Thiéry Constant

The use of terrestrial LiDAR technology in forest science: application fields, benefits and challenges

Abstract• IntroductionThe 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.• ObjectivesTerrestrial 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.• ResultsSince 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.

Detailed analysis of the geometric relationship between external traits and the shape of red heartwood in beech trees (Fagus sylvatica L.)

Frequently occurring red heartwood decisively restricts the volume of light-coloured beech wood, which can be processed to high-grade appearance products. Forestry and wood industry lack suitable means for maximising the yield of light timber, since the intra-tree extent of red heartwood varies considerably. The present study characterised in detail the intra-tree shape of red heartwood and its relationships to tree-external traits (dead branches, branch scars), considered as possible initiation points of red heartwood formation. An experimental method based on log scanning and image analysis was developed and applied. Using its output for three-dimensional visualisation and data analyses, external traits being linked to the local red heartwood shape were identified. Furthermore geometric relationships were established for characterising these external traits and for deriving discriminating criteria.

Forest trees filter chronic wind‐signals to acclimate to high winds

Controlled experiments have shown that trees acclimate thigmomorphogenetically to wind-loads by sensing their deformation (strain). However, the strain regime in nature is exposed to a full spectrum of winds. We hypothesized that trees avoid overreacting by responding only to winds which bring information on local climate and/or wind exposure. Additionally, competition for light dependent on tree social status also likely affects thigmomorphogenesis. We monitored and manipulated quantitatively the strain regimes of 15 pairs of beech (Fagus sylvatica) trees of contrasting social status in an acclimated stand, and quantified the effects of these regimes on the radial growth over a vegetative season. Trees exposed to artificial bending, the intensity of which corresponds to the strongest wind-induced strains, enhanced their secondary growth by at least 80%. Surprisingly, this reaction was even greater - relatively - for suppressed trees than for dominant ones. Acclimated trees did not sense the different types of wind events in the same way. Daily wind speed peaks due to thermal winds were filtered out. Thigmomorphogenesis was therefore driven by intense storms. Thigmomorphogenesis is also likely to be involved in determining social status.

SHINY BEECH WOOD IS CONFIRMED AS AN INDICATOR OF TENSION WOOD

This study sought to develop a method to quantify tension wood areas in stem discs. It was suggested that the shiny appearance of beech (Fagus sylvatica L.) could provide an indication of tension wood. Each of 21 stem discs were digitised and the images analysed. Small areas of the discs were selected, and from these areas, anatomical sections were stained with astra-blue and safranine and the stained sections were compared with the wood disc images. The analysis showed that the shape of shiny areas and tension wood were similar; moreover, the measured ratios of shiny wood and tension wood were in good agreement. This confirms the assumption that in Fagus sylvatica shiny wood corresponds to tension wood.

Approach to the estimation of red heart occurrence in Fagus sylvatica based on geometric relationships between branch scar development and knot dimensions

Abstract The effect of branch scars on the initiation and occurrence of red heart in beech (Fagus sylvatica L.) was studied on 17 trees with red heartwood and 14 trees with no discoloured heartwood, all of which were selected from a 120-year-old high-forest stand in Germany. Logistic regression was used to predict the probability of red heart occurrence for a given tree. The model was based on individual probabilities derived from geometric relationships between scar and knot morphologies, and integrated the diameter at breast height. Using this model, 27 of 31 trees were correctly classified, and the two groups of trees were clearly distinguished by probabilities below 0.25 and above 0.85 (with one exception). Given the constitution of the model, only 17 of 616 scars were found to initiate red heart formation, and the parameters of mechanistic variables were strongly correlated. The results suggest specifying the relationship between scars, knots and red heart, and extending the scope of the model.

Effect of tree size and competition on tension wood production over time in beech plantations and assessing relative gravitropic response with a biomechanical model

PREMISE OF THE STUDY Gravitropic movements are unexpected mechanical processes that could disturb tree design allometries derived from the physics of nonliving bodies. We investigated whether the scaling law of gravitropic performance (power of -2 of stem diameter) derived from integrative biomechanical modeling is disturbed by ontogeny or environment, then discuss the silvicultural and dendroecological consequences. METHODS In a beech (Fagus sylvatica) plantation, four plots with different initial planting densities evolved without any intervention for 26 yr. Regular tree inventories and a silvicultural model were used to monitor competition over time in each plot. The radial production of tension wood was quantified using a cross-section of the stems at 1.30-m height, and an integrative biomechanical model computed the tree gravitropic performance over time. KEY RESULTS All trees developed tension wood over the whole period, with higher amounts at the youngest age, resulting in theoretical lean corrections of ca. 20-30° on the first 4 m of the stem over the whole period. The scaling law of gravitropic performance is slightly larger than the power of -2 of stem diameter. CONCLUSIONS Gravitropic performance in forest ecosystems is mainly limited by size (diameter). Ontogenic acclimation of tension wood formation allows the youngest trees to be more reactive. No additional effect of spacing was found. However, silviculture influences size and, therefore, tree reactivity at a given age. Such results will be helpful for dendroecological approaches that use wood as a marker of environmental disturbances or a trait linked to plant strategies.

Segmentation of forest terrain laser scan data

Modeling of realistic forest scenes is a challenge in virtual reality. This can take benefits from laser scan data acquisitions, where the segmentation of tree objects becomes an important topic. In this paper, we present a new automatic forest segmentation method. In the scan point cloud, based on normal directions, trunks are detected, leading to a correct extraction of single trees from the forest. With the trunk positions, the digital terrain model can be refined. Our technique separates the lower scene points, dedicated to trunk detection, from upper ones, dedicated to crown assignment, making the implementation easy and efficient. In addition, trunks are reconstructed, helping to construct a virtual scene and estimating tree diameter at breast height measurements (DBH). The proposed approach is implemented on real scene data, even including inclined trees, and opens an application to forestry inventories.

Theoretical and experimental study of a mechanical model describing the trunk behaviour of mature beech trees (Fagus sylvatica L.) under the static loading of the crown

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 beam model is proposed and tested theoretically, as well as experimentally, on three mature beech trees. Once the suitability of the model has been established, a simple sensitivity test in proposed that highlights the principal parameters involved in displacement.

Pas de vent, pas de bois. L’apport de la biomécanique des arbres pour comprendre la croissance puis la vulnérabilité aux vents forts des peuplements forestiers

La biomecanique etudie les reactions et adaptations des etres vivants a leur environnement mecanique, par exemple aux oscillations et forces exercees par le vent. Au-dela des theories anciennes de la securite mecanique constante, la mecanobiologie a recemment formalise les signaux mecaniques, la perception des cellules vivantes et les reponses de croissance. Ces mecanismes physiologiques font que l’arbre ne forme vraiment du bois, tissu de soutien, que lorsqu’il est mecaniquement stimule. La croissance est controlee par les deformations mecaniques percues, qui deviennent alors pour l’arbre de bons indicateurs de securite. Pour appliquer ces connaissances aux forets, le projet ANR FOR-WIND a l’ambition de developper une mecanobiologie adaptee aux temps longs et aux conditions naturelles complexes. L’enjeu est de concevoir les pratiques d’amenagement avec de nouveaux indicateurs de vulnerabilite aux vents forts, qui raisonnent l’effet du changement climatique, de la structure du paysage, de la sylviculture ou de l’amelioration genetique au travers des processus cles mais negliges d’endurcissement des arbres aux vents usuels.