173 ARTICLE

Commercial thinning stimulates natural regeneration in spruce–fir stands Matthew G. Olson, Spencer R. Meyer, Robert G. Wagner, and Robert S. Seymour

Abstract: Understanding the response of tree regeneration following commercial thinning treatments can improve planning in managed forests dependent on natural regeneration. We used long-term commercial thinning experiments in eastern spruce–fir stands of Maine, USA, to test two hypotheses: (1) commercial thinning increases the density of tree regeneration and (2) tree regeneration density increases with increasing thinning intensity. A decade after thinning, densities of softwood and hardwood regeneration were 10 times greater in thinned stands than unthinned stands. The abundance of small softwood (0.11–0.60 m tall) was highest in lower intensity thinning treatments, whereas medium (0.61–1.40 m tall) and large (≥1.41 m tall to 8.90 cm diameter at breast height) softwoods increased proportionally with thinning intensity, a pattern related to a higher rate of recruitment in more open stands created by heavier thinning. Hardwood density generally increased with thinning intensity and developed into a significant component of the large size class. Softwood regeneration density was higher in older spruce stands than younger fir stands, which may be due to greater abundance of advance regeneration, higher residual stand mortality, and greater harvest disturbance in older spruce stands. However, acceptable softwood stocking was achieved in all replicates of thinning treatments. Therefore, in addition to providing higher individual-tree growth and merchantable yield, commercial thinning in eastern spruce–fir stands also increases regeneration density. The rate of recruitment also increased as thinning intensity increased, thus stimulating understory regeneration similar to that of a shelterwood establishment cut.

Key words: silviculture, partial cutting, regeneration, softwood, hardwood. Résumé : La compréhension de la réaction de la régénération a` la suite d'une éclaircie commerciale peut améliorer la planifi- cation dans les forêts aménagées qui dépendent de la régénération naturelle. Nous avons eu recours a` des expériences a` long terme portant sur l'éclaircie commerciale dans des peuplements de sapin et d'épinette de l'est du Maine, aux États-Unis, pour tester deux hypothèses : (1) l'éclaircie commerciale augmente la densité de la régénération et (2) la densité de la régénération augmente avec l'intensité de l'éclaircie. Dix ans après l'éclaircie, la densité de la régénération en conifères et en feuillus des peuplements éclaircis était 10 fois plus abondante que celle des peuplements non éclaircis. La plus grande abondance des conifères de petite taille (de 0,11 a` 0,60 m de hauteur) a été observée dans les éclaircies de faible intensité alors que celle

For personal use only. des conifères de tailles moyenne (de 0,61 a` 1,40 m de hauteur) et forte (d'une hauteur de 1,41 m a` un DHP de 8,90 cm) a augmenté proportionnellement a` l'intensité de l'éclaircie, une situation correspondant a` un taux de recrutement plus élevé dans les peuplements plus ouverts a` la suite d'éclaircies fortes. La densité des feuillus augmentait généralement avec l'intensité de l'éclaircie et représentait une partie importante de la régénération de grande taille. La densité de la régénération résineuse était plus forte dans les plus vieux peuplements d'épinette que dans les plus jeunes peuplements de sapin, ce qui a pu être causé par une plus grande abondance de la régénération préétablie, une plus forte mortalité dans les peuplements résiduels et une plus forte perturbation causée par la récolte dans les plus vieux peuplements d'épinette. Toutefois, la densité et la distribution de la régénération résineuse étaient acceptables dans tous les peuplements éclaircis. Par conséquent, en plus d'augmenter la crois- sance des arbres individuels et la production marchande, l'éclaircie commerciale dans les peuplements d'épinette et de sapin de l'est des États-Unis augmente aussi la densité de la régénération. Le taux de recrutement aussi augmente avec l'intensité de l'éclaircie, stimulant ainsi le développement de la régénération de façon similaire a` ce qui est observé a` la suite d'une coupe d'ensemencement du système de coupe progressive. [Traduit par la Rédaction]

Mots-clés : sylviculture, coupe partielle, régénération, conifères, feuillus.

Introduction wood and selection). As there is usually a time lag between the Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 Silvicultural practices are typically prescribed to meet one or more thinning operation and subsequent canopy closure by residual specific objectives. However, the ecological effects of silvicultural crowns (Deal and Farr 1994; Nyland 2002), thinning can increase practices are often not limited only to the desired outcomes. A good environmental resource availability (light, water, and nutrients) in example in this regard is thinning. The primary goal of thinning is to the understory that can stimulate natural regeneration (Deal and concentrate site resources to increase growth on fewer overstory Farr 1994; He and Barclay 2000). trees and to capture early mortality (Smith et al. 1997). Although not Results from thinning studies have shown a variety of understory typically a stated goal, thinning also can trigger a regeneration re- vegetation responses. Several studies have found that thinning alters sponse similar to traditional regeneration methods (e.g., shelter- understory vegetation and generally increases plant abundance

Received 13 June 2013. Accepted 14 December 2013. M.G. Olson. Resource Science Division, Missouri Department of Conservation, West Plains, MO 65677, USA. S.R. Meyer. Center for Research on Sustainable Forests, University of Maine, Orono, ME 04469-5755, USA. R.G. Wagner and R.S. Seymour. School of Forest Resources, University of Maine, Orono, ME 04469-5755, USA. Corresponding author: Matthew G. Olson (e-mail: [email protected]).

Can. J. For. Res. 44: 173–181 (2014) dx.doi.org/10.1139/cjfr-2013-0227 Published at www.nrcresearchpress.com/cjfr on 18 December 2013. 174 Can. J. For. Res. Vol. 44, 2014

(Parker et al. 2001; Chan et al. 2006; Yeo and Lee 2006; Otto et al. 2012). spruce–fir forests to manipulate tree spacing of dense naturally In plantations of the Canary Islands, Otto et al. (2012) observed that regenerated stands, adjust species composition, boost residual commercial thinning significantly increased seedling, sapling, and tree growth, and reduce rotation length (Seymour 1995). Due to juvenile densities of Canary Island pine (Pinus canariensis C. Sm.), the shade tolerance of the spruces and balsam fir, commercial while Yeo and Lee (2006) found higher densities of natural hardwood thinning stands during stem exclusion likely stimulates the estab- regeneration after thinning of conifer plantations in Korea. In other lishment of advance regeneration prior to initiation of regenera- cases, thinning has had no effect on regeneration abundance or de- tion methods. velopment. For example, Lei et al. (2007) found no differences in Successfully regenerating spruce–fir stands in a timely manner regeneration density or recruitment of several species among thin- requires that acceptable stocking of advance regeneration is ning intensity treatments in mixed-conifer plantations of northeast achieved prior to final overstory removal (Seymour 1995). Past re- China. Although thinning of oak-dominated stands did not increase search in eastern spruce–fir has shown that conifer regeneration can total regeneration density, it did increase the density of oak regen- be prolific following partial cutting. Results from a long-term silvi- eration relative to unthinned stands (Ward 1992). Inconsistency in culture experiment in Maine, USA, revealed the abundance of coni- understory regeneration responses to thinning is likely related to fer regeneration in partial-cutting treatments far exceeded the differences among published studies in site conditions, stand his- number needed to ensure dominance in the future stand (Brissette tory, species composition and autecology, thinning treatments, and 1996). Pothier and Prevost (2008) observed substantially higher den- time since thinning. Zald et al. (2008) concluded that regeneration sities of red spruce and balsam fir regeneration in stands treated responses to thinning in Sierra Nevada mixed-conifer forests are with a shelterwood establishment cutting compared with unhar- species-specific and best explained by differences in shade tolerance. vested stands 10 years after treatment in spruce–fir forests of Quebec, Results from studies of understory responses to thinning depend on Canada. We are not aware of any published studies that specifically the temporal scope of the investigation, especially time since thin- address the response of tree regeneration to commercial thinning in ning. Olson et al. (2012) observed a long-term reduction in tree regen- eastern spruce–fir forests. eration density in spruce–fir stands precommercially thinned Our overarching objective was to determine whether a first 24 years earlier, which was likely the result of an initial pulse of commercial thinning (CT) acts similar to an establishment cut of a regeneration stimulated by thinning followed by suppression be- shelterwood system. Using long-term CT experiments established neath the expanding crowns of crop trees and mortality. in young, fir-dominated stands and old, spruce-dominated stands Regeneration responses to thinning are also related to thinning across northern Maine, USA, we evaluated natural regeneration intensity, with increasing thinning intensity generally assumed responses to CT in eastern spruce–fir stands during the first de- to increase regeneration density. For example, a positive relation- cade after treatment. Our two main hypotheses were (1) CT in- ship between thinning intensity and regeneration abundance has creases the density of tree regeneration and (2) tree regeneration been observed in coniferous forests of western North America density increases with increasing CT intensity (or decreasing re- (Deal and Farr 1994; Bailey and Tappeiner 1998; Ares et al. 2010). In sidual stand density). a study that included shelterwood treatments in eastern spruce– fir stands, a positive effect of overstory removal intensity on den- Methods sities of natural conifer regeneration was found (Pothier and Prevost 2008). Increased regeneration density with increasing Study sites thinning intensity in mesic forests has generally been attributed Sites used in this study are part of the University of Maine's

For personal use only. to increasing availability of environmental resources in the un- Commercial Thinning Research Network (CTRN). CTRN is a long- derstory, especially to increased light availability (Hale 2003; Otto term experiment investigating the effects of CT treatments on the et al. 2012). Other studies, however, have shown that reduction in development of eastern spruce–fir stands. The network consists of litter depth (Seiwa et al. 2009) and increased seed production from two complementary experiments established to quantify the the residual stand (Peters and Sala 2008; Otto et al. 2012) also can growth and yield responses following different CT treatments increase regeneration establishment after thinning. across Maine's spruce–fir forest beginning in 2000. The first study The spruce–fir forest type of eastern North America is important was installed in immature balsam fir stands that had previously to the economy and ecology of the northeastern US and eastern received precommercial thinning (PCT; hereafter referred to as fir Canada. The dominant species of this forest type are shade- sites), while the second study was installed in mature spruce tolerant, typically reproducing by advance regeneration that can (mainly red spruce, but also white and black spruce) stands with- persist for decades in the understory before being released by a out previous PCT (herein referred to as spruce sites). In both ex- canopy disturbance (Seymour 1992; Seymour 1995). The high periments, two levels of CT intensity were applied as relative shade tolerance of spruce species (red spruce (Picea rubens Sarg.), density reduction (RDR) treatments: 33% RDR and 50% RDR. Un- white spruce (Picea glauca (Moench) Voss), and black spruce (Picea thinned stands were also included as experimental controls and mariana (Mill.) BSP)) and balsam fir (Abies balsamea (L.) Mill.), cou- are herein referred to as unthinned stands. However, all fir sites pled with a cool, moist climate, often leads to the development of had been previously treated with PCT but were considered as Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 dense understories of advance regeneration, even under a regime unthinned for purposes of this CT investigation. Both experi- of light canopy disturbances (Smith 1991; Seymour 2005). A com- ments were established based on a randomized complete block mon forest management objective for this forest type is high pro- design. For more details on the CTRN studies, see McConville et al. duction of wood products. This objective is often accomplished (2003). through silvicultural systems emphasizing full stocking and shorter rotations than those used in other forest types of the Commercial thinning region (Seymour 1995). Standard silvicultural guidelines for east- The experimental design was uniform among study sites and ern spruce–fir recommend managing for uniform, even-aged consisted of 0.40 ha treatment plots, each with a nested 0.08 ha stand structures (Frank and Bjorkbom 1973; Blum et al. 1983). measurement plot. The CT treatments investigated in this study Even-aged spruce–fir stands are established mainly by natural re- were implemented as crown thinnings and performed primarily generation, and uniform shelterwood methods are recommended using cut-to-length (CTL) systems, most often consisting of a small- to achieve desired regeneration stocking when well-distributed, to mid-sized, single-grip processor with a forwarder. Most of these advance regeneration is lacking (Frank and Bjorkbom 1973; machines had rubber tires with chains; however, in one instance, Seymour 1995). Midrotation treatments, such as precommercial a CTL processor with tracks was used. Forwarder trails were ap- and commercial thinning, are generally recommended in eastern proximately 3.7 m wide and were spaced 30.5 m on center. Each

Published by NRC Research Press Olson et al. 175

treatment plot had one central trail bisecting an interior measure- Table 1. Pretreatment (2000−2001) stand attributes of fir and spruce ment plot plus two outer trails. The pretreatment tree spacing was sites investigated as part of the Commercial Thinning Research Net- such that the small CTL processors (i.e., as narrow as 2.5 m) uti- work in Maine. lized two ghost trails between the forwarder trails to fell, delimb, Age Basal area Density and buck trees in place. Processed logs were then stacked at the Site History (years) (m2·ha−1) (trees·ha−1) edge of the forwarder trails, where the forwarders loaded the logs and yarded them varying distances to the roadside. Slash was left Fir Alder Stream PCT 28 25.0 1344 near the stump where processed. This CTL and forwarder system Weeks Brook PCT 30 30.1 2132 was chosen to minimize forest floor disturbance and represent Penobscot PCT 31 23.2 1784 typical CT operations in the region. CT harvests took place in Experimental Forest winter (i.e., frozen ground covered with deep snow) between 2000 Spruce and 2001. At the time of CT, the stand ages of sites used in this Harlow Road No PCT 70 40.2 2073 study ranged from 28 to 31 years for fir sites and from 54 to Sarah's Road No PCT 54 48.2 4406 70 years for spruce sites (Table 1). Schoolbus Road No PCT 65 47.3 3472 Note: PCT, precommercial thinning. Regeneration study We used measurement plots from the CTRN to examine the influ- ence of thinning intensity on regeneration development 10 years group for tree species were assessed for balsam fir, the spruces, after CT. Specifically, we examined regeneration responses to CT at softwoods, and hardwoods. three fir sites and three spruce sites of the CTRN (see Table 2 for 2011 Analysis of variance (ANOVA) was used to test for the effect of stand attributes). CT on the density of tree regeneration. Because the two experi- ments of the CTRN (i.e., fir sites and spruce sites) were initially Sampling design installed as separate experiments and not designed to be inte- This study was conducted during the summer of 2011. At each grated into a single analysis, we assessed regeneration responses site, a 50% RDR, 33% RDR, and unthinned stand were sampled for to CT treatments by analyzing each experiment individually. Re- a total of 18 stands for this investigation. Regeneration sampling generation data were submitted to a mixed-effects model for a was performed using two partially nested grids of square sample randomized complete block design with the treatment unit as the plots (quadrats) embedded within the long-term 0.08 ha measure- experimental unit and site as the blocking factor. For mixed- ment plots. The first grid consisted of forty-two2m×2mquadrats effects models, CT treatment (50%, 33%, and unthinned) was clas- (small plots) spaced 4 m apart, referred to as the small-plot grid. sified as a fixed effect, while replicate (i.e., site) was classified as a The second grid consisted of twenty-one4m×4mquadrats (large random effect. Before ANOVA models were run, contrasts were plots) spaced 8 m apart, referred to as the large-plot grid. Both constructed for testing the a priori hypothesis that regeneration grids of quadrats were organized into six rows and seven columns density would be greater in thinned (33% and 50%) than un- in which the small-plot grid was centered within the large-plot thinned stands. Pairwise treatment comparisons of regeneration grid. This arrangement produced a partially nested sampling de- density based on Tukey's HSD were interpreted when ANOVA sign as only half of the small plots fell within large plots. In total, detected a significant CT effect. All ANOVA models, contrasts, and 378 small plots and 189 large plots were sampled in each stand mean separations were performed in SAS version 9.2 (SAS For personal use only. Institute Inc. 2008). ANOVA diagnostics revealed that raw data type (spruce and fir), while the sampling effort in a stand type was were either non-normal, heterogeneous, or both. Several transfor- 126 small plots and 63 large plots within each level of thinning. mations were applied to meet distributional assumptions of Collectively, sample grids were used to estimate the number of ANOVA. Data were transformed by taking either the square-root stems per hectare of all trees >0.10 m tall. Small-plot grids cap- or natural log of raw data expanded to the hectare. The a priori tured trees 0.11–1.40 m tall. Sampled vegetation within each small alpha level selected for this investigation was 0.05. plot was stratified into two height classes by species. The smaller Additionally, we were interested in describing substand vari- stratum included trees 0.11–0.60 m tall and the larger stratum ability of tree regeneration in relation to the treatments. For just included trees 0.61–1.40 m tall. Large-plot grids were used to cap- the thinned stands (i.e., 33%–fir, 33%–spruce, 50%–fir, and 50%– ture trees >1.40 m tall. Trees sampled within each large plot were spruce), Spearman correlations were used to evaluate associations further stratified into two size classes based on diameter at breast among regeneration density and overstory variables and run in height (DBH; diameter measured at 1.41 m). Trees >1.40 m tall and R version 2.13.0 (R Development Core Team 2011). Strength of ≤8.90 cm DBH were tallied by species, whereas the species and association was interpreted from correlation coefficients as fol- DBH of each tree >8.90 cm were recorded. Additionally, it was lows: weak (|r| < 0.3), moderate (0.3 < |r| < 0.7), and strong (|r| > 0.7). noted whether or not plots fell within harvest trails used for for- Densities of regeneration within and between forwarder trails in warding logs (i.e., forwarder trails). thinned treatments were compared qualitatively. Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 Analytical approach Results Tree regeneration densities (trees per hectare; TPH) were the primary response variables used for testing our hypotheses about Effect of commercial thinning the effects of CT on natural regeneration. For this investigation, Fir sites data for tree species were analyzed according to size strata as In fir-dominated sites, CT was a significant factor accounting for follows: (i) small regeneration = 0.11–0.60 m tall, (ii) medium re- variability in regeneration densities of small fir and small soft- generation = 0.61–1.40 m tall, (iii) large regeneration ≥ 1.41 m tall to woods (p < 0.05; Table 3). Contrasts indicated that thinning treat- 8.90 cm DBH, and (iv) overstory > 8.90 cm DBH. Except for the ments supported higher densities of small fir, softwood, and most prolific species and groups (including balsam fir, red and hardwood regeneration than unthinned sites (Table 3). According white spruce, and total softwood and hardwood species), there to mean separations, small fir and softwood regeneration densi- were numerous zero-value plots for species-level data. Some spe- ties were significantly higher in both 50% and 33% thinning treat- cies were not found in some treatments units and, in some cases, ments than unthinned stands (Table 4), but no differences were entire sites, which justified pooling of species into groupings detected between levels of thinning (i.e., 33% vs. 50%). Although prior to analysis. Responses at the level of species and species there were no significant differences detected between 33% and

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Table 2. 2011 mean (±1 standard deviation) basal area (m2·ha−1) and stem density (trees·ha−1) of main tree species and groups across thinning treatments at fir and spruce sites of the Commercial Thin- ning Research Network in Maine. Fir sites Spruce sites Attribute by species 50% 33% UT 50% 33% UT Basal area Balsam fir 15.0±16.4 27.7±21.9 29.7±19.7 0 0 0.4±1.9 Spruce 4.0±8.7 0.9±3.6 7.4±13.8 15.5±19.1 21.1±19.3 41.0±21.4 Other softwoods 0 0.4±0.4 0.1±0.1 0.9±0.9 1.6±1.6 9.7±9.7 Hardwoods 0 0 6.6±26.8 0 0 1.3±5.7 Density Balsam fir 417±405 813±641 1290±768 0 0 30±134 Spruce 119±154 40±247 268±445 516±589 724±755 1944±1167 Other softwoods 0 10±79 10±79 10±79 40±154 159±296 Hardwoods 0 0 308±777 0 0 79±307 Note: Treatment codes: 50%, 50% relative density reduction; 33%, 33% relative density reduction; UT, unthinned. Other softwoods is a species grouping of softwood species composed of eastern white pine (Pinus strobus), northern white cedar (Thuja occidentalis), and eastern hemlock (Tsuga canadensis). Hardwoods are composed mainly of red maple (Acer rubrum), the aspens (Populus spp.), and the birches (Betula spp.).

Table 3. P values from ANOVA models (Thinning) and contrasts (T vs. Variation within thinning treatments UT) for effects of thinning treatments on tree regeneration density at Small-scale associations fir and spruce sites of the Commercial Thinning Research Network in Associations among regeneration and overstory density vari- Maine. ables in both 33% and 50% thinning treatments were mostly mod- Fir sites Spruce sites erate to weak, as indicated by Spearman correlations (|r| < 0.70; Species Size Thinning T vs. UT Thinning T vs. UT Tables 5 and 6). Small softwood regeneration density was nega- tively, but moderately (0.30 < |r| < 0.70), correlated with large Balsam fir Small 0.026 0.012 0.530 0.691 hardwood regeneration density in fir sites treated with 33% and Medium 0.141 0.161 0.303 0.444 50% thinning (r = –0.324 and –0.373, respectively). The strongest Large 0.182 0.987 0.190 0.101 Spruce Small 0.262 0.218 0.012 0.005 associations between regeneration and overstory variables were Medium 0.248 0.372 0.104 0.046 indicated by negative, moderate correlations in spruce stands Large 0.570 0.358 0.377 0.266 treated with 33% thinning, specifically, between medium soft- Softwood Small 0.027 0.012 0.022 0.009 wood regeneration density and overstory basal area (r = –0.316), Medium 0.176 0.281 0.152 0.067 medium hardwood regeneration density and overstory basal area Large 0.178 0.803 0.230 0.106 (r = –0.309), and small hardwood regeneration density and over- Hardwood Small 0.071 0.035 0.084 0.041 story density (r = –0.396). In thinned fir stands, all correlations

For personal use only. Medium 0.090 0.068 0.308 0.148 between regeneration and overstory variables were weak and Large 0.258 0.121 0.232 0.107 mostly negative (|r| < 0.30). Note: T, thinned stands; UT, unthinned stands. P values < 0.05 are italicized. Forwarder trails: within vs. between Mean densities of small softwood regeneration were compara- 50% thinning treatments, mean densities of small fir, softwoods, ble within and between forwarder trails in both treatments in fir and hardwoods were nominally greater in the 33% than the stands and in spruce stands thinned to 50% relative density (Fig. 1). 50% thinning treatments, whereas mean densities of medium Except for medium softwood regeneration in 33% thinned fir and large regeneration for these species were nominally greater stands, mean densities of medium and large softwood regenera- tion were consistently greater between than within forwarder in the 50% thinning treatment. trails in both fir and spruce sites. Densities of medium hardwood Spruce sites regeneration within and between trails were comparable for all In spruce-dominated stands, CT was a significant factor ac- treatment and stand type combinations, except for spruce sites counting for variability in regeneration densities of small spruce treated with 33% relative density reduction where density of me- dium hardwood regeneration was greater within forwarder trails. and small softwoods (p < 0.05; Table 3). According to contrasts, the

Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 At fir sites, large hardwood regeneration was more abundant be- regeneration densities of these species size classes, as well as that tween trails in 50% treatments and within trails in 33% treat- of small hardwoods and medium spruce, were higher in the thin- ments. The densities of large hardwood regeneration at spruce ning treatments than in unthinned stands (Table 3). Mean separa- sites were more abundant within trails of 50% treatments and tions revealed that small spruce and softwood regeneration comparable within and between forwarder trails of the 33% treat- densities were significantly higher in both 50% and 33% thinning ments. treatments than in unthinned stands, but no differences were detected between levels of thinning (Table 4). Although no differ- Discussion ences were detected between 50% and 33% thinning treatments, Influence of commercial thinning mean densities of small spruce and softwood regeneration were Our hypothesis that CT increases the density of tree regeneration nominally greater in the 33% treatment than in the 50% treat- during the first decade after treatment was confirmed for several, ment, and mean densities of medium and large spruce and soft- but not all regeneration response variables. A positive response (i.e., wood regeneration were nominally greater in the 50% treatment. increased density) to CT was suggested for small regeneration of Mean densities of all hardwood regeneration size classes were nearly all species within both experiments 10 years post-thinning. nominally greater in the 50% treatment. The regeneration density of small softwoods (a group composed of

Published by NRC Research Press Olson et al. 177

Table 4. Mean (± standard error) number of trees per hectare (TPH) of small, medium, and large regeneration across thinning treatments at fir and spruce sites of the Commercial Thinning Research Network in Maine. Fir sites Spruce sites Species Size 50% 33% UT 50% 33% UT Balsam fir Small 37500±4856a 56825±5828a 3274±784b 1210±193 2738±398 2476±429 Medium 437±109 179±58 119±48 833±207 1857±352 976±245 Large 506±95 208±49 357±74 982±232 1815±416 109±50 Spruce Small 754±170 278±120 20±20 68056±9610a 85286±8067a 3643±619b Medium 40±28 0±0 0±0 6607±854 4143±782 59±7 Large 10±10 20±14 40±24 2004±482 675±197 79±26 Softwood Small 38313±4921a 57242±5863a 3413±791b 73988±9870a 92024±8279a 6595±819b Medium 516±117 179±58 179±64 9404±1025 7167±990 976±245 Large 658±104 327±58 536±97 4097±563 3661±481 188±54 Hardwood Small 2698±326 4921±694 694±223 7619±928 3548±535 381±143 Medium 774±163 337±130 0±0 3194±433 2190±495 24±24 Large 1230±306 982±195 516±114 437±110 317±92 0±0 Note: Treatment codes: 50%, 50% relative density reduction; 33%, 33% relative density reduction; UT, unthinned. Regenera- tion classes: small, 0.11−0.60 m tall; medium, 0.61−1.40 m tall; large, 1.41 m tall to 8.9 cm DBH. For a site, mean TPH within rows with different letters were significantly different at ␣ = 0.05.

Table 5. Spearman correlation coefficients for 33% relative density reduction treatments at fir and spruce sites of the Commercial Thinning Research Network in Maine. Spruce sites Fir sites sSW mSW lSW sHW mHW lHW OBA ODn sSW — −0.193 −0.232 −0.070 −0.136 −0.096 0.202 0.122 mSW −0.157 — 0.295 0.438 0.537 0.216 −0.316 −0.306 lSW −0.071 0.102 — −0.015 0.089 0.021 0.056 0.013 sHW 0.291 −0.157 −0.123 — 0.543 0.130 −0.396 −0.386 mHW 0.149 −0.086 0.015 0.511 — 0.427 −0.271 −0.309 lHW −0.324 0.123 −0.087 −0.154 −0.044 — −0.195 −0.162 OBA 0.065 −0.013 −0.008 −0.087 −0.146 −0.246 — 0.750 ODn 0.059 0.133 0.324 −0.016 −0.090 −0.121 0.782 — Note: sSW, small softwood; mSW, medium softwood; lSW, large softwood; sHW, small hardwood; mHW, medium hardwood; lHW, large hardwood; OBA, overstory basal area; ODn, overstory density (trees per hectare).

Table 6. Spearman correlation coefficients for 50% relative density reduction treatments at fir and

For personal use only. spruce sites of the Commercial Thinning Research Network in Maine. Spruce sites Fir sites sSW mSW lSW sHW mHW lHW OBA ODn sSW — −0.187 −0.246 −0.137 −0.107 −0.121 0.219 0.074 mSW −0.021 — 0.694 0.469 0.569 0.070 −0.046 −0.055 lSW −0.088 0.349 — 0.318 0.640 0.094 −0.169 −0.064 sHW 0.279 −0.048 0.015 — 0.457 0.202 −0.126 −0.065 mHW −0.134 −0.106 0.065 0.219 — 0.175 −0.255 −0.110 lHW −0.373 −0.061 −0.057 −0.072 0.042 — −0.171 −0.200 OBA 0.276 −0.208 −0.050 −0.079 −0.040 0.138 — 0.707 ODn 0.150 −0.053 0.104 −0.130 0.012 0.175 0.792 — Note: See Table 5 for explanation of variable abbreviations.

balsam fir, the spruces, and other softwood species) was greater than Influence of thinning intensity Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 10-times more abundant in thinned stands compared to unthinned We hypothesized a positive response in regeneration density stands. Small hardwood regeneration also benefited from CT as evi- with increasing thinning intensity (i.e., unthinned < light thin- denced by nearly a 10-fold difference in density between thinned and ning < heavy thinning). Statistically, this expectation was not unthinned stands. supported by our study. When compared individually with the Although our analysis only detected one difference in the den- unthinned treatment, densities of small regeneration often were sities of medium or large regeneration classes between thinned significantly greater in 33% and 50% thinning treatments. How- and unthinned stands (e.g., higher density of medium spruce in ever, there were no instances in which regeneration densities thinned stands at spruce sites), densities of medium and large were significantly higher in the heavier 50% thinning than the regeneration classes for nearly all species were nominally greater lighter 33% thinning. Mean densities of all regeneration size in thinned stands, suggesting that commercial thinning may have classes of hardwoods, and medium and large softwood regenera- had a positive effect on nearly all regeneration classes considered tion, generally increased with thinning intensity, but differences in this investigation. Lack of statistical significance in some in- between 50% and 33% treatments were not significant. stances may have been related to high variability in regeneration A positive response in regeneration abundance to overstory densities between treatment replicates, which is often typical of removal intensity has not been consistently observed in past re- regeneration studies (Miina and Heinonen 2008). search. For example, a positive response to cutting intensity has

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Fig. 1. Mean trees per hectare (TPH) of softwood and hardwood regeneration (+ standard error) within and between forwarder trails for thinned (33% and 50%) and unthinned (UT; no trails) treatments at fir and spruce sites of the Commercial Thinning Research Network in Maine. For personal use only.

been documented in coniferous forests of western Oregon, USA ments than in the 33% treatments, indicating a higher rate of ad- (Bailey and Tappeiner 1998; Ares et al. 2010), southeast Alaska, vancement under the heaviest level of thinning tested in this study. USA (Deal and Farr 1994), and southeastern Canada (Pothier and Softwood and hardwood recruitment, however, were attributable to Prevost 2008) but not in conifer plantations in northeastern China different sources of regeneration stimulated by thinning treatments, (Lei et al. 2007). The absence of a positive response in regeneration with softwood recruitment arising from the release of advance re- density to thinning intensity in this study was likely related to generation and hardwood recruitment developing mainly from Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 high variability in regeneration density across replicates. It is also sprouts and advance regeneration to a lesser degree. plausible that the 33% and 50% levels of RDR may not have created For all softwood species and groups, lower densities of small enough of a difference in residual stand structures and associated regeneration in 50% thinning than 33% thinning treatments ap- environmental factors influencing regeneration, particularly peared to be linked to higher densities of medium and large re- light, between treatments to detect significant differences in re- generation in heavier 50% thinning than lighter 33% thinning. generation density. Both of these phenomena may also help to Similar trends among regeneration classes were observed by explain some of the inconsistency in detecting a positive response Pothier and Prevost (2008) under different intensities of shelter- to overstory removal among published studies. wood establishment cutting. This pattern between thinning treat- Past studies of recruitment (i.e., ingrowth of regeneration into ments and among sizes classes was likely due to a higher rate of larger size classes) have observed that the rate of recruitment in- recruitment of softwood advance regeneration into larger size creases with decreasing basal area (Li et al. 2011; Fortin and DeBlois classes, which could depress small regeneration abundance in at 2007). Our results suggested that thinning likely had a similar effect least two ways. First, small regeneration density would decrease on the rates of recruitment of both softwood and hardwood regen- through advancement into larger size classes, which, in turn, eration. For example, medium and large softwood and hardwood would increase abundances of larger size classes. Second, the rapid regeneration densities were nominally greater in 50% thinning treat- advancement of softwood regeneration would capture available

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growing space and, therefore, exclude the establishment of new Fig. 2. Total softwood (SW) regeneration densities arrayed along softwood regeneration. Conversely, slower development of soft- gradients of volume harvested during thinning and 10-year wood regeneration in the 33% thinning would have prolonged the cumulative overstory mortality since thinning at fir and spruce opportunity for softwood colonization, thus favoring the accumu- sites of the Commercial Thinning Research Network in Maine. lation of small softwood regeneration. These interpretations are supported by negative, albeit weak (|r| < 0.3), correlations between small softwood regeneration and both medium and large soft- wood regeneration in the 50% thinning treatment.

Influence of stand type Although we did not statistically compare fir and spruce sites, absolute differences in regeneration densities suggest that stand history, canopy structure, and species composition have influ- enced tree regeneration. Spruce regeneration densities were con- sistently greater in older spruce stands, while differences in balsam fir regeneration between stand types were more compli- cated. Across thinning treatments, small fir density was substan- tially greater in younger fir stands, while densities of medium and large fir were greater in older spruce stands. Greater densities of spruces in spruce-dominated stands and small fir in fir-dominated stands were likely an effect of local seed source. However, higher densities of medium and large fir in older spruce stands was likely due to greater densities of tall fir advance regeneration in place at the time of thinning. Softwood regeneration densities were greater in older spruce stands than in younger fir stands. This difference was likely re- lated to greater stand age and post-thinning, wind-related mortal- ity in older spruce stands. Higher mean densities of medium and achieve desired regeneration stocking (Frank and Bjorkbom 1973; large softwood regeneration in older spruce stands was likely the Frisque et al. 1978; Seymour 1995), which may take 10–15 years result of a greater abundance of advance regeneration at the time to develop (Blum et al. 1983). A main finding of this study was that of CT, suggesting that these stands had reached the understory density and stocking of tree regeneration generally increased un- re-initiation stage before harvest (Oliver and Larson 1996). The der both levels of thinning within the first decade, suggesting that older spruce stands of this study occur on poorly drained sites CT served effectively as a shelterwood establishment cut. This and, coupled with shallow-rooting habit of spruce and fir (Frank finding was more of a surprise for the younger fir stands used in and Bjorkbom 1973; Blum et al. 1983), this likely predisposed these this study, which were approximately 30 years old at the time of stands to higher levels of post-thinning windthrow. Additionally, CT, but not surprising for the older spruce stands where CT took differences in mortality between stand types may be partly an place at a stand age (54–70 years) and stage of stand development

For personal use only. effect of PCT in younger fir stands, as PCT can increase wind- when a shelterwood establishment cutting would likely be con- firmness and therefore reduce a stands susceptibility to wind- sidered. throw after subsequent harvests (Achim et al. 2005). Using this There are two commonly cited standards of acceptable stocking experiment, Pekol (2011) found higher mortality rates in older for eastern spruce–fir forests. According to Frank and Bjorkbom spruce stands than in younger fir stands, which was attributed to (1973), a stocking of 50% or more of sample plots with one or two enhanced wind-firmness in the fir stands that had received PCT. spruce or fir, depending on size, is considered acceptable. Frisque Higher softwood regeneration density in spruce stands was likely et al. (1978) recommended a density of 2500 TPH and 60% of plots associated with not only advanced stand age and greater post- with desirable spruce or fir regeneration as minimum acceptable thinning mortality, but also a greater volume harvested during stocking. These standards for acceptable stocking can be applied thinning (Fig. 2). to assess the efficacy of CT treatments used in this experiment in It is worth restating that stands used in this investigation were developing desirable spruce and fir regeneration 10 years after treated with different intensities of crown thinning; therefore, thinning. Both criteria were met for softwoods, composed mainly the results of this study do not apply to similar spruce–fir forests of spruce and fir, in all thinned treatments included in this study treated with either low or dominant thinning. For example, one (Table 7). Interestingly, softwood regeneration exceeded accept- might anticipate lower regeneration densities and rates of recruit- able stocking standards for both density and percent stocking in ment 10 years after low thinning due to smaller canopy openings unthinned spruce stands and for density in unthinned fir stands. Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 created by thinning and less post-thinning wind-related mortal- Hardwood regeneration density was also increased by thinning, ity, while the opposite would be expected in stands treated with which can negatively impact the future development of spruce dominant thinning (i.e., higher residual stand mortality and re- and fir (Westveld 1931; Seymour 1992). generation densities). Furthermore, the influence of thinning Because taller spruce and fir advance regeneration will usually method on regeneration would also likely depend on stand type, outgrow all but sprouting hardwoods, delaying the final removal with less of an effect in similar fir stands owing to greater wind- until spruce and fir regeneration attains a height of 1–2 m can help firmness imparted by PCT (Achim et al. 2005). mitigate future problems with competing hardwoods, which may require a regeneration period of 10 years or more (Seymour 1995). Role of advance regeneration Applying the more conservative standard of Frisque et al. (1978) It has long been recognized that timely regeneration of eastern for acceptable softwood density (i.e., at least 2500 TPH), large soft- spruce–fir stands depends on adequate stocking of competitive wood regeneration exceeded minimum acceptable density in spruce and balsam fir regeneration before overstory removal both 33% and 50% thinned spruce stands but in neither thinning (Westveld 1931). When acceptable stocking of spruce and fir ad- treatment in fir stands. This result suggests that large softwood vance regeneration is lacking in stands managed as an even-aged regeneration may need more time to develop in thinned fir stands system, uniform shelterwood methods are recommended to included in this study, whereas acceptable density was achieved

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Table 7. Total number of softwood trees 10 years after CT. Correlations between regeneration and over- per hectare (TPH) and percent stocking story density were strongest, and mainly negative, in 33% thinned across thinning treatments at fir and spruce stands, suggesting that substand distribution of regenera- spruce sites of the Commercial Thin- tion was more tightly linked to the overstory where residual stand ning Research Network in Maine. density was greater. Moderate, negative correlations between small Treatment softwood regeneration and large hardwood regeneration in thinned fir stands could be linked to higher abundance of shade-tolerant 50% 33% UT hardwood sprout clumps, mainly red maple (Acer rubrum L.), initi- Fir ated by thinning in fir stands. Competition from tolerant hard- TPH 39487 57748 4128 woods is often more intense on higher quality sites (Seymour % Stocking 86 95 40 1995) such as the fir-dominated sites used in this study. Spruce Overstory removal can impact regeneration in a number of TPH 87489 102852 7759 ways other than enhancing regeneration through the release of % Stocking 99 99 60 growing space. Past research in eastern spruce–fir has shown that Note: Treatment codes: 50%, 50% relative larger advance regeneration is predisposed to mechanical damage density reduction; 33%, 33% relative density and that this damage tends to increase with harvest intensity and reduction; UT, unthinned. Stocking was calcu- is often greater in harvest trails (Frisque et al. 1978; McInnis and lated as percentage of plots with at least one Roberts 1991), which can cause an immediate reduction in regen- softwood regeneration stem (0.11 m tall to eration density after harvesting. Other studies have noted that 8.90 cm DBH). spruce and fir seedlings in harvested stands were often associated with disturbed, mineral substrate (Westveld 1931; Wurtz and in thinned spruce stands within a decade of treatment. Interest- Zasada 2001). We observed that densities of medium and large ingly, balsam fir made up a sizeable component of large softwood softwood regeneration were generally greater between than regeneration in spruce stands. Faster growth rate of fir upon re- within forwarder trails; however, these differences were minor. lease compared with spruce (Westveld 1931; Pothier and Prevost For small softwood regeneration, there were no consistent differ- 2008), coupled with high relative density of large fir regeneration, ences in density with respect to forwarder trails. Frisque et al. nearly assures a future component of balsam fir after final re- (1978) observed less harvest-related damages to advance regener- moval of thinned spruce stands. In thinned fir stands, the density ation following a forwarder operation than skidding operations. of large hardwood regeneration was greater than that of soft- Forwarders were used in this experiment, which may help explain woods. Thus, hardwood control may be warranted in similar fir minor differences in softwood regeneration densities within and stands treated with CT if the long-term objective is to maintain between forwarder trails. Interestingly, Pothier and Prevost (2008) spruce–fir dominance. observed greater percent stocking of large (1–4 m tall) red spruce regeneration within than between forwarder trails 10 years after Influence on species composition shelterwood establishment cutting, which was likely attributed to A common objective of silviculture in eastern spruce–fir is to lower damage incurred by forwarding logs over protective snow increase the spruce component, particularly red spruce, relative cover and faster growth driven by higher availability of resources to balsam fir while maintaining acceptable stocking. Due to the in forwarder trails. In this study, it is plausible that damage in

For personal use only. susceptibility of balsam fir to numerous disease-causing agents, harvest trails may have reduced advance regeneration density reducing the proportion of fir can increase stand-level resistance shortly after thinning, but densities of medium and large soft- to insect and pathogen damages (Frank and Bjorkbom 1973). In- wood regeneration in forwarder trails have largely recovered over creasing the red spruce component in spruce–fir forests through 10 years. the release of natural advance regeneration can be a challenge, as there appear to be no understory light conditions that favor red Silvicultural implications spruce over balsam fir (Moores et al. 2007). Owing to high densi- Overall results from this study indicate that in addition to the ties of spruce regeneration, there is a much higher probability expected increase in individual-tree growth and yield, commer- that final removal alone will maintain spruce dominance in cial thinning in eastern spruce–fir can also increase regeneration thinned spruce stands in this study. However, because spruce density over unthinned stands and increase the rate of recruit- regeneration was a minor component of fir-dominated stands in ment as thinning intensity increases. Thus, CT can effectively also this experiment, follow-up treatments may be warranted after serve as a de facto shelterwood establishment cutting in eastern final removal to maintain spruce, e.g., precommercial thinning to spruce–fir. favor spruce and discriminate against fir. This shelterwood effect of CT has implications for the sustain- It is worth noting that the lower densities of spruce regenera- ability of eastern spruce–fir managed under more intensive, even- tion observed in fir-dominated stands is at least partially a legacy aged systems (i.e., those that integrate intermediate treatments). Can. J. For. Res. Downloaded from www.nrcresearchpress.com by University of Maine on 02/20/14 of how early PCT operations were implemented in northern Traditional shelterwood methods may not be necessary in inten- Maine. Crop-tree selection rules during PCT favored leaving dom- sively managed even-aged spruce–fir as CT can be used to simul- inant stems regardless of species (Seymour 1992). Because fir is taneously improve residual tree growth and accumulate softwood more likely to outgrow spruce early in stand development, these advance regeneration to an acceptable stocking level, which can early PCT operations tended to favor fir over spruce and create also reduce rotation length by eliminating the regeneration pe- nearly pure fir stands. Not only did this PCT procedure tend to riod needed to develop acceptable stocking under a shelterwood. reduce spruce abundance, but it also would have reduced spruce's However, the shelterwood method may be appropriate for fir regeneration potential on these sites by depleting future seed stands similar to those not treated with CT in this study, as soft- sources for spruce regeneration. Therefore, it is likely that the low wood regeneration stocking was below minimum standards. The abundance of spruce regeneration in fir-dominated stands is a rapid development of hardwood competition after CT may war- long-term consequence of early PCT practices in Maine. rant additional control treatments if the long-term objective is to maintain spruce–fir dominance. If softwood advance growth gets Variability within thinning treatments too tall by the end of the rotation, a final overstory removal Our analysis of within-stand variability of regeneration revealed may kill or severely damage a significant portion of acceptable weak associations between regeneration and substand factors softwood stocking. Therefore, careful logging to protect larger

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advance regeneration may be necessary in stands treated with Miina, J., and Heinonen, J. 2008. Stochastic simulation of forest regeneration CT to avoid losses of future spruce and fir growing stock. establishment using a multilevel multivariate model. For. Sci. 54: 206–219. Moores, A.R., Seymour, R.S., and Kenefic, L.S. 2007. Height development of shade-tolerant conifer saplings in multiaged Acadian forest stands. Can. J. Acknowledgements For. Res. 37(12): 2715–2723. doi:10.1139/X07-110. This research was jointly funded by the Cooperative Forestry Nyland, R.D. 2002. Silviculture: concepts and applications. 2nd ed. Waveland Research Unit and the U.S. Forest Service, Northeastern States Press, Inc., Long Grove, Illinois. Oliver, C.D., and Larson, B.C. 1996. Forest stand dynamics. Updated edition. John Research Cooperative. We thank the landowners of the CFRU for Wiley and Sons, Inc., New York. access to CTRN study sites and assistance with the harvesting on Olson, M.G., Wagner, R.G., and Brissette, J.C. 2012. Forty years of spruce–fir stand their properties. We also thank Brian Roth with the CFRU for development following herbicide application and precommercial thinning assistance throughout the duration of this study, as well as the in central Maine, USA. Can. J. For. Res. 42(1): 1–11. doi:10.1139/X11-132. Otto, R., Garcia-del-Rey, E., Mendez, J., and Fernandez-Palacios, J.M. 2012. Effects 2011 field crew for data collection. of thinning on seed rain, regeneration, and understory vegetation in a Pinus canariensis plantation (Tenerife, Canary Islands). For. Ecol. Manage. References 280: 71–81. doi:10.1016/j.foreco.2012.05.027. Achim, A., Ruel, J.-C., and Gardiner, B.A. 2005. 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