Forest Ecology and Management 323 (2014) 148–157

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Forest Ecology and Management

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Shelterwood cutting to release coniferous advance growth and limitaspen sucker development in a boreal mixedwood stand

http://dx.doi.org/10.1016/j.foreco.2014.03.0150378-1127/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +1 (418) 643 7994x6615; fax: +1 (418) 643 2165.E-mail address: marcel.prevost@mrn.gouv.qc.ca (M. Prévost).

Marcel Prévost ⇑, Josianne DeBloisMinistère des Ressources naturelles du Québec, Direction de la recherche forestière, 2700 rue Einstein, Québec, QC G1P 3W8, Canada

a r t i c l e i n f o

Article history:Received 21 November 2013Received in revised form 5 March 2014Accepted 6 March 2014Available online 29 March 2014

Keywords:Ecosystem-based forest managementHardwood conversionStratified species mixtureCareful loggingConifer advance growth

a b s t r a c t

Stratified mixtures of pioneer hardwoods sheltering shade-tolerant conifers are commonly encounteredin the southern boreal forest. We used two-step shelterwood cutting to release conifer advance growthand limit trembling aspen (Populus tremuloides Michx.) development in a stratified mixed aspen–coniferstand in Quebec, Canada. This paper presents 10-year regeneration dynamics after the establishment cutapplied with different cutting intensities (0%, 35%, 50%, 65%, and 100% basal area removal). Aspen suck-ering was proportional to cutting intensity (p < 0.001) and its survival was limited under all four densitiesof residual cover. After 10 years, aspen density was 6600 stems/ha in the 0%, 35%, and 50% cuts,1600 stems/ha in the 65% cut and 4700 stems/ha in the clearcut. Adequate protection of balsam fir (Abiesbalsamea (L.) Mill.) and spruce (Picea glauca (Moench) Voss and Picea mariana (Mill.) BSP) advance regen-eration strongly contributed to limiting aspen development. The release treatment affected conifer sap-ling height growth, stem diameter, crown length growth and live crown ratio. Generally, the growthreaction to canopy removal was better in the clearcut than in partial cuts. Balsam fir response was goodin the two initial height classes studied (130–300 and 301–500 cm), but a significant spruce responseoccurred only in the smallest class. The final cut will be necessary to assess the overall effect of two-stepshelterwood cutting on species composition and growth of the new cohort.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction cover, as a combination of surviving advance regeneration and

Maintaining natural diversity of tree species at the landscape le-vel is a basic element of the concept of ecosystem-based forestmanagement (Plotkin, 2004). At this scale, the landscape may ormay not include mixed-species stands, provided that it containsstands dominated by different species (Cavard et al., 2011). Mixed-wood forests dominated by shade-intolerant hardwoods, however,are highly susceptible to hardwood conversion following clearcut-ting to the detriment of coniferous species (Greene et al., 2002;Grondin et al., 2003). In Canada, for instance, in mature stands thatcontain a trembling aspen (Populus tremuloides Michx.) compo-nent, this pioneer species can invade cutover sites through rootsuckering (Doucet, 1979; Man et al., 2008; Gradowsky et al.,2010). Because they are supported by the parent-tree root system,aspen asexual reproduction and rapid height growth confer a hier-archical advantage over conifers from the early stage of standdevelopment (Greene et al., 1999). Hence, a common situation fol-lowing harvest in the Canadian boreal mixedwood forest is thatshade-tolerant conifers, such as balsam fir (Abies balsamea (L.)Mill.) and spruce (Picea spp.), are present under a dense aspen

individuals established post-cut. Although stem density and spe-cies composition may be adjusted with an adapted precommercialthinning for this type of mixture (e.g. Prévost and Gauthier, 2012),such intervention is costly and its long-term effects are still un-known. Otherwise, these stands rapidly develop into an even-agedstratified mixture (Smith et al., 1997), with aspen potentially dom-inating conifers for decades. A sound silvicultural approach for ma-ture aspen–conifer mixtures should primarily aim to modulateestablishment of the new cohort, by limiting aspen developmentand promoting conifer species. Accelerating natural forest succes-sion to limit hardwood conversion would be in line with the con-cept of ecosystem-based forest management.

Part of the even-aged silvicultural system, shelterwood cuttingis used to establish a new cohort under a partial overstory that lim-its light and temperature extremes in the understory (Matthews,1989; Nyland, 2002). This regeneration system is comprised of atleast one partial cut, usually a seed cut, followed 5–15 years laterby a final cut that removes the residual overstory to release thenew cohort. A possible variant is that the first cut aims to releaseadvance growth under partial shade during the regeneration phase(Smith et al., 1997; Raymond et al., 2013). In Canada, the shelter-wood seed cut has been used to regenerate shade tolerant ormid-tolerant conifers, such as red spruce (Picea rubens Sarg.)

M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157 149

(e.g. Fajvan and Seymour, 1993; Pothier and Prévost, 2008; Prévostand Gauthier, 2013) and white spruce (Picea glauca (Moench) Voss)(e.g. Youngblood, 1991; OMNR, 1998; Raymond et al., 2000),among others. Although practices to protect advance growth arebeing widely applied (Groot et al., 2005), the two-step shelterwoodapproach based on advance regeneration still remains to bedocumented.

In the shelterwood system, the establishment cut leaves maturetrees of desired species as seed sources, which in turn limit lightavailability in the understory. Hence, residual trees may also limitdevelopment of shade-intolerant species, which is the basichypothesis of our silvicultural approach in mixed aspen–coniferstands. In a previous experiment, a seed cut that removed up to50% of merchantable basal area (BA) greatly limited aspen develop-ment in an aspen-dominated mixedwood stand (Prévost and Pothi-er, 2003). In this stand containing only sparse coniferousregeneration, residual trees in the main cover cast enough shadeto be detrimental to aspen suckers (Pothier and Prévost, 2002).Conversely, the seed cut was a failure in terms of establishmentand growth of coniferous regeneration, because fast-growingdeciduous species, such as aspen, white birch (Betula papyriferaMarsh), pin cherry (Prunus pensylvanica L.f.), and red maple (Acerrubrum L.), outgrew conifers in the new cohort (Prévost and Pothi-er, 2003). It was hypothesized, however, that post-cut regenerationdynamics would have been different if dense advance coniferregeneration had been present, as young conifers are generallyknown to react positively to release (Thorpe and Thomas, 2007),especially balsam fir (Davis, 1989). This assumption was reinforcedby a recent analysis indicating that conifers in intermediate andsuppressed strata had the strongest growth reaction after 10 yearson this site (Prévost et al., 2010).

This study aims to assess the two-step shelterwood cuttingbased on advance regeneration in an aspen–conifer stand with asignificant density of tall coniferous advance growth. The first cutwould be a means of promoting conifer advance growth (e.g. Kra-sowski and Wang, 2003) while limiting aspen development in thisstand (e.g. Prévost and Pothier, 2003). While the success of the re-lease cut may be related to the current height of advance growth(Pothier et al., 1995) and level of pre-cut suppression (Krasowskiand Wang, 2003), it is likely that the shade from a well-developedregeneration stratum may be harmful to emerging aspen suckers.The rational is that prioritizing the harvesting of aspen would trig-ger suckering (Schier, 1973), but subsequent light levels should beinsufficient for their growth and survival.

We present the 10-year effects of the establishment cut onregeneration dynamics, with emphasis on aspen suckering andunderstory conifer development in relation to partial cutting inten-sity. We hypothesized that (i) aspen growth and survival will belimited under the shade of the residual forest cover, (ii) the shadefrom tall advance growth will supplement the shade from theresidual cover to limit even more aspen development, (iii) post-cut growth of released coniferous regeneration will be proportionalto cutting intensity, (iv) growth of coniferous regeneration will berelated to tree size, and (v) growth response will differ between firand spruce regeneration. Stratified aspen–conifer mixtures arecommon in the Canadian boreal mixedwood forest and alternativesto clearcutting must be developed to maintain their softwoodcomponent.

2. Materials and methods

2.1. Site description

The study was carried out in a boreal mixedwood stand (Mac-Donald, 1995) located in the balsam fir–yellow birch bioclimatic

domain, region 4d, of the High Hills of Charlevoix and Saguenay(Saucier et al., 1998), approximately 160 km northeast from Qué-bec City, Quebec, Canada (47�550N, 70�030W). Mean temperatures(1971–2000, station La Malbaie) vary from �13.0 �C in January to17.8 �C in July, with a mean annual temperature of 2.3 �C. The re-gion receives 805 mm in average annual precipitation with 20%falling as snow.

The experiment was established in 2001, in a mixed aspen–conifer stand with an average basal area (BA) of 26.4 ± 6.2 m2/ha,composed of 53% aspen, 28% white birch, 11% balsam fir, 3% whitespruce, 2% black spruce (Picea mariana (Mill.) B.S.P.), and 2% redmaple (Table 1). Before the cut, mature aspen dominated the maincanopy whereas conifers generally occupied the intermediatestory, although some stems were in the dominant class. The under-story contained a coniferous regeneration that averaged5435 stems/ha, with 27% of stems P1 m in height, and was com-posed of 90% balsam fir and 10% spruce. Small red maple seedlingswere numerous (14,000 stems/ha) with 80% less than 30 cm inheight. The shrub layer was mainly composed of hazelnut (Coryluscornuta Marsh.) and mountain maple (Acer spicatum Lam.), with avariable cover of striped maple (Acer pensylvanicum L.). The soilis a loamy till with a mesic drainage on a less than 10% south-east-facing slope.

2.2. Experimental design

The experiment was made up of four complete randomizedblocks, each one containing five treatments: three partial cuttingintensities (uniform removal of 35%, 50%, and 65% of BA), a carefullogging around advance growth clearcut (100%) that also protectedsmall merchantable conifers (9–15 cm DBH), and an uncut control(0%). For the conifer fraction (16% of BA), the 100% cut roughly cor-responds to CPPTM (Coupe avec Protection des Petites TigesMarchandes) in Quebec (Ruel et al., 2013) and to HARP (Harvestingwith Regeneration Protection) in Ontario (Groot et al., 2005).Blocking was based on pre-cut density of advance coniferousregeneration (P1.3 m in height) (Block A = 1425–2000 stems/ha;B = 1650–4475; C = 675–1225; D = 475–625), as previously mea-sured in each experimental unit. The five treatments were thereaf-ter assigned at random within each of these density classes. Thislayout was designed to assess the different levels of BA removalunder comparable levels of shade from the protected advance coni-fers. Corresponding approximately to homogenous zones withinthe stand in terms of softwood merchantable BA (Block A = 0.5–2.5 m2/ha; B = 3.1–7.4; C = 3.4–6.6; D = 4.3–10.5), density of ad-vance growth tended to decrease with increasing conifer BA.

Each experimental unit (EU) measured 50 m � 50 m (0.25 ha inarea) and contained a 20 m � 20 m central plot (400 m2) fromwhich precise measurements were taken (Fig. 1). Before cutting,all stems P1.3 m in height and <9.1 cm diameter at breast height(DBH) were tallied by species and DBH class (1, 2, 4, 6, and8 cm), while stems P9.1 cm DBH were measured with a diametertape in each 400-m2 plot. This initial inventory was used to markthe stems to be removed in the partial cuts in order to preciselycontrol the % BA removal in the central plot. The equivalent mark-ing was then done in the 15-m wide buffer strip, with removal reg-ularly checked using a prism with a BA factor of 2.0 m2. The choiceof stems to be cut followed these criteria: (1) aspen (which was al-most mature); (2) white birch or red maple, both for fuelwood (theless vigorous of the two species); (3) mature balsam fir and (4)spruce (mature or declining only). Tree marking was designed totarget spruce as seed trees, to retain small merchantable conifersand to create a uniform canopy without excessively large openings.In the clearcut, all hardwoods P9.1 cm DBH and conifersP15.1 cm DBH were marked in a single operation in the50 m � 50 m unit. Cutting was done during August and early

Table 1Density of merchantable stems (N, stems/ha) and basal area (BA, m2/ha) related to cutting intensity (CI, removal of 0%, 35%, 50%, 65%, and 100% of BA). Experimental blocks weredesigned in order to initially assign comparable density of conifer advance regeneration within a complete replication, which roughly corresponds to homogeneous zones withinthe site regarding softwood merchantable BA.

All species Trembling aspen White birch Balsam fir Spruce Conifer regeneration

Treatment Pre-cut Post-cut Pre-cut Post-cut Pre-cut Post-cut Pre-cut Post-cut Pre-cut Post-cut Pre-cut density (stems/ha)

Block CI (%) N BA BA N BA BA N BA BA N BA BA N BA BA

A 0 850 17.3 16.9 275 8.9 8.6 450 7.4 7.3 125 1.0 0.9 0 0.0 0.2 2000A 35 850 21.1 14.3 325 11.4 5.1 475 9.1 8.3 0 0.0 0.2 50 0.5 0.9 1625A 50 900 18.7 10.9 175 8.7 2.4 475 7.4 5.9 175 1.4 1.5 75 1.1 1.1 1675A 65 850 23.3 8.6 275 11.9 0.5 475 9.7 6.4 0 0.0 0.0 100 1.7 2.0 1825A 100 675 31.4 1.1 525 29.5 0.0 75 1.0 0.0 50 0.5 0.7 0 0.0 0.0 1425

B 0 1450 32.8 31.8 375 18.4 17.8 500 7.0 6.8 475 6.4 6.2 100 1.0 1.1 1675B 35 700 19.6 12.7 175 9.9 3.3 200 6.0 5.8 275 3.0 2.9 50 0.7 0.7 1650B 50 750 21.4 10.9 150 10.1 3.6 400 8.3 4.9 125 1.9 1.5 75 1.0 1.1 1925B 65 1150 21.9 8.1 200 7.8 0.0 625 10.3 4.4 250 2.7 2.6 75 1.1 1.1 2800B 100 650 29.0 2.2 425 24.3 0.0 25 0.5 0.0 50 2.5 0.6 125 1.6 0.9 4475

C 0 825 25.3 26.3 250 15.5 15.5 175 3.7 3.7 325 3.9 4.8 25 0.3 0.5 875C 35 1125 24.9 16.9 225 10.5 5.0 600 10.3 8.4 250 2.9 2.9 50 1.3 0.2 875C 50 1225 25.2 13.7 175 8.2 3.1 550 10.3 5.6 325 3.8 3.2 175 2.8 2.0 675C 65 875 28.7 11.5 150 11.3 0.9 375 11.1 6.1 200 3.4 3.0 0 0.0 0.0 1225C 100 775 22.1 1.6 300 11.3 0.0 225 5.7 0.0 125 0.9 1.0 125 4.2 0.6 950

D 0 1200 37.0 38.0 225 15.5 17.3 425 10.8 10.0 425 8.4 8.4 100 2.1 2.1 475D 35 650 26.2 17.9 225 16.8 9.5 200 4.2 3.6 200 4.8 4.5 25 0.3 0.5 525D 50 1075 27.4 14.8 275 13.5 4.0 450 7.6 5.5 200 2.1 2.2 75 2.2 1.7 525D 65 950 33.6 11.9 150 13.4 0.0 300 11.5 6.6 300 4.8 3.6 25 0.2 0.0 625D 100 1075 40.5 2.3 325 24.1 0.0 325 7.4 0.0 375 7.0 2.3 25 1.7 0.0 600

Mean 0 1081 28.1 28.2 281 14.6 14.8 388 7.2 6.9 338 4.9 5.1 56 0.9 0.9 125635 831 23.0 15.5 238 12.2 5.7 369 7.4 6.5 181 2.7 2.6 44 0.7 0.6 116950 988 23.2 12.6 194 10.1 3.3 469 8.4 5.5 206 2.3 2.1 100 1.8 1.4 120065 956 26.9 10.0 194 11.1 0.3 444 10.7 5.9 188 2.7 2.3 50 0.8 0.8 1619

100 794 30.7 1.8 394 22.3 0.0 163 3.6 0.0 150 2.3 1.1 69 1.9 0.5 1863

Note: Conifer advance regeneration includes stems P1.3 m in height. Shorter regeneration was not tallied before cutting. Pre-cut measurements were made in June 2001 andpost-cut measurements were made in October 2001.

Fig. 1. Layout of Experimental Units (EUs) in the complete randomized block design. Each EU contains a 400-m2 central plot to measure residual stems of commercial species(P1.3 m in height) and sixteen 4-m2 regeneration quadrats within the central plot. For each EU, the upper case letter is the block and the value is the cutting intensity (% ofmerchantable basal area).

150 M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157

September 2001, using a chainsaw for felling. Trees were deb-ranched and slashed on site, to be transported to the landings witha F4 Dion track forwarder, from 50-m equidistant trails withoutcirculating in the 400 m2 central plots.

2.3. Vegetation monitoring

Within each 400-m2 plot, sixteen 2 m � 2 m (4 m2) quadratswere established in 2002 to do post-cut regeneration surveys.

M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157 151

Commercial species (balsam fir, black spruce, white spruce, trem-bling aspen, white birch, red maple) and the principal non-com-mercial species (hazelnut, mountain maple, striped maple,mountain ash (Sorbus americana Marsh.), willow (Salix spp.)) weretallied by height class (1–5, 6–30, 31–60, 61–100, 101–200, 201–300, and >300 cm). These surveys were carried out at years 1–3(2002–2004), 5 (2006), 7 (2008), and 10 (2011) post-cut.

After the cut in 2001, all residual stems of commercial speciesP1.3 m in height were numbered in the 400-m2 plots. Species,DBH, total height, crown height, and four crown radius (N, E, Sand W) were then recorded. On the same occasion, height growthof advance balsam fir and spruce (stems 65 m in height) was mea-sured by annual whorls on the main stem to characterize conifer-ous development from 1997 to 2001, the 5 years before treatment.At years 1–3, 5, 7, and 10 post-harvest, DBH of all numbered stemswas measured. Meanwhile, height growth of advance conifers65 m in height was measured by annual whorls to characterizetheir development from 2002 to 2011, 1–10 years post-harvest.In 2007, we increased our sampling to obtain a minimum of fivefirs and five spruces in each of the two sapling height categories(small = 130–300 cm; large = 301–500 cm) per experimental unit.We therefore had to localize some stems outside the 400-m2 plotbut inside the 15-m buffer strip of the same unit. These new stemswere numbered and recorded for species, DBH, total height, andheight growth of preceding years by measuring annual whorlsfrom 1997 to 2007. These additional sample saplings were thereaf-ter included in the 2008 and 2011 regular surveys. In the fall of2011, total height, crown height and four crown radius (N, E, Sand W) were finally recorded for all numbered stems.

2.4. Statistical analyses

Regeneration density measurements, repeated 1, 5, and10 years after cutting, were analyzed separately for each commer-cial species. A linear mixed effects model for repeated measure-ments was used, with a variance–covariance matrix to take intoaccount the correlation between measurements done on the sameEUs. The choice of this matrix was made to minimize the likelihoodvalue of the model while having as few parameters as possible.Cutting intensities, measurement years, and their interaction wereintroduced in the model as fixed effect factors, whereas blockswere considered as a random effect factor. Data for balsam firand white birch were log-transformed to meet the underlyingassumptions of the model, but were presented in their originalform in the results section. For all regenerating species, the useof aspen pre-cut BA (Table 1) was rejected as a covariate(p P 0.138).

The effects of cutting intensities and species (trembling aspenand conifers) on regeneration density among height classes (A:6–100 cm, B: 101–200 cm, C: 201–300 cm and D: >300 cm) wereevaluated 1, 5 and 10 years after cutting, using a classic four-wayanalysis of variance with cutting intensities, species and heightclasses as fixed effects and blocks as a random effect factor. Squareroot transformation was used and values were presented in theiroriginal scale.

Growth of conifer advance regeneration data were analyzedwith the same classic four-way analysis of variance, 5 or 10 yearsafter cutting, except that height classes were replaced by initialsapling sizes (small: 130–300 cm in height, large: 301–500 cm).Pre-cut measurements (2001) were used as a covariate when re-quired, to account for potential differences prior to cutting.

All analyses were performed on EU values using the MIXED pro-cedure of SAS (v. 9.2, SAS Institute Inc., Cary, North Carolina) andSatterthwaite’s method for approximating the denominator de-grees of freedom. In all cases where a factor or an interactionwas statistically significant (p < 0.05), a simulation-based approach

was used to assess differences (Westfall et al., 1999). For significantinteractions between main factors, levels of one factor were com-pared at a fixed level of the other factors. Homogeneity of varianceswas assessed using standard graphical methods, while normality ofthe residuals was confirmed with the Shapiro–Wilk’s statistic.

3. Results

3.1. Regeneration dynamics

3.1.1. Aspen suckeringAspen suckering was related to cutting intensity and the treat-

ment effect changed with time as indicated by the CI � Y interac-tion (p = 0.004, Table 2 and Fig. 2). One year after treatment,density of suckers was higher in the 100% cut (63,400 stems/ha)than in all other cutting intensities (15,400–29,200 stems/ha,p 6 0.029, results of multiple comparison tests not presented).Thereafter, aspen density strongly decreased with time in all treat-ments, mortality affecting 80% or more of the stems at year 5 post-cut. At this time, the 100% cut still contained a higher density ofsuckers (13,200 stems/ha) than the control and 35% cut (respec-tively 500 and 1600 stems/ha, p 6 0.045), while the 50% and 65%cuts had intermediate densities (4200 and 5200). After 10 years,mortality rate of aspen suckers exceeded 90% and density was6600 stems/ha in the 0%, 35%, and 50% cuts, 1600 stems/ha inthe 65% cut and 4700 stems/ha in the clearcut. Treatment effectwas no longer significant.

3.1.2. Regeneration of other speciesA time effect was found in balsam fir (p < 0.001, Table 2 and

Fig. 3), white birch (p = 0.008, data not shown), and red maple(p = 0.017, data not shown) densities. For balsam fir, mean densitywas higher at year 10 (36,400 stems/ha) compared to years 1(10,600) and 5 (11,300, p 6 0.001). For white birch, densities werehigher at years 1 (2000 stems/ha) and 5 (1000) than at year 10(400, p 6 0.016). For red maple, density was higher at year 5(28,400 stems/ha) compared to years 1 (22,300) and 10 (19,300,p 6 0.034). No significant effects were found for spruce density.

3.1.3. Height classes of aspen vs. conifersOne year following treatment, aspen density was higher

(p 6 0.012) than conifer density in the 6–100 cm height class ofthe 50% (21,000 stems/ha vs. 7000 stems/ha), 65% (25,300 vs.8200) and 100% (44,200 vs. 19,400) cuts, and in the 101–200 cmclass of the 100% cut (19,200 vs. 3600) (CI � Sp � H, p = 0.055, Ta-bles 3 and 4). For aspen, density in these two classes was higher inthe 100% cut than in all other treatments (p 6 0.008), except for thelowest class in the 65% cut (p = 0.088). In every treatment, aspendensity was highest in the lowest class (p 6 0.036). For conifers,density in the 6–100 cm class was only higher in the 100% cut rel-ative to the control (p = 0.002). In the 35%, 50%, and 100% cuts,conifer density was highest in this lowest class (p 6 0.049). Inthe 65% cut, density was higher in the 6–100 cm class than inthe 201–300 (p = 0.002) and >300 cm classes (p = 0.006).

At year 5, total stem density (aspen and conifers of all heightclasses combined) was higher in the 100% cut (35,400 stems/ha)than in all other treatments (6400–17,800), and higher in the65% (17,800) cut than in the control (6400, p 6 0.036, Table 4).All treatments combined, conifer density in the 6–100 cm class(7400 stems/ha) was higher than in taller classes (900–1900) andhigher than aspen density in this same class (800) (Sp � H,p < 0.001, Table 3). For aspen, density was higher in the 101–200 cm class (1600 stems/ha) than in the >300 cm class (500,p = 0.002).

Table 2Analysis of variance and associated probabilities (P > F) for regeneration density (stems/ha) measurements repeated 1, 5, and 10 years after cutting (0%, 35%, 50%, 65%, and 100%basal area removal).

Source of variation df Trembling aspen White birch Red maple Spruce Balsam fir

Cutting intensity (CI) 4 <0.001 0.388 0.484 0.467 0.471Year (Y) 2 <0.001 0.008 0.017 0.105 <0.001CI � Y 8 0.004 0.122 0.445 0.485 0.138

Note: df = degrees of freedom of the numerator. Denominator degrees of freedom according to Satterthwaite: CI = 8.4–13.7, Y = 10.9–14, CI � Y = 11.2–17.3. Variance–covariance matrix: UN(1) for trembling aspen and UN for white birch, red maple, spruce, and balsam fir. Analyses were done on log-transformed data for balsam fir and whitebirch.

Fig. 2. Density of aspen suckers (stems/ha) related to cutting intensity (% ofmerchantable basal area), at years 1, 5, and 10 post-treatment. The solid barindicates the mean and the empty bar indicates the standard error.

Fig. 3. Density of balsam fir seedlings (stems/ha) related to cutting intensity (% ofmerchantable basal area), at years 1, 5, and 10 post-treatment. The solid barindicates the mean and the empty bar indicates the standard error.

Table 3Analysis of variance and associated probabilities (P > F) for regeneration density(stems/ha) related to cutting intensity (0%, 35%, 50%, 65%, and 100% basal arearemoval), species (trembling aspen or conifers), and height class (A = 6–100 cm;B = 101–200 cm; C = 201–300 cm; D = >300 cm) 1, 5, and 10 years after treatment.

Source of variation df Year 1 Year 5 Year 10

Cutting intensity (CI) 4 <0.001 <0.001 <0.001Species (Sp) 1 0.010 <0.001 <0.001CI � Sp 4 0.004 0.165 0.373Height class (H) 3 <0.001 <0.001 <0.001CI � H 12 <0.001 0.261 0.003Sp � H 3 <0.001 <0.001 <0.001CI � Sp � H 12 0.055 0.322 0.540

Note: df = degrees of freedom of the numerator. Denominator degrees of freedomaccording to Satterthwaite: CI = 15–120, all other factors and interactions = 105–120. Analyses were done on the square root of regeneration density.

152 M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157

At year 10, height class interacted with treatment (CI � H,p = 0.003, Tables 3 and 4) and species (Sp � H, p < 0.001). Aspenand conifers combined, stem density in the 6–100 cm class washigher than in all the taller classes in the 0%, 35%, and 50% cuts(all p < 0.001), and higher than in the 201–300 (p = 0.003)and > 300 cm (p = 0.017) classes in the 65% cut. Meanwhile, their

combined density did not differ among the four height classes inthe 100% cut; this treatment had more stems 101–200 cm in height(total of 5500 stems/ha) than the 0% (500) and 35% (1100) cuts, andmore stems >300 cm (5500) than the 0%, 35%, and 50% cuts (700–1000) (p 6 0.029). Overall, conifer density decreased from the firstto the third height class (from 14,900 to 800 stems/ha, p 6 0.039),but was higher than aspen density up to the second class (bothp < 0.001). At this time, aspen density was comparable amongclasses.

3.2. Growth of conifer saplings

3.2.1. DBH, height growth, and total heightFive years following treatments, mean conifer DBH, initial sap-

ling sizes of balsam fir and spruce combined, was higher in theclearcut than in other treatments (p 6 0.046, Fig. 4). Spruce DBH(7.7 cm) was higher than fir DBH (6.5 cm) for the large sapling cat-egory (Sp � Si, p < 0.001, Table 5). At year 10, single effect of allthree factors was significant (p 6 0.002). Mean DBH was higherin the 100% cut (8.0 cm) than in the 0% (5.6 cm), 35% (6.5 cm),and 50% (6.6 cm) cuts (p 6 0.018), and still higher for spruce thanfor fir of both sizes combined (p < 0.001).

Single effect of cutting treatment and initial sapling size wasfound in conifer height growth at years 5 and 10 (all p < 0.001, Ta-ble 5 and Fig. 4). For years 5 and 10, mean growth of conifers washighest in the 100% cut (36 and 40 cm/year, respectively,p 6 0.012), lowest in the control (both years, 17 cm/year), and be-tween these values in the 50% and 65% cuts (24–29 cm/year). Glob-ally, large conifers had higher 10-year height growth (30 cm/year)than small conifers (22 cm/year) (p < 0.001).

Total height was subjected to different interactions among fac-tors at the end of each 5-year period (p = 0.008–0.049, Table 5 andFig. 4). At year 5, fir was taller in the 100% cut than in the controlfor the two initial sizes (p 6 0.009), while spruce height was simi-lar among treatments (p P 0.062). Spruce was taller than fir in thelarge sapling category of the 0% and 50% cuts (p 6 0.026). At year10, fir in the 100% cut was taller than in the 0%, 35%, and 50% cuts

Table 4Density of aspen suckers and coniferous species (stems/ha) by height class (A = 6–100 cm; B = 101–200 cm; C = 201–300 cm; D = >300 cm) related to cutting intensity (CI, % ofmerchantable basal area) at years 1, 5, and 10 post-treatment.

CI (%) Year 1 Year 5 Year 10

A B C D A B C D A B C D

Trembling aspen0 6031 78 0 39 469 39 0 0 313 0 0 0

35 13946 1407 39 0 196 899 430 78 195 0 39 11750 21017 2383 0 0 977 2461 703 78 430 117 0 3965 25273 3867 39 0 664 2031 1992 547 39 156 430 938

100 44211 19196 39 0 1146 4636 4688 2696 117 625 1055 2891

Conifers (mainly balsam fir)0 3901 435 391 625 4492 391 274 742 10509 547 352 781

35 7266 938 508 352 6524 1211 508 899 21875 1133 743 89950 7032 742 391 352 6524 1289 274 469 17383 1289 430 66465 8243 1758 391 547 8516 2539 899 625 12266 2852 860 1211

100 19441 3644 391 560 13034 6081 2253 886 12657 4844 1641 2617

Fig. 4. Evolution of diameter at breast height, height growth, and total height for two initial sizes of conifer saplings (small = 130–300 cm in height, large = 301–500 cm),related to cutting intensity (0%, uncut control = dot; 35% = circle; 50% = empty square; 65% = triangle; 100% = solid square) from 5 years before cutting to 10 years post-cut.Because treatments were applied during late summer 2001, year 0 was 2001, year 1 post-cut was 2002, and year 1 before the cut was 2000.

Table 5Analysis of variance and associated probabilities (P > F) for diameter at breast height (DBH, mm), height growth (cm/yr), and total height (cm) related to cutting intensity (0%, 35%,50%, 65%, and 100% basal area removal), species (balsam fir or spruce), and initial sapling size (small = 130–300 cm in height; large = 301–500 cm) 5 and 10 years after treatment.

Source of variation df DBH Height growth Total height

Year 5 Year 10 Year 5 Year 10 Year 5 Year 10

Cutting intensity (CI) 4 <0.001 0.002 <0.001 <0.001 0.008 <0.001Species (Sp) 1 <0.001 <0.001 0.148 0.684 <0.001 0.001CI � Sp 4 0.631 0.169 0.246 0.378 0.008 0.097Initial sapling size (Si) 1 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001CI � Si 4 0.787 0.641 0.230 0.127 0.043 0.136Sp � Si 1 <0.001 0.138 0.843 0.082 0.049 0.617CI � Sp � Si 4 0.113 0.058 0.459 0.981 0.059 0.037Covariate 1 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Note: df = degrees of freedom of the numerator. Denominator degrees of freedom according to Satterthwaite: CI = 11.9–55.6, Sp = 37.1–55.3, CI � Sp = 37–55.8, Si = 35.8–55.3,CI � Si = 35.9–55.4, Sp � Si = 35.8–55.3, CI � Sp � Si = 35.7–55.9, Covariate = 38.4–56.6.

M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157 153

154 M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157

for small saplings (4.6 m vs. 3.0–3.7 m, p 6 0.045), and taller thanin the 0%, 35%, and 65% cuts for large saplings (7.8 m vs. 5.4–6.5 m, p 6 0.026). For spruce, small saplings were taller in the100% cut than in the 0% and 35% cuts (4.7 m vs. 3.3–4.0 m,p 6 0.011), while height of large saplings did not differ amongtreatments (6.7–7.2 m, p P 0.962).

3.2.2. Live crown characteristicsTen-year crown length growth was related to cutting intensity

and initial sapling size (both p < 0.001, Table 6 and Fig. 5). Meanswere higher in the 100% cut (28 cm/year) than in other treatments(11–19 cm/year, p 6 0.025), except in the 50% cut (21 cm/year,p = 0.081). Growth in this latter treatment was higher than in thecontrol (p = 0.018). Similar to height growth, crown length growthwas higher for large saplings than for small saplings (p < 0.001).

An interaction was found between cutting intensity and initialsapling size for crown diameter growth (CI � Si, p = 0.041, Table 6and Fig. 5). Large saplings had higher gains than small ones in the100% cut only (14 vs. 9 cm/year, p < 0.001). Moreover, large saplingmean gain was higher in this treatment as compared to all othertreatments (7–9 cm/year) (p 6 0.004).

Table 6Analysis of variance and associated probabilities (P > F) for 10-year live crown growth(cm) and live crown ratio at year 10 related to cutting intensity (0%, 35%, 50%, 65%,and 100% basal area removal), species (balsam fir or spruce), and initial sapling size(small = 130–300 cm in height; large = 301–500 cm).

Source of variation df Crown lengthgrowth

Crown diametergrowth

Live crownratio

Cutting intensity (CI) 4 <0.001 0.005 0.024Species (Sp) 1 0.217 0.975 0.001CI � Sp 4 0.552 0.361 0.211Initial sapling size (Si) 1 <0.001 <0.001 0.424CI � Si 4 0.886 0.041 0.378Sp � Si 1 0.111 0.646 0.004CI � Sp � Si 4 0.327 0.501 0.259Covariate 1 – – <0.001

Note: For crown length and crown diameter, pre-cut value (2001) was not signifi-cant as covariate and was removed from the model. df = degrees of freedom of thenumerator. Denominator degrees of freedom according to Satterthwaite: CI = 11.3–12.5, Sp = 36.1–39.2, CI � Sp = 35.6–38.8, Si = 35–37.5, CI � Si = 35.2–37.4,Sp � Si = 35.1–37.5, CI � Sp � Si = 34.7–37.4, Covariate = 45.9.

Fig. 5. Ten-year live crown growth related to cutting intensity (0%, 35%, 50%, 65%, and(small = 130–300 cm in height; large = 301–500 cm). The letters on the x axis are the fivsevere = S; 100%, total = T. The solid bar indicates the mean and the empty bar indicates

At year 10, live crown ratio was related to cutting intensity(p = 0.024, Table 6, data not shown). Overall mean was higher inthe clearcut (0.80) than in the control (0.68, p = 0.016). An interac-tion was found between species and initial size (Sp � Si, p = 0.004).Live crown ratio was higher for spruce (0.79) than for fir (0.70) inthe small sapling category (p < 0.001). A tendency for a higher ratioin small saplings (0.79) than in large saplings (0.74) was also de-tected for spruce (p = 0.054).

4. Discussion

4.1. Aspen development

4.1.1. First-year suckeringFirst-year aspen density was in the lower range of those re-

ported in other silvicultural studies. For example, density in theclearcut reached 131,000 stems/ha in northeastern Ontario (Grootet al., 2009), 103,000 stems/ha in northwestern Quebec (Brais et al.,2004), and 74,000 stems/ha in Alberta (Lennie et al., 2009), as com-pared to 63,400 stems/ha in the present study. As expected, aspensuckering was proportional to cutting intensity (Fig. 2), which con-curs with results of Brais et al. (2004) following a comparablerange of BA removal. However, overall density of stems 1 yearpost-cut was 25% higher than in the previous experiment thatcompared the same treatments in Lower Appalachians (Prévostand Pothier, 2003). Aspen pre-cut BA was slightly higher in thepresent study (14 vs. 11 m2/ha in the Appalachians), which mayhave amplified suckering (Frey et al., 2003). In addition, this resultcould be closely related to clonal differences between the sites(Zasada and Schier, 1973), as we clearly observed higher aspen vig-our and tree quality in the present study, presumably with stron-ger suckering ability (Tew, 1970). For instance, the aspen BA wasmade up by only 260 stems/ha in High Hills of Charlevoix as com-pared to 425 stems/ha in the Appalachians. It is also possible thatsoil warming may have increased suckering (Maini and Horton,1966), because the experiment was conducted on a south-east-fac-ing slope allowing more solar radiation to reach the understory(e.g. Prévost and Raymond, 2012). We believe that it occurred onlyin places, however, as the dense coniferous regeneration stratumlikely restricted energy exchanges at the soil surface. Otheruncontrolled site factors, such as site index, soil disturbance, slash

100% basal area removal), species (balsam fir or spruce), and initial sapling sizee cutting intensities: 0%, uncut control = C; 35%, light = L; 50%, moderate = M; 65%,the standard error.

M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157 155

retention and timing of harvest, may all have contributed to thisdifference in initial suckering.

4.1.2. The role of advance growthOur first hypothesis, asserting limited sucker growth and sur-

vival under a residual cover, was confirmed. In all partial cuts, as-pen height (Table 4) and total density (Fig. 2) were lower than inthe 100% cut. Results show that the establishment cut allowed tolimit aspen development up to 65% BA removal. At this relativelyhigh level of partial cutting, aspen density was comparable to thedensities at lower intensities at year 10. At the same time, densitywas not statistically higher in the clearcut than in other treat-ments. This is a noticeable result considering that complete over-story removal has eliminated all overstory shade from maturecrowns. We presume that aspen self-thinning was driven by com-petition for light (Shepperd, 1993), and for nutrients and carbohy-drates through clonal connections (Krasny and Johnson, 1992;DesRochers and Lieffers, 2001; Baret and DesRochers, 2011). It isclear, however, that a part of aspen mortality was also attributableto competition from conifers in the new cohort. Besides drawingon the same soil-based resources as aspen, tall advance regenera-tion cast additional shade to this intolerant species, especiallysince small merchantable conifers were retained. This confirmsour second hypothesis regarding the impact of conifer advancegrowth on understory light environment and aspen development.Moreover, our experimental design allows a good insight into therelationship between advance conifer regeneration and aspen sur-vival. The two blocks with the highest densities of pre-establishedconifers (A and B, Table 1) contained much less aspen at year 10,when all treatments are combined (mean: 5200 and 2700 stems/ha, respectively), than the two blocks with the lowest densities(C and D: 15,200 and 7000 stems/ha). The same trend was foundfor clearcut experimental units, with two- to threefold fewer aspenin blocks A (3800 stems/ha) and B (2200) than in blocks C (7000)and D (5800). It is worth mentioning that small merchantable coni-fers certainly contributed to understory shade in clearcut experi-mental units, but no relationship between the residual BA (1.1–2.3 m2/ha) and aspen mortality was found. Our results clearly sup-port protection of conifer advance growth as part of a silviculturalstrategy for limiting hardwood conversion in the boreal mixed-wood forest.

4.1.3. Residual BAThis experiment was conducted in a mixed stand dominated by

high quality aspen. Despite tree vigour that conferred strong suck-ering ability (Tew, 1970), aspen development was limited up to65% BA removal, as compared to 50% in a lower quality stand inthe Appalachians (Prévost and Pothier, 2003), situated 150 kmsouth of the present site. Residual BA in the 65% cut averaged10.0 m2/ha in Charlevoix, which is comparable to that in the 50%cut in the Appalachians (10.4 m2/ha). Hence, this confirms that aresidual density around 10 m2/ha, proposed in Prévost and Pothier(2003), could limit aspen development in such a high quality standwith a total BA of 26 m2/ha. Besides confirming the role of advancegrowth on understory shade and aspen development, results in the65% cut allow another estimation with respect to residual BA. Interms of aspen mortality, the best results were obtained in thetwo lowest residual BAs (8.6 and 8.1 m2/ha, respectively in blocksA and B). Therefore, in aspen–conifer stands with dense pre-estab-lished conifer regeneration, an establishment cut that removes 65%of BA and leaves a residual BA of between 8 and 10 m2/ha would beadequate to limit aspen recruitment. Our data indicate that theminimal density of pre-cut conifer regeneration should be around2000 stems/ha, in the 1.3–5 m height class. Adequate protection ofthis advance regeneration during harvest is certainly a premise ofthis approach. Although skid trails were not sampled in this

experiment, and therefore post-treatment conifer densities areprobably overestimated, we consider our harvesting system asmost effective regarding careful logging.

4.2. Height classes of aspen vs. conifers

Periodic stem numbering by species and height class permittedus to highlight some aspects of regeneration dynamics over the 10-year period. At year 1 post-cut, density of both aspen and coniferswas highest in the 6–100 cm height class. Asexual reproductioncapacity of aspen (Greene et al., 1999) allowed this species to out-number conifers in this class in the three highest BA removal, aswell as in the taller class in the clearcut (Table 4). Hormonal imbal-ance in the root system (Schier, 1973) was then the driving factorfor aspen reaction over the degree of shading. From year 5, conifersoutnumbered aspen in the 6–100 cm class of all treatments. As thisshift was not coupled with an increased proportion of aspen re-lated to conifers in taller classes, low light availability was proba-bly the limiting factor for aspen to reach upper levels. It is alsolikely that residual aspens inhibited sucker initiation and drawon the interconnected root system (Smith et al., 1997; DesRochersand Lieffers, 2001; Baret and DesRochers, 2011). Year 10 was char-acterized by an important recruitment of new balsam fir seedlingsin all but the clearcut treatment (Fig. 3). Our field observations atyear 7 (not presented) suggested that year 5 or 6 had mast seed-rain that enriched the fir seedling bank under favorable weatherconditions. Furthermore, a favorable micro-environment may ex-plain why fir regenerated well under a residual cover (Prévost,2008). At year 10, conifers largely outnumbered aspen up to200 cm tall in all treatments. Above 200 cm, aspen recruitmentwas poor except in the 65% (1400 stems/ha) and 100% cuts(3900 stems/ha), where conifers were also well represented(2100 and 4300 stems/ha, respectively). Therefore, conifers madeup the major part of the new cohort in all treatments includingthe clearcut in this aspen-dominated mixedwood stand. Our re-sults support the use of partial cutting to promote conifer advanceregeneration in boreal mixedwood stands, contrary to conclusionsof MacDonald et al. (2004).

4.3. Growth response of conifer saplings

We measured different aboveground conifer attributes to reflectgrowth performance under the different treatments. Of all vari-ables, height growth was the one for which the response was bestrelated to the cutting approach, independent of sapling size or spe-cies. Compared to the control, 10-year height growth was im-proved by about 50% in the partial cuts (Fig. 4). Contrary to ourthird hypothesis, however, this effect was not proportional to par-tial cutting intensity. Additionally, the relative growth gain evenreached 125% in the clearcut. Such a positive height growth re-sponse is in line with findings of Brais et al. (2013) for balsam firsaplings in eastern boreal mixedwood stands. This result goesagainst findings of Krasowski and Wang (2003) for subalpine fir(Abies Lasiocarpa (Hook.) Nut) following different levels of releasein a mixed-species stand in western Canada. Their intermediatetree size (1.5–5.0 m in height), which nearly encompasses oursmall and large sapling categories, responded best to a partialcut, while complete overstory removal caused a 12% height growthloss over a 3-year period. In the present study, balsam fir andspruce did not experience stronger postrelease stress in the clear-cut than in partial cuts. A clear height growth reaction rather oc-curred at year 3 in all cutting intensities. Moreover, highestoverall DBH in the 100% cut at years 5 and 10 indicate that stemsalso allocated resources to radial growth. This effect was particu-larly clear for balsam fir from year 2 in the large sapling category.Similarly, complete overstory removal stimulated radial growth of

156 M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157

balsam fir (Brais et al., 2013) and subalpine fir (Krasowski andWang, 2003).

Height and density of advance conifer regeneration were impor-tant factors in this experiment. Besides influencing understorylight, they both reflect a pre-cut degree of suppression. In addition,pre-release height growth is a good indicator of post-release re-sponse (Ruel et al., 2000). We used regeneration density for block-ing, but one of our goals was to determine how the response torelease treatment differed with sapling size. As well, it was inter-esting to compare growth responses of balsam fir and spruce,two species with different functional traits that coexist in this for-est type (Messier et al., 1999). For the two species, a 36% higherheight growth of large saplings versus small saplings clearly indi-cates that their hierarchical position was an advantage upon re-lease. This result confirms our fourth hypothesis and suggeststhat tallest regeneration at time of release might maintain its dom-inance for decades, as already reported by Pothier et al. (1995) forblack spruce.

For other growth variables, data interpretation was compli-cated by different interactions between factors. Regarding ourfifth hypothesis, spruce had a certain advantage over fir in thelarge sapling category for DBH and total H at years 5 and 10.Therefore, spruce simply maintained its pre-cut dominance overfir in this size category. As for height growth, results for totalheight indicate that both species performed best in the clearcut,but differently between initial sizes. Balsam fir response was de-tected in both size categories (analysis for year 5), while spruceresponse was detected later (year 10) and only in the small sap-ling category. Nevertheless, patterns of height growth indicatethat a reaction took place at year 3 in all three cases (Fig. 4). Sim-ilar sapling growth stagnation following release has been re-ported in boreal silvicultural experiments (Bose et al., 2013).Comparatively, spruce in the large sapling category did not reactbefore year 6 post-cut. We believe that, in the small sapling cat-egory, crowding and sheltering by larger trees lessened sprucephysiological shock following complete overstory removal. Thisfaster response of small conifers to release conforms to the liter-ature for the boreal forest (Ruel et al., 2000). Overall data stillsuggest that balsam fir would surpass spruce in the clearcutand confirm its strong ability to react to release (Davis, 1989).Precommercial thinning may therefore become necessary to ad-just stem density and species composition.

Crown growth of advance conifers had a direct impact on re-sults of this study, through the obvious effect of crown volumeon the degree of shading in the understory. As expected, 10-yeargrowth results for crown length were comparable to those forheight growth, i.e. independent of species, highest in the clearcut,and highest for large saplings all treatments combined (Fig. 5).Data for crown diameter indicate that the highest lateral growthwas also for large conifers, but this occurred only in the 100%cut. Hence, significant radial crown expansion of large saplings isin line with their ability to capture the growing space and maintaina dominant position in the clearcut (Pothier et al., 1995). Livecrown ratio, which is a good indicator of tree vigour for shade-tol-erant conifers (Ruel et al., 2000), was improved by complete over-story removal, with an 18% overall increase as compared to thecontrol (0.80 vs. 0.68). Unlike crown length and diameter, this ratiopermitted to differentiate crown reaction between species and sizecategory. For spruce, higher live crown ratio in small saplings thanin large saplings is indicative of a better vigour, which is in linewith growth results. Higher live crown ratio of spruce than fir inthe small sapling category, however, may be related to species’ dif-ferences in propensity to maintain long live crown or expand later-ally (Messier et al., 1999; Ruel et al., 2000). Nevertheless, similar toheight growth, live crown ratio suggests that spruce in this layeracclimatized well to new growing conditions.

5. Conclusion

Conditions following the first cut of a two-step shelterwood ap-proach suggest that aspen regeneration can be limited in an aspendominated mixedwood stand presenting a dense conifer regenera-tion. Application of the establishment cut with an adequate ad-vance growth protection permitted to limit aspen developmentup to 65% BA removal. Conifer saplings greatly benefited of canopyopening with a good reaction of balsam fir in both size categories,and of spruce in the smallest category. Overall growth responsewas better in the clearcut than in partial cuts in our experimentalunits of 0.25 ha in area. This allows us to conclude that, as an alter-native to the two-step shelterwood, small patch-clearcut wouldpermit to maintain a proportion of conifers in the new cohort inpresence of a dense conifer regeneration 130–500 cm in height.Nevertheless, results of the establishment cut indicate that a par-tial cut, even of high intensity, would allow a much greater propor-tion of conifers to be maintained in the long term in aspendominated mixedwood stands. Therefore, the two-step shelter-wood securing conifer advance regeneration should be consideredas an ecosystem-based forest management strategy to limit hard-wood conversion in the boreal mixedwood forest.

Overall results of this study are based on an adequate protec-tion of coniferous advance growth during the establishment cut.In this forest type, this was a very important point for giving coni-fers a competitive position with respect to aspen suckers. The con-tribution of skillful tree fellers and the use of an easilymanoeuvrable forwarder undoubtedly permitted a high rate of ad-vance growth survival. The extent to which these results would beobtained operationally will mainly depend upon stand suitabilityfor this approach and reproduction of a comparable careful har-vesting methodology. This might raise questions about cost-effec-tiveness of logging operations with regard to protection of advancegrowth. It is important to note that the scenario must be com-pleted with a final cut, to evaluate the overall merit of the two-stepshelterwood based on advance growth. The new conifer cohort,with individuals now close to reaching 10 m in height, have to beefficiently protected again. Further surveys will be necessary be-fore solid conclusions can be drawn.

Acknowledgements

We thank Patricia Raymond for revising an earlier version ofthis manuscript, Daniel Dumais for computing density of mer-chantable stems and basal area related to treatment, and DebraChristiansen-Stowe for her linguistic revision. Two anonymousreviewers are also acknowledged for their helpful comments. Theauthors are also indebted to Coop Quatre Temps, Abitibi-Consoli-dated Inc. (Resolute Forest Products) and regional staff of the Min-istère des Ressources naturelles for their excellent collaboration atdifferent stages of this study. Heartfelt thanks are extended toJean-Pierre Lapointe, Maurice Gagnon, Julie Forgues, Carlo Gros-Louis, Daniel Guimond, Serge Williams, Pascal Lainé, Étienne DuBerger, Christian Villeneuve and many summer students for theirexcellent fieldwork over the years. This study is part of Project112310016 of the Direction de la recherche forestière, MRNQ.

References

Baret, M., DesRochers, A., 2011. Root connections can trigger physiologicalresponses to defoliation in nondefoliated aspen suckers. Botany 89, 753–761.

Bose, A.K., Harvey, B.D., Brais, S., Beaudet, M., Leduc, A., 2013. Constraints to partialcutting in the boreal forest of Canada in the context of natural disturbance-based management: a review. Forestry 1–18. http://dx.doi.org/10.1093/forestry/cpt047.

Brais, S., Harvey, B.D., Bergeron, Y., Messier, C., Greene, D., Belleau, A., Paré, D., 2004.Testing forest ecosystem management in boreal mixedwoods of northwestern

M. Prévost, J. DeBlois / Forest Ecology and Management 323 (2014) 148–157 157

Quebec: initial response of aspen stands to different levels of harvesting. Can. J.For. Res. 34, 431–446.

Brais, S., Work, T.T., Robert, É., O’Connor, C.D., Strukelj, M., Bose, A., Celentano, D.,Harvey, B.D., 2013. Ecosystem responses to partial harvesting in eastern borealmixedwood stands. Forests 4, 364–385.

Cavard, X., Macdonald, S.E., Bergeron, Y., Chen, H.Y.H., 2011. Importance ofmixedwoods for biodiversity conservation: evidence for understory plants,songbirds, soil fauna, and ectomycorrhizae in northern forests. Environ. Rev. 19,142–161.

Davis, W.C., 1989. The role of released advance growth in the development ofspruce-fir stands in eastern Maine. PhD Diss., Yale Univ., New Heaven, CT. pp.104.

DesRochers, A., Lieffers, V.J., 2001. Root biomass of regenerating aspen (Populustremuloides) stands of different densities in Alberta. Can. J. For. Res. 31, 1012–1018.

Doucet, R., 1979. Méthodes de coupe et de préparation de terrain pour favoriser larégénération naturelle de quelques tremblaies de l’est du Québec. For. Chron.55, 133–136.

Fajvan, M.A., Seymour, R.S., 1993. Canopy stratification, age structure, anddevelopment of multicohort stands of eastern white pine, eastern hemlock,and red spruce. Can. J. For. Res. 23, 1799–1809.

Frey, B.R., Lieffers, V.J., Landhäusser, S.M., Comeau, P.G., Greenway, K.J., 2003. Ananalysis of sucker regeneration of trembling aspen. Can. J. For. Res. 33, 1169–1179.

Gradowsky, T., Lieffers, V.J., Landhäusser, S.M., Sidders, D., Volney, J., Spence, J.R.,2010. Regeneration of Populus nine years after variable retention harvest inboreal mixedwood forests. For. Ecol. Manage. 259, 383–389.

Greene, D.F., Zasada, J.C., Sirois, L., Kneeshaw, D., Morin, H., Charron, I., Simard, M.-J.,1999. A review of the regeneration dynamics of North American boreal foresttree species. Can. J. For. Res. 29, 824–839.

Greene, D.F., Kneeshaw, D.D., Messier, C., Lieffers, V., Cormier, D., Doucet, R., Coates,K.D., Groot, A., Grover, G., Calogeropoulos, C., 2002. Modelling silviculturalalternatives for conifer regeneration in boreal mixedwood stands (aspen/whitespruce/balsam fir). For. Chron. 78, 281–295.

Grondin, P., Bélanger, L., Roy, V., Noël, J., Hotte, D., 2003. Envahissement desparterres de coupe par les feuillus de lumière (enfeuillement). In: Grondin, P.,Cimon, A. (Eds.), Les enjeux de biodiversité relatifs à la composition forestière. .Min. Res. nat. Fau. Parcs Qué., Dir. rech. for. et Dir. env. for. pp. 131–174.

Groot, A., Lussier, J.-M., Mtchell, A.K., MacIsaac, D.A., 2005. A silvicultural systemsperspective on changing Canadian forestry practices. For. Chron. 81, 50–55.

Groot, A., Man, R., Wood, J., 2009. Spatial and temporal patterns of Populustremuloides regeneration in small forest openings in northern Ontario. For.Chron. 85, 548–557.

Krasny, M.E., Johnson, E.A., 1992. Stand development in aspen clones. Can. J. For.Res. 22, 1424–1429.

Krasowski, M.J., Wang, J.R., 2003. Aboveground growth responses of understoryAbies lasiocarpa saplings to different release cuts. Can. J. For. Res. 33, 1593–1601.

Lennie, A.D., Landhäusser, S.M., Lieffers, V.J., Sidders, D., 2009. Regeneration of aspenfollowing partial and strip understory protection harvest in boreal mixedwoodforests. For. Chron. 85, 631–638.

MacDonald, G.B., 1995. The case for boreal mixedwood management: an Ontarioperspective. For. Chron. 71, 725–734.

MacDonald, G.B., Cherry, M.L., Thompson, D.J., 2004. Effect of harvest intensity ondevelopment of natural regeneration and shrubs in an Ontario borealmixedwood stand. For. Ecol. Manage. 189, 207–222.

Maini, J.S., Horton, K.W., 1966. Vegetative propagation of Populus spp. I. Influence oftemperature on formation and initial growth of aspen suckers. Can. J. Bot. 44,1183–1189.

Man, R., Kayahara, G.J., Rice, J.A., MacDonald, G.B., 2008. Eleven-year responses of aboreal mixedwood stand to partial harvesting: light, vegetation, andregeneration dynamics. For. Ecol. Manage. 255, 697–706.

Matthews, J.D., 1989. Silvicultural Systems. Clarendon Press, Oxford, UK (pp. 284).Messier, C., Doucet, R., Ruel, J.-C., Claveau, Y., Kelly, C., Lechowicz, M.J., 1999.

Functional ecology of advance regeneration in relation to light in boreal forests.Can. J. For. Res. 29, 812–823.

Nyland, R.D., 2002. Silviculture: Concepts and Applications, second ed. WaverlandPress, Inc., Long Grove (IL, USA) (pp. 682).

Ontario Ministry of Natural Resources (OMNR), 1998. Silvicultural guide of theGreat Lakes-St. Lawrence conifer forest in Ontario. Ont. Min. Nat. Resour.Queen’s Printer for Ontario. Toronto, pp. 424.

Plotkin, R., 2004. Ecosystem based forest management, reality or rhetoric ? – anassessment template and a case study. The Sierra Club of Canada. Ottawa, ON,Canada.

Pothier, D., Prévost, M., 2002. Photosynthetic light response and growth analysis ofcompetitive regeneration after partial cutting in a boreal mixed stand. Trees 16,365–373.

Pothier, D., Prévost, M., 2008. Regeneration development under shelterwoods in alowland red spruce – balsam fir stand. Can. J. For. Res. 38, 31–39.

Pothier, D., Doucet, R., Boily, J., 1995. The effect of advance regeneration height onfuture yield of black spruce. Can. J. For. Res. 25, 536–544.

Prévost, M., 2008. Effect of cutting intensity on microenvironmental conditions andregeneration dynamics in yellow birch – conifer stands. Can. J. For. Res. 38, 317–330.

Prévost, M., Gauthier, M.-M., 2012. Precommercial thinning increases growth ofoverstory aspen and understory balsam fir in a boreal mixedwood stand. For.Ecol. Manage. 278, 17–26.

Prévost, M., Gauthier, M.-M., 2013. Shelterwood cutting in a red spruce – balsam firlowland site: effects of final cut on water table and regeneration development.For. Ecol. Manage. 291, 404–416.

Prévost, M., Pothier, D., 2003. Partial cuts in a trembling aspen–conifer stand:effects on microenvironmental conditions and regeneration dynamics. Can. J.For. Res. 33, 1–15.

Prévost, M., Raymond, P., 2012. Effect of gap size, aspect and slope on available lightand soil temperature after patch-selection cutting in yellow birch–coniferstands, Quebec, Canada. For. Ecol. Manage. 274, 210–221.

Prévost, M., Dumais, D., Pothier, D., 2010. Growth and mortality following partialcutting in a trembling aspen–conifer stand: results after 10 years. Can. J. For.Res. 40, 894–903.

Raymond, P., Ruel, J.-C., Pineau, M., 2000. Effet d’une coupe d’ensemencement et dumilieu de germination sur la régénération des sapinières boréales riches deseconde venue du Québec. For. Chron. 76, 643–652.

Raymond, P., Legault, I., Guay, L., Godbout, C., 2013. Chapitre 19 – La coupeprogressive régulière. In: Le guide sylvicole du Québec, Tome 2 – Les concepts etl’application de la sylviculture. Ouvrage collectif supervisé par C. Larouche, F.Guillemette, P. Raymond et J.-P. Saucier, Min. Res. nat. Qué., Les Publications duQuébec. pp. 410–453.

Ruel, J.-C., Messier, C., Doucet, R., Claveau, Y., Comeau, P., 2000. Morphologicalindicators of growth response of coniferous advance regeneration to overstoreyremoval in the boreal forest. For. Chron. 76, 633–642.

Ruel, J.-C., Fortin, D., Pothier, D., 2013. Partial cutting in old-growth boreal stands:an integrated experiment. For. Chron. 89, 360–369.

Saucier, J.-P., Bergeron, J.-F., Grondin, P., Robitaille, A., 1998. Les régions écologiquesdu Québec méridional (3e version): un des éléments du système hiérarchiquede classification écologique du territoire mis au point par le ministère desRessources naturelles du Québec. L’Aubelle no. 124, L’Ordre des Ingénieursforestiers du Québec, Québec.

Schier, G.A., 1973. Origin and development of aspen root suckers. Can. J. For. Res. 3,45–53.

Shepperd, W.D., 1993. Initial growth, development, and clonal dynamics ofregenerated aspen in the Rocky Mountains. USDA For. Serv. Res. Pap. RM-312.

Smith, D.M., Larson, B.C., Kelty, M.J., Asthon, P.M.S., 1997. The Practice ofSilviculture: Applied Forest Ecology, ninth ed. John Wiley & Sons Inc., NewYork, NY (pp. 537).

Tew, R.K., 1970. Root carbohydrate reserves in vegetative reproduction of aspen.For. Sci. 16, 318–320.

Thorpe, H.C., Thomas, S.C., 2007. Partial harvesting in the Canadian boreal: successwill depend on stand dynamic responses. For. Chron. 83, 319–325.

Westfall, P.H., Tobias, R.D., Wolfinger, R.D., 1999. Multiple comparisons andmultiple tests using the SAS System. SAS Institute Inc., Cary, NC (pp. 416).

Youngblood, A.P., 1991. Radial growth after shelterwood seed cut in a mature standof white spruce in interior Alaska. Can. J. For. Res. 21, 410–414.

Zasada, J.C., Schier, G.A., 1973. Aspen root suckering in Alaska: effect of clone,collection date and temperature. Northwest Sci. 47, 100–104.