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Pour une valorisation optimisée de la biomasse forestière basée sur une connaissance de la variabilité de la masse volumique dans l’arbre

Abstract : Basic density is an important physical property of wood as it helps to explain the behaviour of the tree in situ and the wood in use. Currently, the databases containing mean wood basic density are mainly composed of mean stem wood calculated on core samples taken at 1.30 meters. Thus, variations in basic density within the stem, between components and within components are completely ignored. However, these variations can have an important impact on the calculation of wood aboveground biomass. Moreover, as bark and knots are components of the tree rich in extractives, it is important to know the available fraction of these components in order to test the feasibility of an extractives chemistry pathway. This thesis contributes to the ExtraFor_Est project, which aims to characterise and quantify the extractible resource of forests in the Grand Est and Bourgogne-Franche-Comté regions.In this thesis, three softwood and three hardwood species were studied: silver fir, Abies alba Mill., Norway spruce, Picea abies (L.) H. Karst., Douglas fir, Pseudotsuga menziesii (Mirb.) Franco, pedunculate oak, Quercus robur L., sessile oak, Quercus petraea L. and European beech, Fagus sylvatica L. For softwood species, trees were sampled in plots testing contrasting thinning intensities. In the end, only softwood species could be studied during the time of the thesis, but the other data were prepared for the continuation of the project. Four components of the tree were analysed: stem wood, stem bark, knots and branches. An X-ray tomograph was used to obtain the basic density of the components.In a first step, a comparison between the mean basic density of the stem wood measured at 1.30 m and the mean basic density of the studied components was carried out and the impact of the observed difference on the biomass calculation. It was found that the mean stem wood basic density measured at 1.30 m is significantly higher than the mean stem wood basic density for Abies alba and Pseudotsuga menziesii. Knots, branches and bark are mostly denser than stem wood measured at 1.30 m. These differences in basic density between components lead to an underestimation or overestimation of biomass depending on the component and the species studied, the difference being up to more than 40% for knots, for example.In a second step, the variation of basic density with height in the tree was analysed for stem wood. The results show that the direction of variation depends on the species. Two forms of descriptive models were developed for Abies alba and Pseudotsuga menziesii, one taking as input the mean basic density measured at 1.30 m and the other not taking it into account. The relative RMSEs for Abies alba and Pseudotsuga menziesii are 9.9% and 8.1%, respectively, for the model without the mean basic density measured at 1.30 m and 7.6% and 5.9%, respectively, for the model with the mean basic density measured at 1.30 m.Finally, the study of longitudinal and height-dependent variations in the basic density of stem bark, knots and branches was started. For bark, the basic density decreased with height for Abies alba, decreased and then increased for Picea abies and increased and then decreased slightly for Pseudotsuga menziesii. For branches, it was found that basic density decreases rapidly in the first 50 cm from their insertion on the stem and then remains stable until their apex.
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Submitted on : Thursday, July 28, 2022 - 5:34:12 PM
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Antoine Billard. Pour une valorisation optimisée de la biomasse forestière basée sur une connaissance de la variabilité de la masse volumique dans l’arbre. Ecologie, Environnement. AgroParisTech, 2021. Français. ⟨NNT : 2021AGPT0019⟩. ⟨tel-03740044⟩

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