The mechanics of wood as a material for construction, furniture, pulp and other uses have been addressed in a very large number of papers and are a well-established field for both research and technical applications (for example among many others, see Pöhler et al., 2006 for beechwood). In addition to such approaches that derive from material sciences, further developments based on similar physical concepts addressed the questions raised by the biomechanics of the standing and the growing tree which requires some degree of postural control and sensing of specific signals (gravity, movements…; see for instance Fournier et al. 2013; Dlouha et al., 2025). Within this field of research, the question of the correlation of wood anatomy (diameter of xylem tracheids or vessels, fibre content and angles, vessel wall thickness… ) and biomechanical properties is of prime importance, and specific responses of wood and bark components have been identified over the last decades. In particular the occurrence of reaction wood generates local strains and contributes to the postural control (Ruelle, 2014).

In this preprint, Almeras et al. address a complementary question related to the properties of juvenile wood in trees. During the first years of the growth of young trees, the annual tree rings display quite specific properties (large tree rings, less dense wood, …) that gradually change with age and dimensions of the trees until reaching a range of values typical for adult trees. During the first years, the interannual changes might follow an ontogenetic trajectory mainly related to age (and dimensions) while in later stages, they appear to be strongly controlled by environment (wind, soil fertility, site index, irradiance, water availability, ...). All these changes result in radial profiles along tree rings (from the pith to the bark) of three main features that govern the biomechanical properties of wood, namely the width of the annual tree ring, the local specific gravity (wood density), and the specific modulus which contributes with density to the local modulus of elasticity (Fournier et al. 2013). Such gradients of local wood properties within stems have been analysed and synthesised in the last years (Lachenbruch et al. 2011, Meinzer et al. 2014). 

Here, the authors address the question of local variations of such properties within tree stems as a function of the distance to the pith (inversely related to the age of the trees when the ring was formed) in a broadleaved species, European Beech (Fagus sylvatica L.). They checked whether ring width, specific gravity and specific modulus display systematic trends from pith to bark across tree stems, and whether these trends enable the detection of a general ontogenic (age-related) effect with very similar patterns in juvenile wood of different individuals, or whether adaptive factors (modulated by the environment and by the mechanical constraints induced by the postural control of growth) dominate already in juvenile wood, like it does at later stages. Such questions were already analysed in the wood of some coniferous species (softwood with tracheids), but less frequently in hardwood species (angiosperms, like Beech with its diffuse porous wood anatomy).

Before starting the analysis of age-related tree ring properties in juvenile wood, the authors addressed the potential impact of duraminisation, which affects the oldest tree rings in the inner wood (that is those formed during the juvenile growth stages). Duraminisation results from local deposition of a number of secondary metabolites and results in the build-up of heartwood; in the case of beech however, reddish heartwood is less present than in other species (Knoke, 2003). Almeras et al showed here that the occurrence of reddish wood did only marginally affect the mechanical properties and contributed only marginally to the observed variations among trees 

The very solid experimental design enabled the authors to clearly assign a fraction of the observed variation in the three parameters to (i) the site where trees had grown, (ii) to the individuals within these sites and (iii) to the position of the ring within the stem. The intraindividual component of the variation was much larger than the former. However, the observed asymmetry in the patterns of ring properties in juvenile wood, and the large variability in these patterns among trees led the authors conclude that the ontogenic juvenility effects, visible in ring width were largely dominated by other effects influenced by the local environment. In this respect, the results differ from those that were recorded earlier with Pinus taeda L. in a plantation (i.e., trees of the same age and homogenous spatial distribution, Bendtsen and Senft, 1986). 

The recommended version of the preprint is very original as it shows how local (radial) variations of biomechanical wood properties can be addressed in a systematic way. This lead to novel approaches that share light on the processes governing wood formation in trees.

The first version of the preprint was submitted over a year ago. The recommended version differs in many respects from the initial one. The two rounds of reviews with external reviewers, and the additional one with the recommender resulted in an in-depth reorganisation of the statistical analysis and of the demonstration. This took some time, but shows also the benefits that may be gained during an open peer review process like the one developed by the Peer Community in…. 

References

Bendtsen BA, Senft J. 1986. Mechanical and anatomical properties in individual growth rings of plantation-grown eastern cottonwood and loblolly pine. Wood Fiber Sci 18: 23-38. 

Dlouhá J, Moulia B, Fournier, M et al. 2025 Beyond the perception of wind only as a meteorological hazard: importance of mechanobiology for biomass allocation, forest ecology and management. Ann For Sci 82, 1. https://doi.org/10.1186/s13595-024-01271-6 

Fournier M, Dlouha J, Jaouen G, Alméras T. 2013. Integrative biomechanics for tree ecology: beyond wood density and strength. J Exp Bot, 64, 4793-4815. https://doi.org/10.1093/jxb/ert279

Knoke T. 2003 Predicting red heartwood formation in beech trees (Fagus sylvatica L.)? Ecol. Model. 169, 289-312. https://doi.org/10.1016/S0304-3800(03)00276-X 

Lachenbruch, B., Moore, J.R., Evans, R. (2011). Radial Variation in wood structure and function in woodypPlants, and hypotheses for its occurrence. In: Meinzer, F., Lachenbruch, B., Dawson, T. (eds) Size- and age-related changes in tree structure and function. Tree Physiology, vol 4. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-1242-3_5 

Longuetaud F, Mothe F, Santenoise P, Diop N, Dlouha J, Fournier M, Deleuze C. 2017. Patterns of within-stem variations in wood specific gravity and water content for five temperate tree species. Ann For Sci 74:64. https://doi.org/10.1007/s13595-017-0657-7

Meinzer, F., Lachenbruch, B., Dawson, T. (eds). 2014. Size- and age-related changes in tree structure and function. Tree Physiology, vol 4. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-1242-3_5 

Pöhler E, Klingner R, Künniger T. 2006. Beech (Fagus sylvatica L.) - Technological properties, adhesion behaviour and colour stability with and without coatings of the red heartwood. Ann For Sci 63: 129-137. https://doi.org/10.1051/forest:2005105

Ruelle J. 2014. Morphology, anatomy and ultrastructure of reaction wood. In: Gardiner B, Barnett J, Saranpää P, Gril J (eds) The Biology of Reaction Wood. Springer Series in Wood Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-10814-3_2 

Cite the recommended preprint: 

Almeras Tancrède, Jullien Delphine, Liu Shengquan, Loup Caroline, Gril Joseph, Thibaut Bernard (2025) The diversity of radial variations of wood properties in European beech reveals the plastic nature of juvenile wood. HAL, ver.6 peer-reviewed and recommended by PCI Forest and Wood Sciences https://hal.science/hal-04133248

Scientific context

A large part of deforestation in Ivory Coast is due to cocoa plantations, but the need for timber is increasing with population growth. A good compromise for farmers should be for agroforestry to combine cocoa and timber income. Tree for timber management may come from natural regeneration by the large diversity of available seed stock in the cocoa plots, fostering the safeguarding of biodiversity (Vroh et al. 2019, Kouassi et al. 2024)

Questions, hypotheses, methodology

Besides trade-off with cocoa productivity, this very new way of practising agroforestry raises many questions about timber productivity, the geometry of trunks or timber quality in connection with population needs.

Agroforestry trees should be less slender with lower branches than forest trees. This was studied here by Kouassi et al. (2024) using a very large sampling (150 plots) that covered the whole cocoa production zone on the south Ivory Coast. The description of many trees, including diameter at breast height, log height and tree age, allowed accurate modelling of tree growth and building of growth trajectories for 23 species of commercial interest.

Main results and interpretations

Using modelled diameter growth and bole volume trajectories, associated with the ongoing rule for harvesting based on minimum logging diameter, makes it possible to predict timber harvesting at different ages of the cocoa plantation.

Moreover, natural regeneration proves to be more efficient than plantation using saplings, but logs are more conical and with lower branches than their counterparts in natural forests, as expected. 

Recommendation

This is a very interesting paper on agroforestry for fruit (cocoa) and timber production, full of interesting data coming from a large number of study areas (150 plots) where both spontaneous (natural regeneration from past forest seeds) or planted fast-growing trees are used. Using a Bayesian scheme, models are built to predict (1) the changes in diameter with tree age, (2) the relationship between diameter and height, and (3) the relationship between tree dimensions and bole volume. Predictions about the change in bole volume with age are deduced from these models. This makes it possible to build scenarios for adding value to the wood produced by the agroforestry system, depending on the current or future technical and socio-economic context.

References

Kouassi AK, Zo-Bi IC, Hérault B, Konan IK, Dago MR, Lasbats B, Schmitt S, N'Guessan AE, Aussenac R (2024) Tree growth in West African cocoa agroforestry systems: high timber yields and superior performance of natural regeneration. HAL, ver.2 peer-reviewed and recommended by PCI Forest and Wood Sciences https://hal.science/hal-04638492

Vroh TA, Abrou NEJ, Gone BI, Adou Yao CY (2019). Système agroforestier à cacaoyers en Côte d’Ivoire : connaissances existantes et besoins de recherche pour une production durable. Rev. Mar. Sci. Agron. Vét. (2019) 7 (1): 99-109.