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Personne :
Achim, Alexis

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Achim

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Alexis

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Université Laval. Département des sciences du bois et de la forêt

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ncf11852316

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  • PublicationRestreint
    Wood quality of black spruce and balsam fir trees defoliated by spruce budworm: A case study in the boreal forest of Quebec, Canada
    (Elsevier, 2019-02-01) Paixao, Carlos; Achim, Alexis; Krause, Cornelia; Morin, Hubert
    Spruce budworm (SBW – Choristoneura fumiferana Clem.) is one of the most damaging defoliating insects in the coniferous forests of eastern North America. In Canada, the widely distributed balsam fir (Abies balsamea L. (Mill)) and black spruce (Picea mariana B.S.P. (Mill)) are its most important hosts. Defoliation by SBW reduces growth in the host trees and can lead to host mortality. Although SBW impacts on growth are well documented, much less is known about changes in wood properties resulting from defoliation. To address this knowledge gap, we sampled 36 SBW-infested stands located in the boreal forest of Quebec (Canada) to determine whether defoliation modifies the wood quality of affected trees. The selected stands had been subjected to one to four years of SBW defoliation. For both species, we assessed ring growth, wood density, and the anatomical characteristics of stem wood formed during the outbreak years. We determined that rings formed during the SBW outbreak had a significant and progressive loss of biomass production with a longer duration of defoliation. SBW significantly reduced latewood density in the second and third year of defoliation for black spruce and the third and fourth year for balsam fir. Average ring density was reduced only in black spruce and only after four years of defoliation. The observed changes in growth and density were associated with changes in anatomical features. While the cellular characteristics of the earlywood remained fairly constant, significant reductions occurred in latewood cell-wall thickness after three years of defoliation. Our study shows that not only do SBW outbreaks reduce annual radial growth, but the cellular characteristics in latewood cells are also modified momentarily. Thus, SBW outbreaks affect wood density and quality in both black spruce and balsam fir.
  • PublicationRestreint
    Xylogenesis in stems and roots after thinning in the boreal forest of Quebec, Canada.
    (Oxford University Press, 2017-06-28) Lemay, Audrey; Achim, Alexis; Krause, Cornelia; Rossi, Sergio
    The reduction of competition through thinning increases radial growth in the stem and roots of many conifer species. However, not much is known about the effect of thinning on the dynamics of wood formation and intra-annual development of the growth ring, especially in the roots, which are an essential part of the tree for stability and resource acquisition. The aim of this study was to evaluate the effect of an experimental thinning on the dynamics and phenology of xylogenesis in the stem and roots of black spruce and balsam fir. Experimental and control trees were selected in two mature even-aged stands, one black spruce (Picea mariana (Mill.) BSP) and one balsam fir (Abies balsamea (L.) Mill.). Wood microcores were collected weekly in the stem and roots from May to October for a period of 4 years. The onset and ending of each cell differentiation phase were computed, as well as growth rate and total cell production. Results show that thinning increased the cell production rate of stem and roots of black spruce and balsam fir. This higher daily growth rate caused an increase in the total number of cells produced by the cambium. The intensity of the treatment was sufficient to significantly increase light availability for residual trees, but insufficient to modify soil temperature and water content to a point at which a significant change in the timing or duration of xylogenesis would be induced. Thus, thinning increased cell production rate and total number of cells produced in both stem and roots, but did not result in a change in the phenology of wood formation that could lead to increased risks of frost damage in the spring or autumn.
  • PublicationRestreint
    Relating mechanical strength at the stem level to values obtained from defect-free wood samples
    (2010-08-22) Ruel, Jean-Claude; Cloutier, Alain; Espinoza-Herrera, Raul; Achim, Alexis
    Stem or branch failure is a recurrent problem in silviculture and arboriculture. The risk of rupture varies with species in relation to the inherent mechanical properties of the species and the presence of defects. In general, calculations of critical loads for breakage are based on mechanical properties determined from defect-free samples and adjustment factors that try to scale up to full trees that include defects. This study aims at developing an objective method to scale up mechanical resistance to breakage from defect-free samples to full trees, including different types of defects. It combines two approaches. In the first one, a correction factor is determined from a meta-analysis of various tree-pulling studies involving balsam fir [Abies balsamea (L.) Mill.], white spruce [Picea glauca (Moench) Voss], jack pine [Pinus banksiana Lamb.], and black spruce [Picea mariana (Mill.) B.S.P.]. The second approach consists in obtaining empirical data from three-point bending tests using 8-foot (2.44 m) logs with various amounts of decay. Results show that the correction required varies according to the species and the presence of some defects. For balsam fir, which was the species showing the most important difference between whole log and small sample values, differences in correction factors were found between tree-pulling tests and three-point bending tests. Data from winching tests tend to underestimate the stem’s resistance to breakage since they likely represent the weakest trees among those tested. No relationship was found between the adjustment factors and different indices used in arboriculture to account for decay, showing the complexity of mechanical resistance at the stem level.