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Old-growth forests play a major role in conserving biodiversity, protecting water resources, sequestrating carbon, and these forests are indispensable resources for indigenous societies. To preserve the ecosystem services provided by these boreal ecosystems, it becomes necessary to develop novel silvicultural practices capable of emulating the natural dynamics and structural attributes of old-growth forests. The success of these forest management strategies depends on developing an accurate understanding of natural regeneration dynamics. Our goal was therefore to identify the main patterns and the drivers involved in the regeneration dynamics of old-growth forests, placing our focus on boreal stands dominated by black spruce (Picea mariana(L.) Mill.) and balsam fir (Balsam fir(L.) Mill.) in eastern Canada. We sampled 71 stands in a 2200 km2study area located within Quebec’s boreal region. For each stand, we noted tree regeneration (seedlings and saplings), structural attributes (diameter distribution, deadwood volume, etc.), and abiotic (topography and soil) factors. We observed that secondary disturbance regimes and topographic constraints were the main drivers of balsam fir and black spruce regeneration. Furthermore, the regeneration dynamics of black spruce appeared more complex than those of balsam fir. We observed distinct phases of seedling production first developing within the understory, then seedling growth when gaps opened in the canopy, followed by progressive canopy closure. Seedling density, rather than the sapling density, had a major role in explaining the ability of black spruce to fill the canopy following a secondary disturbance. The density of balsam fir seedlings and saplings was also linked to the abundance of balsam fir trees at the stand level. This research helps explain the complexity of old-growth forest dynamics where many ecological factors interact at multiple temporal and spatial scales. This study also improves our understanding of ecological processes within native old-growth forests and identifies the key factors to consider when ensuring the sustainable management of old-growth boreal stands.

Forest ecosystem management heads towards the use of partial cuttings. However, the wide variation in growth response of residual trees remains unexplained, preventing a suitable prediction of forest productivity. The aim of the study was to assess individual growth and identify the driving factors involved in the responses of residual trees. Six study blocks in even-aged black spruce [Picea mariana (Mill.) B.S.P.] stands of the eastern Canadian boreal forest were submitted to experimental shelterwood and seed-tree treatments. Individual-tree models were applied to 1039 trees to analyze their patterns of radial growth during the 10 years after partial cutting by using the nonlinear Schnute function on tree-ring series. The trees exhibited different growth patterns. A sigmoid growth was detected in 32% of trees, mainly in control plots of older stands. Forty-seven percent of trees located in the interior of residual strips showed an S-shape, which was influenced by stand mortality, harvested intensity and dominant height. Individuals showing an exponential pattern produced the greatest radial growth after cutting and were edge trees of younger stands with higher dominant height. A steady growth decline was observed in 4% of trees, represented by the individuals suppressed and insensitive to the treatment. The analyses demonstrated that individual nonlinear models are able to assess the variability in growth within the stand and the factors involved in the occurrence of the different growth patterns, thus improving understanding of the tree responses to partial cutting. This new approach can sustain forest management strategies by defining the best conditions to optimize the growth yield of residual trees.