The irregular shelterwood system as an alternative to clearcutting to achieve compositional and structural objectives in temperate mixedwood 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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2022, Forest Ecology and ManagementCitation Excerpt :Primarily in eastern Canada, but also in New England (Peterson and Maguire, 2004) and to some degree the Lake States (Helman et al., 2021), irregular shelterwoods have been proposed as a hybrid of traditional even and uneven-aged management for mixedwood and hardwood forests (Raymond et al., 2009). Irregular shelterwood systems are an approach to capture the ecological complexity, particularly species abundance and structure, of forests following mesoscale disturbances (Raymond and Bédard, 2017). The continuous cover and expanding gap versions of this system have increased spatial and temporal flexibility relative to traditional shelterwood, single-tree, and group selection methods to meet these objectives (Raymond et al., 2009).
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2021, Forest Ecology and ManagementCitation Excerpt :In the United States, mixed oak-hardwood forest types make up 51% of all forests (Smith et al. 2009; Dey 2014). For these forest types, shelterwood regeneration harvests are common and known to be successful in regenerating oak, hickory and other heavy-seeded shade-tolerant to shade mid-tolerant tree species, while uneven-aged management techniques such as those often practiced in northern hardwoods may fail to regenerate the less shade-tolerant species in these systems (Loftis 1990; Brose 2011; Raymond and Bédard 2017; Ashton and Kelty 2018). Soil studies on shelterwoods in these forest types are, however, few and far between and have yielded mixed results.
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