Understanding the processes allowing trees to orientate their stems and branches requires an understanding of the mechanical properties of individual cells. As the cells are formed, maturation stresses are created that can lead to the reorientation of the tree. But measuring the properties within the different wood cells produced in normal wood, tension wood or compression wood requires measurements at very fine spatial resolution and the wood cells must remain in-situ so that the cell mechanical characteristics are preserved. In the article of Arnould et al (2022), measurements of the mechanical characteristics of poplar tension wood were measured in comparison to normal wood at different distances from the cambium and therefore different levels of maturation. The work required incredible care to embed the wood in resin, to cut the wood with extremely sharp microtone blades in order to minimize artefacts in the measurements, and then ultra-careful atomic force microscope (AFM) measurements across cell walls from the edge of the lumen to the middle lamella at extremely high spatial resolution. The result is a detailed picture of the kinetic development and maturation of tension wood cells in a tree. The measurements showed that the G-layer reaches close to its final stiffness long before its final thickness, and this is different from the maturation kinetics of other cell wall layers where thickening and stiffening are generally synchronous. Finally, although the G-layer in poplar tension wood fibres and in flax phloem fibres are in many ways very similar there are clear differences in the kinetics of their development and maturation. The detailed information presented in this paper can therefore help to clarify the different hypothetical mechanisms proposed to explain excess stress generation in the tension wood of trees and help move us towards a full understanding of how the “muscles” of trees work.

References

Arnould O, Capron M, Ramonda M, Laurans F, Alméras T, Pilate G, Clair B (2022) Mechanical characterisation of the developing cell wall layers of tension wood fibres by Atomic Force Microscopy. bioRxiv, 2021.09.23.461481, ver. 4 peer-reviewed and recommended by Peer Community in Forest and Wood Science. https://doi.org/10.1101/2021.09.23.461481

Trees are constantly subjected to mechanical stresses (gravity, wind, storms) but they have a remarkable ability to remain upright despite their great size. Straightness is also major characteristic that greatly determines the quality and market value of a log. For some species, maritime pine in particular, this is even a default that geneticists are trying to correct through dedicated breeding programs (Bartholomé et al. 2016). If trees are able to maintain a straight trunk, or to return to straightness after a growth accident, for example, it is thanks to an "engine" whose mechanisms are now better known (Moulia et al. 2021). This mechanism lies in the structure of the trunk itself and the ability of trees to produce cells and tissues that display different mechanical properties during their maturation. Some fibers will "pull" the trunk in one direction, known as tension wood, while others will « push » it in the opposite direction (compression wood). The posture control of a tree is therefore directly related to the growth process of the trees and the placement of this reaction wood at specific points in the trunk. The internal structure of the trunk will therefore retain the memory of these growth constraints throughout its life and, if we are able to read it, we can envisage reconstructing its history over the years. This source of information contained in tree rings is still largely unexplored. However, it can reveal insights into the evolution of the climate, or help foresters to select the most valuable trees. Sophisticated approaches exist to measure these growth forces in wood, but the major difficulty remains our ability to read the mechanical properties with simpler, more widely used methods. The article by Thibaut and Gril (2021) proposes such a methodology.

The approach used here is similar to the one used for other wood functions, such as sap transport: linking the mechanical function of wood to its structural properties. The transport capacity of wood is for example well explained by the distribution of vessel sizes. However, other interesting properties, such as resistance to cavitation, are only very weakly explained by the same anatomical characteristics. The authors, after analyzing the wood properties of many species, both tropical and temperate, conclude that growth forces can be deduced from variables that are relatively simple to measure, such as wood density or moduli of elasticity.  The article provides a series of generic and more specific equations and relationships that allow these growth forces to be estimated. 

I recommend this article to people who want to learn about the principles and concepts of tree biomechanics. I also recommend it to people who want to further explore the mechanical properties of trees and who will be able to characterize them thanks to the method proposed here by the authors.  

References

Bartholomé J, Bink MC, Heerwaarden J van, Chancerel E, Boury C, Lesur I, Isik F, Bouffier L, Plomion C (2016) Linkage and Association Mapping for Two Major Traits Used in the Maritime Pine Breeding Program: Height Growth and Stem Straightness. PLOS ONE, 11, e0165323. https://doi.org/10.1371/journal.pone.0165323

Thibaut B, Gril J (2021) Tree growth forces and wood properties. HAL, hal-02984734, ver. 4 peer-reviewed and recommended by Peer community in Forest and Wood Sciences. https://hal.archives-ouvertes.fr/hal-02984734

Moulia B, Douady S, Hamant O (2021) Fluctuations shape plants through proprioception. Science, 372. https://doi.org/10.1126/science.abc6868