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The effect of the self-weight increment, caused by annual growth, can be one of the principal processes affecting the development
of growth stresses linked to the formation of reaction wood. This paper presents a preliminary study focusing on the relationship
between the whole weight of the crown and the trunk displacement it creates. A mechanical...
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Citations
... From a more mechanistic and functional point of view, our results concerning the morphological variables that statistically explain growth stress variation are in agreement with the mechanosensing theory (i.e., the formation of reaction wood through the perception of bending strains due to mechanical stimulation), already established for very young trees (Coutand 2010;Coutand and Moulia 2000). Previous studies have developed models and experiments of bending forces and strains induced by the tree selfweight (Barbacci et al. 2009;Fournier et al. 1990Fournier et al. , 2013. Such models combine the trunk inclination, the crown shape, the stem slenderness and stiffness to define the mechanical loading (i.e., the bending strains) exerted by gravity in peripheral wood. ...
Key message
According to biomechanical processes, tree morphology (trunk inclination, height-to-diameter ratio and crown area) explains statistically silvicultural effects on growth stress variation.
Abstract
Growth stresses constitute the main mechanism allowing the tree to control its posture against its mechanical environment, but are also among the most important factors contributing to the depreciation of timber value. This study aimed at assessing the link between stand planting density and growth stress level in European beech (Fagus sylvatica) stands. Beech seedlings were planted in four plots corresponding to four planting densities: 2,500, 5,000, 10,000 and 40,000 stems/ha. They were left to grow for 26 years without any intervention, resulting in trees with highly different morphologies but of the same age and provenance. After 26 years of growth, both the tree morphology and growth stress indicators were measured on the standing trees in each plot and an attempt was made to establish a link between them. Our results showed that initial stand density influences growth stresses of the first order as a result of its impact on tree morphology. The best predictors of high growth stresses were high trunk inclination, high height-to-diameter ratio (slenderness factor) and low crown area. According to mechanosensing theories, these morphological criteria emphasised that growth stresses are due to a global mechanical stimulation rather than to local stem inclination alone. Research now has to be undertaken on new methods that combine the integrative assessment of tree morphology as well as its monitoring over time.
... Mechanical engineering principles have been applied under static loading conditions in order to quantify tree stability. The application of beam theory has been used at length to describe tree displacement due to gravity and wind where the main stem is modeled as a tapered pole (Wilson and Archer, 1979;Niklas, 1992;Barbacci et al., 2009;Ennos and van Casteren, 2010;Peltola, 2006). Using this model, under an applied load, a tree responds in four ways: bending sways, torsional sways, longitudinal sways, and coupled sways (Wilson and Archer, 1979;Mayer, 1987). ...
Wind is a dominant abiotic factor that a tree experiences throughout its lifetime and can cause severe tree damage, resulting in risk of injury to humans, and economic and ecological losses. It is thought that trees develop structural properties and architectures that help withstand loading conditions by dissipating wind energy through damping mechanisms. The role of branch motion in reducing potential dangerous wind loads has been the focus of relatively few studies. Even fewer studies have examined tree sway response to natural wind loaded conditions. In this investigation, branch frequencies were calculated for three Fraxinus americana using a three-dimensional motion capture system for both wind loading and hand loading conditions. Individual branch frequencies and phase angle values were calculated after portions of the tree crown mass were removed. Wind loaded branch sway frequencies ranged between 0.2 and 1.1 Hz while the pull and release test induced mean frequencies of 0.2 Hz. There was no significant difference between phase angle shifts or frequencies after the removal of tree crown mass. The hypotheses tested require further investigation as the interference with neighboring tree crowns prevented desired tree sway dynamics to occur.
... Other developments could concern the way trees and forests develop a mechanically efficient and acclimatised woody structure in their natural environment, which is a major constraint (and challenge) for forest ecosystems (Chave et al. 2009). Tree biomechanical response to wind and unbalanced biomass allocation during growth, which have major consequences on wood functional properties, have been studied mainly from the theoretical point of view and applied to only a small number of trees (Barbacci et al. 2009; Brüchert and Gardiner 2006; Moulia and Fournier 2009). However, this requires wide-scale measurements of tree shape and biomass, which could benefit from T-LiDAR scanning. ...
• Introduction
The use of terrestrial LiDAR (light detection and ranging) scanners in forest environments is being studied extensively at present due to the high potential of this technology to acquire three-dimensional data on standing trees rapidly and accurately. This article aims to establish the state-of-the-art in this emerging area.
• Objectives
Terrestrial LiDAR has been applied to forest inventory measurements (plot cartography, species recognition, diameter at breast height, tree height, stem density, basal area and plot-level wood volume estimates) and canopy characterisation (virtual projections, gap fraction and three-dimensional foliage distribution). These techniques have been extended to stand value and wood quality assessment. Terrestrial LiDAR also provides new support for ecological applications such as the assessment of the physical properties of leaves, transpiration processes and microhabitat diversity.
• Results
Since 2003, both the capabilities of the devices and data processing technology have improved significantly, with encouraging results. Nevertheless, measurement patterns and device specifications must be selected carefully according to the objectives of the study. Moreover, automated and reliable programmes are still required to process data to make these methodologies applicable specifically to the forest sciences and to fill the gap between time-consuming manual methods and wide-scale remote sensing such as airborne LiDAR scanning.
... It has been argued that the sheer diversity of mechanical challenges and the fact that they all change in quality and intensity during growth actually prevent a genetically fixed mechanical design from being competitive (except species colonizing a very specific niche; Fournier et al. 2006; Moulia et al. 2006 ). Therefore , a phenotypical developmental plasticity has been selected that includes (1) acclimation of the plant's load-bearing structure through thigmomorphogenesis, and (2) a process of active recovery and posture control through bending motors (Moulia et al. 2006; Barbacci et al. 2009). As the integrative mechanobiology of tropisms and postural controls has recently been reviewed in detail (e.g., Moulia et al. 2006; Moulia and Fournier 2009; Almeras and Fournier 2009), this chapter focuses on the integrative mechanobiology of thigmomorphogenetic acclimation. ...
Mechanosensitive control of plant growth is a major process shaping how terrestrial plants acclimate to the mechanical challenges set by wind, self-weight, and autostresses. Loads acting on the plant are distributed down to the tissues, following continuum mechanics. Mechanosensing, though, occurs within the cell, building up into integrated signals; yet the reviews on mechanosensing tend to address macroscopic and molecular responses, ignoring the biomechanical aspects of load distribution to tissues and reducing biological signal integration to a “mean plant cell.” In this chapter, load distribution and biological signal integration are analyzed directly. The Sum of Strain Sensing model S
3m is then discussed as a synthesis of the state of the art in quantitative deterministic knowledge and as a template for the development of an integrative and system mechanobiology.
... En effet pour l'usinage des bois, on recherche des troncsà la fois rectiligne et dont le bois est homogène en terme de propriétés mécaniques (Sierra-De-Grado et al. [2008]). Ce problème est particulièrement aigu chez certaines espèces comme le pin maritime (Sierra-De-Grado et al. [2008]) ou le hêtre (Barbacci et al. [2009]). Or chez les arbres le moteur des mouvements actifs est la production de bois de réaction, présentant des propriétés physiques, densité, et mécaniques, pré-contraintes de maturations, très différentes (Barbacci et al. [2009]). ...
... Ce problème est particulièrement aigu chez certaines espèces comme le pin maritime (Sierra-De-Grado et al. [2008]) ou le hêtre (Barbacci et al. [2009]). Or chez les arbres le moteur des mouvements actifs est la production de bois de réaction, présentant des propriétés physiques, densité, et mécaniques, pré-contraintes de maturations, très différentes (Barbacci et al. [2009]). Un arbre présentant une mauvaise régulation de son mouvement pourra présenter une forme 9. Communication personnelle de Bruno Moulia 10. ...
... Ainsi on peut raisonnablement espérer compléter et amplifier l'analyse des mécanismes de régulation de la forme des troncs (et de production de bois de réaction) déjà entreprise et accélérer la sélection de génotypes assurant une forme favorable dans des conditions de croissance variées (Sierra- De-Grado et al. [2008]). Comme nous l'avons vu en introduction ceci peutêtre très important pour une meilleure valorisation des ressources boisées pour plusieurs essences forestières majeures (Sierra-De-Grado et al. [2008], Barbacci et al. [2009]). Il faut insister ici sur le fait que le caractère adimensionnel et synthétique de B, allié au caractère indéfini de la morphogenèse des végétaux souligné en introduction (chapitre 1), permet desétudes sur des plantes aux stades jeunes avec bon espoir de généralisation sur l'ensemble des stades, y compris adultes. ...
Most of the plants are mostly vertical and straight on all or a part of their aerial systems. To reach this postural shape, stems display complex movements related to environmental perceptions, gravity for instance, or internal perceptions, the own deformation of the organ. When a stem is tilted from the vertical, without light, it starts to curve locally in order to align each part of its axis vertically. During this movement, different transitory shapes can be observed, depending on the species. Some stems never overshoot the vertical whereas others display C-shape or even S-shape. A minimal dynamical modeling of the gravitropic movement has been developed. This modeling only depends on two parameters. A first parameter describes the way the plants curve in order to reach the vertical, the graviceptive term. The second term describes the proprioception, tending to reduce the curvature of the organ. A gravipropriotropic steady state, between straightening and curvature of the stem, is then reached. An adimensionless number, B, can then be defined as the ratio between the graviceptive and propioceptive sensitivities . This unique number describes at the same time, the steady state shape and the transient movements displayed. It allows to compare several species even if their characteristic time and length are quite different. An experimental estimation of B is proposed as the rate of the length of the curved zone on the final shape on the effective length of the stem. The comparison of the estimation of B with a simple description of the transitory shapes of the movement was in accordance with the gravirpopiceptive model. Moreover,this sets a phenotyping protocole for plant gravitropism. More advanced analysis of deviations between the model and the experiments demonstrates that elongation and apical influences are second order. We also found a non trivial relation between oscillating propagative behaviour of elongation rate and curvature rate. This suggests a better regulation than expected probably linked to auxin polarised transport and cell elongating motor cycle.
... The first step of our study was to establish a reference 3D model for each tree and to manually estimate their woody volume, for the stem and for each branch (Figure 1). The method used was derived from the methodology already developed for other studies (Constant et al. 2003;Barbacci et al. 2009). This manual method allowing for the 3D reconstruction of trees resulted from the coupling of tacheometer (Leica TCR 307) measurements, i.e. 3D points, to manual measurements (i.e. ...
The assessment of the spatial distribution of the tree biomass by means of terrestrial LiDAR scanning in forest environment still remains a challenge. This paper aims at presenting a method coupling terrestrial LiDAR measurements (FARO LS 880 HE) to a point cloud processing software (InnovMetric Software Inc., PolyWorks) allowing for tree architecture and wood volume distribution modeling, at the tree scale, for two Beech trees (Fagus sylvatica). This method is compared to a detailed and labour-intensive way of tree biomass distribution assessment (tacheometer and manual measurements), considered here as the reference. Our results quantify the appropriateness of the T-LiDAR method to estimate wood volumes and to correctly represent the three-dimensional architecture of a tree and the spatial distribution of its woody biomass. However, some measurement parameters must be carefully considered and further refinements are still needed to automate the extraction of information from point clouds.
To test for different stem reorientation strategies in leaning seedlings of maritime pine (Pinus pinaster Ait.) and loblolly pine (Pinus taeda L.), we inclined dormant plants at 0°, 45° and 90° for 35 days in optimal growth conditions. Stem angle to the vertical was measured at regular intervals. After 22 and 35 days, bending tests were carried out to determine stem elasticity and stiffness. The quantity of compression wood (CW) formed was also measured. Results showed that after 24 h, the apices of leaning maritime pine stems had completely reoriented to the vertical, whereas no such primary tropic response was observed in loblolly pine. After 22 days, the significantly stiffer loblolly pines had begun straightening up from the stem base, but not maritime pine. After 35 days, the overall secondary reorientation process was more efficient in loblolly pine, with significantly more CW produced. In both species, CW had formed immediately on tilting stems, i.e. no early wood cells had formed. As maritime pine originates from a dry to semi-arid region, having less CW will mean that water uptake will be greater for the same xylem area compared to loblolly pine which possesses more CW. In establishing maritime pine seedlings, rapid apical orientation with regard to light and gravity will compensate for a slower straightening up mechanism in the more lignified parts of the stem. Therefore, juvenile needle exposure to patches of light will be maximised at all times.
From an ecological point of view, the tension or compression stresses developed by wood maturation during growth allow the tree to maintain its vertical posture against gravity. These stresses, called growth stresses, results from the formation of a particular wood called reaction wood. From a mechanical point of view, the asymmetry of growth stresses between the two opposite faces of the tree trunk causes its reorientation, characterised by a curvature. This work aims at developing new theoretical and metrological tools to assess the link between tree morphology and growth stresses in a biomechanical and ecological framework. The first approach aimed at establishing the link between growth dynamics and competition of the stand with tree morphology and reaction of posture control. It is based on data taken in a long-term forestry experiment (beech plantations of different initial planting densities that grew during 26 years). The retrospective analysis of reaction wood production on wood discs taken on the trees allowed to assess the variation of their gravitropic performance over time. The scaling law established between curvature rate and stem circumference showed (i) the leading effect of the diameter of a stem on its reactivity, and (ii) the absence of additional effects of competition. At the end of the experimentation, the assessment of the tree morphology allowed to confirm the link between stem leaning and slenderness with growth stresses indicators. From a mechanical point of view, tree morphology can be assess by (i) the shape of its stem (leaning, curvatures), and (ii) the spatial distribution of its biomass, that can provide biomechanical variables for stimulus-response models. Therefore, an important methodological work was performed, based on terrestrial LiDAR technology (a promising tool for forest measurements based on 3D laser digitisation) coupled to geometrical modelling. It allowed to obtain accurate 3D mocks-up representing the woody structure of trees of variables species. The mocks-up allowed to model the bending stress exerted by the aerial biomass of the trees with the aim of linking it to the growth stresses indicators. The results show that bending stress is a promising variable for assessing the degree of reaction of trees. The developed methodology also gives many perspectives for monitoring tree morphology over time with the aim of biomechanical interpretation.
