Primary and Secondary Branch Growth in Black Spruce and Balsam Fir after Careful Logging around Small Merchantable Stems (CLASS)

Abstract

Careful logging around small merchantable stems (CLASS) is a partial cutting treatment that consists of the harvest of 70%–90% of the merchantable volume of an irregular coniferous stand. In this treatment, regeneration, saplings and small merchantable stems (DBH < 15 cm) are preserved and can continue to grow and develop into the dominant layer of the new stand. The aim of this project was to examine the effects of CLASS on the primary and secondary growth of branches, as well as on branch diameter in black spruce and balsam fir trees in the boreal forest of Quebec, Canada. Primary and secondary growth were measured on five branches per tree while branch diameter was analysed from 15 whorls distributed within the crown of the 48 black spruce and 48 balsam fir trees sampled. Branch primary and secondary growth significantly increased after CLASS in the lower part of the crown in both species, and both types of growth increased proportionally. These findings suggest that CLASS may delay crown recession as the lower branches tend to survive and grow for a longer period. However, although radial growth increased in the years post-CLASS, this did not significantly influence the final branch diameter and should not lead to lumber downgrade.

Maximum Branch Diameter in Black Spruce Following Partial Cutting and Clearcutting

“…Moreover, black spruce has a very slow growth rate, especially for older individuals [2]. Thus, even if branches of our sampled trees increased their growth after treatment, as was observed in other studies [15,44], this increase was so small that the effect was minimal at the time of our sampling. The most important effect will likely be branch longevity, as lower branches may remain alive longer [45].…”

“…We divided crown length into five sections, each having 20% of the total crown length, and we collected at least three whorls from each of these five sections, as in Lemay, Krause, and Achim [15]. The relative height of a branch in the live crown was determined as follows:…”

“…The latter reduce stand density and increase the growth of residual trees, thereby affecting branch properties. For instance, thinning, which reduces competition and increases light availability for residual trees, can increase branch size in many conifer species [12,13], especially in the lower part of the crown that was previously shaded by competing neighbors [14,15]. However, the changes in environmental conditions are correlated with harvesting intensity [16]; therefore, residual trees in partial cutting or clearcutting stands may not be influenced in the same manner.…”

Influence of Stem Taper on the Yield of Black Spruce Stands and Trees Following Commercial Thinning

“…), Goudiaby et al [43] also detected a positive correlation between stem volume growth, growth efficiency (stem to foliage biomass ratio), and foliage biomass at mid-crown. However, for shade-tolerant species such as black spruce, opening the canopy may not necessarily affect branch development, taper, or stem shape in the same way [49]. Thus, Goudiaby et al [43] again observed no change in growth efficiency, foliage biomass, or biomass density after thinning.…”

“…Ever since theories related to stem form were first proposed [45], foresters have concurred that silvicultural practices favoring growth increment also produce changes in stem shape. Various authors have suggested that the patterns of wood deposition along the stem are affected by variations in crown size, which in turn are affected by tree characteristics and stand conditions (particularly stand density) [46][47][48][49]. This hypothesis has been verified for some species, including Douglas fir and Sitka spruce (Picea sitchensis), for which Mitchell [17] found that basal allocation temporarily increased after thinning, suggesting a period of acclimatized growth during which trees re-equilibrate with post-thinning wind loads.…”