.05) (Table 1). Comparatively, the coarse woody debris of intact classes (1–3) was more abundant in the uncut, while that of well-decayed classes (4–5) appeared to be more abundant in the partial cut and uncut.
The Shannon species diversity based on the percentage cover of understory woody species (trees and shrubs) did not differ significantly among treatments (Table 1).

3.3. Overstory Trees

Overstory trees (>4.0 m tall) were 1800 stems/ha in the uncut control, significantly lower than those in the harvest treatments (3500–4000 stems/ha, Table 2). The difference was mainly due to pine and poplar trees that were generally less abundant in the uncut. Between years 17 and 21, overstory density did not change in the preharvest spray and partial spray, but increased by 10%–20% in the partial cut and broadcast spray, mainly due to increases in birch, maple, spruce, and fir, despite a decrease in poplars in all treatments (Figure 4 and Figure S1). The temporal change in pine density in the overstory varied with treatments, with a slight decrease in the preharvest spray and broadcast spray, no change in the uncut and partial spray, and a >10% increase in the partial cut.
The overstory DBH or height of all species did not differ significantly among treatments (Table 2). However, poplar, birch, maple, and pine trees in the uncut were generally larger and fir trees were smaller, compared to those in the harvest treatments. The overstory BA was about 20 m2/ha in all treatments (Table 2), and increased by 4–7 m2/ha from the levels of year 17, except for the uncut, which did not change. The BA of all species groups increased, except for the uncut, where the poplar BA decreased and the pine BA did not change between years 17 and 21. Birch and maple had a higher BA in the uncut and the leave corridor of the partial cut, due to the contribution of advance growth and residual canopy trees, while the pine BA from postharvest regeneration was the highest in the broadcast spray, followed by the preharvest spray, partial spray, partial cut, and uncut (Figure 4, Table 2).
The lead overstory species (>20% in density) varied by treatments: pine, poplar, and maple in the preharvest spray and partial spray; maple and poplar in the partial cut; pine and maple in the broadcast spray; and maple in the uncut (Table 2; Figure 4). The overstory composition by BA was slightly different, due to relatively small maple trees. Among the four harvest treatments, the proportion of pine trees with a DBH ≥10 cm was highest in the preharvest spray (59%), followed by the broadcast spray (52%), partial spray (45%), and partial cut (18%) (Figure 4). The proportion of poplars reaching DBH ≥10 cm was 14%, 8%, 19%, and 12%, respectively, in the four harvest treatments.
Between treatment corridors in the partial spray, a higher proportion of pine trees reached a DBH ≥10 cm in the sprayed corridors, while more poplar trees, along with more maple trees, reached a DBH ≥10 cm, in the unsprayed corridors (Figure 5). In the partial cut, the harvested corridors had higher densities of poplar and pine from postharvest regeneration, while the leave corridors had more spruce and fir trees from advance growth or residual overstory.
The Shannon species diversity for stem counts appeared to be higher in the preharvest spray and partial cut relative to the uncut, while the Shannon structural diversity by 10 cm diameter classes was the opposite (Table 2). None of the differences reached the level of significance of 0.05.

4. Discussion

4.1. Postharvest Regeneratuion Dynamics

Across all treatments, the overstory trees at year 21 had 20%–30% of the regeneration densities at year 11 postharvest (Figure 1; Table 2), following the principle of self-thinning [28]. The proportion of seedlings and saplings entering the overstory decreased with increases in total density, with the highest (21%) in the preharvest spray and the lowest (12%) in the partial cut (Figure 1 and Figure 4), leading to similar overstory density in the four harvest treatments. Convergence of overstory density over time is observed elsewhere in aspen-dominated stands [29,30,31].
A partial forest canopy does not help the survival and growth of jack pine seedlings, relative to clearcut along with herbicide use [19]. Although the planting density was the same (1500 seedlings/ha), the partial cut had lower pine density than the partial spray at both years 6 and 11 postharvest, due to poorer natural regeneration [8,19]. With overstory and understory trees combined, about 40% of the year 11 pine seedlings survived at year 21 in the partial cut, compared to 75% in the partial spray. Other than the low light level [8], the high pine mortality in the partial cut may have resulted from winter browsing of snowshoe hares that use nearby leave trees as shelters [32], instead of direct suppression by competitive vegetation. Although jack pine seedlings girdled by hares at the base generally resprout, as noticed in field surveys, damaged seedlings are heavily suppressed and may die quickly from insufficient light [33,34,35].
Between the two herbicide uses, about 60% of the year 11 pine seedlings survived in the preharvest spray, compared to 104% in the broadcast spray; the survival rate of the latter is likely due to continuous postharvest natural regeneration [8,19]. Other than competition with more and larger competitive broadleaves, shrubs, and herbs, the low survival in the preharvest spray may also be related to greater number of natural jack pine seedlings shortly after harvest [8,19], which grow more slowly than planted trees [36,37] and can suffer more from a competitive disadvantage.
The density change from the year 11 regeneration to the year 21 overstory canopy was highest for poplars (−65%) and lowest for maple trees (−15%). Birch, maple, spruce, and fir are more tolerant to low light [35], and therefore showed a continuous ingrowth into the overstory between years 17 and 21, compared to a lack of poplar and pine recruitment, except for pine trees in the partial cut, which had greater competitive suppression and size differentiation (Figure 4 and Figure 5).
The taller understory trees in the preharvest spray at 21 years postharvest relates to higher postharvest vegetation abundance [20] that would have restricted further ingress of natural seedlings, while the higher understory aspen density in the uncut likely resulted from a recent reduction in overstory aspen, which would have reduced apical dominance over root suckering [38,39].

4.2. Overstory Composition and Treatment Objectives

At the time of harvest, trembling aspen, jack pine, and white birch aged about 80 years [18] and showed signs of dying branches and crowns, indicating a canopy transition to gap dynamics [40]. By year 17 postharvest, the overstory aspen composition by BA in the uncut dropped to 15% from the preharvest level of 51%, and further decreased to 10% at year 21 (Table 2). Twenty-one years after harvest, the overstory BA in the four harvest treatments reached 75% of the preharvest level (26 m2/ha) [18], while the BA in the uncut dropped to 70% of the preharvest level.
Consistent with expectations, the preharvest spray produced a blended aspen and pine mixture (22% and 57% by BA, respectively). The higher pine mortality at the early stage [19] did not continue, based on density changes between years 17 and 21. The reduction in aspen density between years 17 and 21 suggests that the pine dominance will persist.
As expected, the postharvest broadcast spray produced a pine-dominated stand (74% by BA) with variable amounts of tolerant conifers (spruce and fir, 4%) and hardwoods (birch, aspen, and maple, 22%). The postharvest use of herbicide reduced the density and size of all three competitive hardwoods, relative to the trees in the preharvest spray and partial cut treatments, and allowed for good growth and survival of pine seedlings [19]. In contrast to the observations by Fu et al. [41], a substantial increase in grasses did not occur after harvest [8], which can balance out the benefits of hardwood reduction [41]. Given the relatively high pine density and dominance in size, the future jack pine composition will likely reach 90%.
The jack pine composition in the partial spray was 41% by BA, more than expected from the proportion of area treated, aiming for a mosaic of alternating hardwood and conifer corridors [18]. The growth and mortality of the planted jack pine was similar between the postharvest broadcast spray and the sprayed corridors of the partial spray [19]. The relatively small average size of pine trees in the partial spray resulted from natural regeneration in the unsprayed corridors, where natural pine seedlings were overtopped by fast-growing aspen.
The partial harvest did not produce a mosaic of alternating conifer and hardwood corridors, in contrast to expectations [19]. Both harvested and leave corridors were dominated by hardwoods (birch, aspen, and maple), accounting for 68% of the total BA, greater than the equal conifer and hardwood mixtures projected based on 6th-year observations [8]. Apparently, planted pine seedlings are less competitive than aspen, white birch, and red maple, which can either reproduce in high abundance from vegetative regeneration [3,42,43], or are more tolerant to low light [35].

4.3. Vegetation and Diversity

Although the differences in the abundance of vegetation groups among treatments were no longer significant, some trends observed in the first 6 years [8] persisted at 21 years postharvest. The higher amount of shrubs in the preharvest spray and higher amount of grasses in the harvested corridors of the partial cut likely resulted from low aspen regeneration density and a lack of exposure to herbicide, while the higher mosses in the leave corridors of the partial cut and uncut were due to the absence of disturbance [8] or herbicide use [15]. The higher amount of coarse woody debris of the intact classes in the uncut, mostly aspen, suggests a more recent overstory decline [22], while the difference in well-decayed classes among treatments indicates continuous residual tree mortality from aging and windthrow [8].
Neither the preharvest spray nor the partial cut had higher species and structural diversity, inconsistently with the expectation that the preharvest spray would have reduced aspen dominance and increased resources for other species, or that the partial cut would produce more structurally diverse stand conditions and provide habitats for shade-intolerant, pioneering species, as well as shade-tolerant, understory species [44,45]. The reduced aspen density with preharvest spray increased the abundance, but not the richness, of understory woody species [20]. The greater woody species diversity in the partial cut was found only at year 6 postharvest, despite the fact that a similar trend persisted from the 11th-year regeneration [20] to the 21st-year overstory canopy (Table 1). Our results, along with those from others [45,46], suggest that the effects of partial harvest on species diversity generally show a temporal shift from short-term positive [46] to long-term neutral [45,47]. With the increase of the overstory canopy and the reduction in shade-intolerant species (Figure 1), the diversity of both overstory and understory woody species tended to converge.

5. Conclusions

Silvicultural systems for regenerating and managing boreal mixedwood forests are generally based on mixtures of intolerant hardwoods with tolerant conifers through effective management of understory conifers [7,17]. Comparatively, options for intolerant conifers are limited, and require more management interventions.
The results of this study suggest that the partial harvest system, commonly used for shade-tolerant conifers [7,17], does not produce desirable results for intolerant jack pine. However, if leave strips had been removed at year 6 postharvest, according to the initial plan [18], jack pine might have had better survival and growth [19]. As a result, the leave corridors would have been dominated by aspen after the removal of leave trees [8], instead of being dominated by maple, and the harvested corridors would be hardwood-dominated mixtures, based on year 6 observations [8], not conifer-dominated mixtures, as initially targeted [18]. The partial harvest would produce more desirable results if the target conifers were shade-tolerant spruce [35].
Spraying the mature aspen canopy before harvest was more effective than doing so for the partial harvest in establishing intolerant jack pine. Although both partial harvest and preharvest spray provide immediate control over aspen regeneration density and size [8,19], relative to postharvest spray, which has a delay [15], the existence of leave strips substantially reduced understory light, compared to the level in the preharvest spray [9]. Preharvest spray allowed for adequate growth of both trembling aspen and jack pine, and reduced the need for resorting to a high planting density to maintain jack pine quality and subsequent stand density management to achieve adequate growth [48,49], ultimately lowering management costs.
The postharvest use of herbicide in broadcast or strips produced desirable results in terms of maximizing jack pine growth. As indicated by Deighton et al. [14], postharvest spray does not cause a loss of diversity in the overstory or understory layers. However, surviving hardwoods will have a low growth quality from herbicide-induced wood stain and rot and stem dieback, and may not serve the needs of commercial timber quality [9,13].
The results of the 21-year postharvest assessments indicate that preharvest spray is an effective silvicultural option for establishing an intimate mixture of trembling aspen and jack pine after clearcut, a mixedwood stand that resembles natural aspen–pine mixtures. Due to the absence of exposure to herbicide, the aspen trees in the preharvest spray were not compromised in terms of growth quality, compared to trees in the partial spray–sprayed corridors and the broadcast spray. Partial spray reduced herbicide use and produced spatial mosaics of trembling aspen and jack pine, with herbicide damage on aspen occurring in sprayed corridors, but not in unsprayed corridors. Partial harvest may be an option for establishing spatial mosaics of trembling aspen and jack pine if leave strips are removed after year 6. These conclusions are based on 21 years of postharvest assessments, and may need to be validated with longer-term assessments (e.g., 30 or 40 years).
This study tests the effectiveness of five silvicultural options in the renewal of trembling aspen–jack pine mixedwood stands after harvest. While the uncut served mostly as a control for the overstory canopy, clearcut with site preparation and planting may be included as a control for regeneration. This option, however, would unlikely be effective from the perspective of regeneration success based on jack pine performance in partial spray–unsprayed corridors and partial cut–harvested corridors.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/f16040683/s1: Figure S1: Diameter distribution of overstory trees (>4.0 m tall) by species groups and treatments at year 17 postharvest; Table S1: Parameters of Chapman–Richards model in estimating total tree height from DBH.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

I want to thank Amy Shewchuk, Amy Bolduc, Mya Rice, John Schnare, Jennifer Dacosta, Emily Rennie, Graedyn Smith, Nathan Hawkes, Riley Stobie, Nathan Keranen, Nathan Perkins, David Maxfield, Tashi Lhamo, Erika Mihell, Cole Barban, Ana Kostic, Sierra Rutland, Erika Mihell, Justin Viljakainen, and Madison McCaig of Ontario Ministry of Natural Resources and Forestry for assistance with the year 17 and 21 postharvest assessments, and Mike Brienesse of Ontario Ministry of Natural Resources and Forestry and three anonymous reviewers for providing constructive suggestions for improving earlier drafts of the manuscript. The financial support from Ontario Ministry of Natural Resources and Forestry is greatly appreciated.

Conflicts of Interest

The author declares no conflict of interest.

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Forests 16 00683 g001
Figure 1. Postharvest dynamics of regeneration densities (understory trees ≥0.1 m and ≤4.0 m tall) by species groups and treatments. Preharvest herbicide spray was applied before harvest and postharvest operations included site preparation at year 1, planting at year 2, and postharvest spray at year 3. The regeneration survey was not conducted for the uncut from year 1 to 11, the levels of which presumably resembled the level prior to harvest (9000 stems/ha, mostly maple 58%, fir 21%, and aspen 16%), or for the partial cut and partial spray at year 1, the levels of which were assumed to be similar to that of the postharvest broadcast spray before herbicide use.
Figure 1. Postharvest dynamics of regeneration densities (understory trees ≥0.1 m and ≤4.0 m tall) by species groups and treatments. Preharvest herbicide spray was applied before harvest and postharvest operations included site preparation at year 1, planting at year 2, and postharvest spray at year 3. The regeneration survey was not conducted for the uncut from year 1 to 11, the levels of which presumably resembled the level prior to harvest (9000 stems/ha, mostly maple 58%, fir 21%, and aspen 16%), or for the partial cut and partial spray at year 1, the levels of which were assumed to be similar to that of the postharvest broadcast spray before herbicide use.
Forests 16 00683 g001
Forests 16 00683 g002
Figure 2. Height distribution of understory trees (≥0.1 m and ≤4.0 m tall) by species groups and treatments at year 21 postharvest.
Figure 2. Height distribution of understory trees (≥0.1 m and ≤4.0 m tall) by species groups and treatments at year 21 postharvest.
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Forests 16 00683 g003
Figure 3. Height distribution of understory trees (≥0.1 m and ≤4.0 m tall) by species groups and treatment corridors of partial spray and partial cut at year 21 postharvest.
Figure 3. Height distribution of understory trees (≥0.1 m and ≤4.0 m tall) by species groups and treatment corridors of partial spray and partial cut at year 21 postharvest.
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Forests 16 00683 g004
Figure 4. Diameter distribution of overstory trees (>4.0 m tall) by species groups and treatments at year 21 postharvest.
Figure 4. Diameter distribution of overstory trees (>4.0 m tall) by species groups and treatments at year 21 postharvest.
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Forests 16 00683 g005
Figure 5. Diameter distribution of overstory trees (>4.0 m tall) by species groups and treatment corridors of partial spray and partial cut at year 21 postharvest. It should be noted that overstory plot was wider than treatment corridors, which may have reduced actual difference between treatment corridors.
Figure 5. Diameter distribution of overstory trees (>4.0 m tall) by species groups and treatment corridors of partial spray and partial cut at year 21 postharvest. It should be noted that overstory plot was wider than treatment corridors, which may have reduced actual difference between treatment corridors.
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Table 1. Comparisons of understory tree density and height, vegetation cover, and woody species diversity among treatments at year 21 postharvest. Means with different letters are significantly different (p < 0.05, bold).
Table 1. Comparisons of understory tree density and height, vegetation cover, and woody species diversity among treatments at year 21 postharvest. Means with different letters are significantly different (p < 0.05, bold).
AttributeTreespPreharvest SprayPartial SprayPartial CutBroadcast SprayUncut
Density (stems/ha)All tree0.6045642773415,182723110,243
Poplar0.00235 b174 b269 b96 b972 a
Birch0.6779383651068582556
Maple0.72034295738898461556632
Pine0.279785252170
Spruce0.44572012593082243486
Fir 0.45844314817271391597
Height (cm)All tree0.014113 a70 b69 b92 ab76 b
Cover (%)Shrub0.86951.147.144.746.345.4
Herb0.14618.123.510.217.115.0
Grass0.0660.41.43.51.20.3
Fern0.1399.511.210.68.218.5
Moss0.2342.33.49.74.811.6
CWD10.0031.1 b0.6 b1.9 b1.4 b8.7 a
CWD20.2210.80.21.50.51.1
Woody species diversityShannon species0.9981.451.431.421.441.42
Table 2. Comparisons of overstory density, DBH, height, basal area (BA), and species and structural diversity among treatments at year 21 postharvest. Means with different letters are significantly different (p < 0.05, bold).
Table 2. Comparisons of overstory density, DBH, height, basal area (BA), and species and structural diversity among treatments at year 21 postharvest. Means with different letters are significantly different (p < 0.05, bold).
AttributeTreespPreharvest SprayPartial SprayPartial CutBroadcast SprayUncut
Density (stems/ha)All trees0.0074036 a4024 a3500 a3812 a1801 b
Poplar0.285967142585157240
Birch0.512626383454494261
Maple0.807100093614568691121
Pine<0.0011181 a1076 a244 b1690 a25 b
Spruce0.955718210494115
Fir0.77119212139092239
DBH (cm)All trees0.0757.37.27.67.49.6
Poplar0.0097.6 b7.4 b7.2 b6.2 b23.9 a
Birch<0.0014.7 c5.0 c7.9 b5.2 c10.9 a
Maple0.0034.5 b4.4 b6.9 ab5.0 b8.7 a
Pine<0.00110.1 b9.9 b8.9 b10.8 b27.2 a
Spruce0.0675.87.213.75.510.9
Fir0.0766.07.66.05.43.2
Height (m)All trees0.0956.66.76.96.38.9
Poplar0.0028.5 b8.1 b8.3 b6.9 b17.8 a
Birch<0.0015.7 b5.8 b7.1 b5.9 b9.8 a
Maple0.0024.9 b4.4 b6.5 ab4.9 b8.8 a
Pine0.0067.0 b6.8 b6.4 b7.5 b18.5 a
Spruce0.0904.14.08.93.48.0
Fir0.0364.6 ab5.1 a4.2 ab4.1 ab1.6 b
BA (m2/ha)All trees0.93119.720.020.720.118.7
Poplar0.4294.47.24.31.91.9
Birch0.0041.3 b1.0 b3.2 a1.1 b3.2 a
Maple0.0181.8 b2.4 ab6.5 ab1.5 b9.0 a
Pine<0.00111.2 b8.3 b2.1 c14.9 a1.9 c
Spruce0.2230.30.52.60.42.5
Fir0.6420.70.61.90.40.3
Overstory diversityShannon species0.091.411.301.411.311.06
Shannon structure0.110.830.880.900.961.07
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Man, R. Effectiveness of Silvicultural Options in Renewal of Trembling Aspen–Jack Pine Mixedwood Stands, 21 Years After Treatment. Forests 2025, 16, 683. https://doi.org/10.3390/f16040683

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Man R. Effectiveness of Silvicultural Options in Renewal of Trembling Aspen–Jack Pine Mixedwood Stands, 21 Years After Treatment. Forests. 2025; 16(4):683. https://doi.org/10.3390/f16040683

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Man, Rongzhou. 2025. "Effectiveness of Silvicultural Options in Renewal of Trembling Aspen–Jack Pine Mixedwood Stands, 21 Years After Treatment" Forests 16, no. 4: 683. https://doi.org/10.3390/f16040683

APA Style

Man, R. (2025). Effectiveness of Silvicultural Options in Renewal of Trembling Aspen–Jack Pine Mixedwood Stands, 21 Years After Treatment. Forests, 16(4), 683. https://doi.org/10.3390/f16040683

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