Elsevier

Forest Ecology and Management

Volume 398, 15 August 2017, Pages 91-100
Forest Ecology and Management

The irregular shelterwood system as an alternative to clearcutting to achieve compositional and structural objectives in temperate mixedwood stands

https://doi.org/10.1016/j.foreco.2017.04.042Get rights and content

Highlights

  • Compared regeneration dynamics under irregular and even-aged silvicultural systems.

  • Conifers poorly regenerated in clearcuts because of heavy interspecific competition.

  • Partial shade by all shelterwood treatments restricted competition expansion.

  • Irregular shelterwood treatments established a new cohort of diverse species.

  • Irregular shelterwood achieved best compositional and structural objectives.

Abstract

Even-aged systems including clearcutting and its variants have been used for decades in North America for managing conifer-dominated stands with the goal to achieve sustainable wood production. Although clearcutting has been successfully applied in conifer-dominated stands of the boreal forest, it has proved maladapted in the temperate mixedwood forest where hardwood and shrub competition is more intense. Over time, regulated even-aged management has simplified the structure and composition of the temperate mixedwood forest which used to be dominated by late-successional stands with high conifer proportions. This study assesses alternative silvicultural systems inspired by natural dynamics for balsam fir (Abies balsamea [L.] Mill.) – yellow birch (Betula alleghaniensis Britton) stands growing in Québec, Eastern Canada. In absence of stand-replacing disturbances, spruce budworm outbreaks (Choristoneura fumiferana [Clem.]) and windthrow trigger regeneration phases through gap dynamics that result in multi-cohort, irregular stand structures. Here, we assess irregular shelterwood systems aiming at regenerating red spruce (Picea rubens Sarg.), balsam fir and yellow birch. In 2009, we established an experiment made of four randomized blocks comparing a gradient of treatments of increased cutting intensity: (i) uncut control (0% removal, 30 m2/ha); (ii) continuous cover irregular shelterwood (19 m2/ha of residual basal area [RBA] at the first cut, 25–30 years intervals without final cut), (iii) extended irregular shelterwood (RBA 17 m2/ha, final cut at year 30), (iv) uniform regular shelterwood (RBA 15 m2/ha, final cut at year 10) and (v) clearcut (harvest of merchantable trees >9 cm DBH). We monitored light transmission, seedbed quality and lower vegetation (<3 m) during the first 5 years postcut. Shelterwood treatments diversified both understory light and seedbed conditions, thereby increasing the array of available regeneration niches. Shade-tolerant red spruce and balsam fir established more successfully in the less intense irregular shelterwood treatment (continuous cover) and with a higher spruce: fir proportion. All cutting treatments increased seedling density >30 cm of mid-tolerant yellow birch. Interspecific competition dominated by pin cherry (Prunus pensylvanica L.f.) and red maple (Acer rubrum L.) was the main factor limiting conifer establishment and was more abundant in the clearcut. Maintaining a partial cover in shelterwood treatments successfully limited competition expansion while regenerating target species. Even if relatively short term, results show that irregular shelterwood systems could be a sound alternative to clearcutting for regenerating and maintaining the structure of late-successional balsam fir-yellow birch stands.

Introduction

Even-aged systems including clearcutting and its variants have been used for decades in North America for managing conifer-dominated stands with the goal to achieve sustainable wood production (Seymour and Hunter, 1992, Groot, 2002). Although clearcutting has been successfully applied in conifer-dominated stands of the boreal forest, it has proved maladapted in the temperate mixedwood forest where hardwood and shrub competition is more intense (Archambault et al., 1998, Laflèche et al., 2000). Over time, regulated even-aged management has simplified the structure and composition of the temperate mixedwood forest, which used to be dominated by late-successional stands with high conifer proportions (Lorimer, 1977, Barrette and Bélanger, 2007, Boucher et al., 2009). Specifically, the heavy cutting intensity associated with clearcutting systems trigger the rapid development of competing species such as red maple (Acer rubrum L.), pin cherry (Prunus pensylvanica L.f.) and mountain maple (Acer spicatum Lam.) (Archambault et al., 1998, Hébert, 2003). Moreover, current clearcutting practices protecting soil and advance regeneration generate little favorable seedbeds for the germination of small-seeded species such as yellow birch (Betula alleghaniensis Britton) and red spruce (Picea rubens Sarg.) (Laflèche et al., 2000). Additionally, regeneration deficiencies of the shade-tolerant conifers red spruce and balsam fir (Abies balsamea [L.] Mill.) is compounded by harvesting practices that remove seeds trees over large scales (Gordon, 1996, Fortin et al., 2003a). Regeneration failures are particularly frequent for red spruce that is generally outcompeted by balsam fir (Moores et al., 2007, Olson and Wagner, 2011, Dumais and Prévost, 2014). Therefore, developing silvicultural systems better adapted to natural dynamics could contribute to maintain species and structural diversity found in late-successional stands (Gendreau-Berthiaume et al., 2012, Kuuluvainen and Grenfell, 2012).

North American natural mixedwood forests are characterized by small-scale and moderate disturbance dynamics occurring between long fire-return intervals (Lorimer, 1977, Fraver and White, 2005, Kneeshaw and Prévost, 2007). Insect epidemics, wind and natural senescence cause partial stand mortality and opportunities for regeneration establishment and release (Bouchard et al., 2008, Amos-Binks and MacLean, 2016). In the specific case of balsam fir-yellow birch stands, cyclic spruce budworm (Choristoneura fumiferana [Clem.]) epidemics drive natural dynamics and generate multi-cohort, irregular stand structures (Bouchard et al., 2006, Bouchard et al., 2007). Silvicultural systems using repetitive and irregular partial cuttings like the irregular shelterwood system could help to emulate this process and thereby to establish and release regenerating cohorts (Seymour et al., 2002, Raymond et al., 2009). Also called “Femelschlag”, the irregular shelterwood system was first developed by Gayer in Central Europe during the 19th century to promote mixed-species natural regeneration (Silvy-Leligois, 1953, Schütz, 1999). This system has been assessed to regenerate species mixtures in Europe (Grassi et al., 2004, Klopcic and Boncina, 2012) but it’s only recently that irregular shelterwood systems have been developed in North America (Arseneault et al., 2011, Bédard et al., 2014, Suffice et al., 2015). Knowledge of their effects and applicability remains therefore limited, especially in conifer-dominated stands.

Here we present the first 5-yr results of a long-term experiment comparing 5 silvicultural systems representing a gradient of cutting intensity for balsam fir-yellow birch irregular stands. Clearcut of all merchantable trees (>9 cm DBH, diameter at breast height 1.3 m aboveground) was the most intense scenario. It was followed by uniform regular shelterwood, which aimed at establishing a mixture of shade-tolerant and mid-tolerant species under a partial cover during a short period of time (i.e. 1/5 or less of rotation length) to generate stands with a single age class (Hannah, 1988, Matthews, 1989). After these two more intense even-aged systems there were two variants of the irregular shelterwood cutting system (extended and continuous cover; Raymond et al., 2009). Their goal was to establish regeneration while maintaining a sheltering cover to provide partial shade, seed sources and structural attributes over an extended period of time (>1/5 of rotation length). They both resulted in stands with two or more age classes (Raymond et al., 2009). The continuous cover variant is less intense than the extended variant because of the lighter harvest and the absence of final cut. At the end of the gradient, the uncut control represented natural stand development. Because the maintenance of irregular stand structures relies on the assumption that each establishment cut enhances microenvironmental conditions to install a new cohort of seedlings and stimulate the development of older cohorts, our study focused on establishment conditions and regeneration dynamics. We predicted that by enhancing light and seedbed conditions, irregular shelterwood treatments will (1) enable successful regeneration of the three target species (red spruce, balsam fir and yellow birch), while (2) maintaining high red spruce proportions relative to balsam fir and (3) limiting the interspecific competition by deciduous shrub and tree species.

Section snippets

Site description

We set this experiment in the Portneuf County, Eastern Canada, approximately 80 km northwest of Quebec City. The research areas are located at the southern margins of the balsam fir – yellow birch bioclimatic domain, the transition zone between the boreal and deciduous forests (Saucier et al., 2009). The disturbance regime is characterized by moderate and small-scale disturbances by spruce budworm, hemlock looper (Lambdina fiscellaria [Guen.]), birch dieback, wind and natural senescence causing

Diameter distributions

Diameter distributions varied greatly among treatments, from the most complex observed in control plots to the simplest in the clearcut (Fig. A.1). Overall stem distribution was more similar to the control in the two irregular shelterwood treatments (continuous cover and extended). In these three treatments, softwoods followed a reverse-J distribution, while hardwoods a bell shape distribution. The uniform regular shelterwood and clearcut treatments greatly simplified the diameter distributions

Stand structure

One of the anticipated benefits of the irregular shelterwood system is to maintain complex and irregular stand structures (Schütz, 2002, Raymond et al., 2009). As predicted, our results show that immediately after the first establishment cut, the two irregular shelterwood treatments could maintain a stand structure close to that of control plots. Even though this will require to be monitored over the long term, these treatments should have a sufficient number of saplings (1–10 cm DBH) to sustain

Management implications

This study aimed at assessing the effects of irregular shelterwood systems as alternative to clearcutting in balsam fir-yellow birch stands. As expected, irregular shelterwood systems used to emulate partial disturbance dynamics by spruce budworm successfully established a new cohort of diverse species including red spruce, balsam fir and yellow birch, while maintaining natural stand structures. Irregular shelterwood, and in particular the continuous cover variant, appears promising as

Acknowledgements

We wish to thank Stéphane Tremblay, Vincent Roy and Catherine Larouche for their contribution on the development of the Irregular Shelterwood System research program. Special thanks to Éric Michaud and Éric Cantin (Groupement forestier de Portneuf), Scierie Éloi Moisan and the MFFPQ Portneuf-Laurentides Management Unit for their willingness and support to develop irregular shelterwood systems in a field experiment. We are grateful to all the technicians and students that contributed to

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