Abstract
Forest regeneration is a key element in achieving sustainable forest management. Partial harvest methods have been used extensively in temperate broadleaf and mixedwood ecosystems to promote regeneration on poorly stocked sites and to maintain forest composition and productivity. However, their effectiveness in promoting conifer establishment has yet to be demonstrated in unmanaged boreal forests, especially those dominated by black spruce (Picea mariana (Mill.) BSP) where constraints for regeneration differ from those found in more meridional regions. We aimed to evaluate conifer seedling density and dimensions, 10 years after the onset of a gradient of silvicultural treatments varying in harvesting intensities, and to identify the critical factors driving the regeneration process. Study blocks of even-aged black spruce stands in the eastern Canadian boreal forest were submitted to three variants of shelterwood harvesting: a seed-tree harvest, a clear-cut and an untreated control. Shelterwood and seed-tree harvesting were combined with spot scarification to promote regeneration. Shelterwood and seed-tree harvesting produced a density of conifer regeneration sufficient to maintain forest productivity, but they did not promote seedling growth. Black spruce was the predominant species in terms of regeneration density, with proportions 3–5× higher than that for balsam fir (Abies balsamea (L.) Mill.). Ten years after treatment, seed-origin black spruce seedlings were abundant in skidding trails, while layers dominated the residual strips. Balsam fir density was not influenced by treatment nor by tree position relative to skidding trails. Balsam fir and black spruce had different responses to treatment in terms of height and diameter, the former exhibiting a better growth performance and larger diameter in the residual strips. Spot scarification created micro-sites that had a significant impact on the regeneration process. Overall, our results support that shelterwood and seed-tree harvesting combined with scarification enable adequate regeneration in black spruce stands, confirming these treatments as viable silvicultural alternatives to clear-cutting when required by sustainable forest management objectives.
Highlights
Seedling establishment is of crucial importance to the persistence, productivity and resilience of forest ecosystems; adequate regeneration is critical in the sustainable management of the boreal biome (Prévost, 1996; Gauthier et al, 2009)
Black spruce regeneration density 10 years a.c. was significantly affected by treatments in interaction with stand types (Figure 4)
CC showed the lowest density of seedlings 10 years a.c., while in older stands, the values were similar to the other studied treatments, with the exception of ministrip harvesting (MS) (Figure 4)
Summary
Seedling establishment is of crucial importance to the persistence, productivity and resilience of forest ecosystems; adequate regeneration is critical in the sustainable management of the boreal biome (Prévost, 1996; Gauthier et al, 2009). The shelterwood and seed-tree systems are silvicultural alternatives to clear-cutting that can potentially address these concerns (Kern et al, 2017). The shelterwood system favors the establishment and growth of regeneration through a uniform opening of the canopy, while limiting the growth of competing vegetation (Nyland, 2016). This system could be appropriate to ensure the regeneration of boreal conifers, maximize wood production and maintain biodiversity due to the high retention levels of forest stands (Vanha-Majamaa et al, 2007; Gauthier et al, 2009; Montoro Girona et al, 2017). The remaining trees are chosen to provide sufficient seed sources after harvesting; the remaining cover is low, enabling light to reach the soil surface
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