How does bark contribution to postural control change during tree ontogeny? A study of six Amazonian tree species.

Bruno Clair,Romain Lehnebach,Tancrède Alméras

doi:10.1093/jxb/eraa070

Abstract

Recent works revealed that bark is able to produce mechanical stress to control the orientation of young tilted stems. Here we report how the potential performance of this function changes with stem size in six Amazonian species with contrasted bark anatomy. The potential performance of the mechanism depends both on the magnitude of bark stress and the relative thickness of the bark. We measured bark longitudinal residual strain and density, and the allometric relationship between bark thickness and stem radius over a gradient of tree sizes. Constant tensile stress was found in species that rely on bark for the control of stem orientation in young stages. Other species had increasing compressive stress, associated with increasing density attributed to the development of sclereids. Compressive stress was also associated with low relative bark thickness. The relative thickness of bark decreased with size in all species, suggesting that a reorientation mechanism based on bark progressively performs less well as the tree grows. However, greater relative thickness was observed in species with more tensile stress, thereby evidencing that this reduction in performance is mitigated in species that rely on bark for reorientation.

Highlights

Bark is a multifunctional tissue (Angyalossy et al, 2016; Rosell, 2019)
Scaling of inner bark thickness with stem size Studying a much broader species diversity, Rosell (2016) reported a similar distribution of bark thickness, indicating that our sampling is representative of other tropical rainforests
Our analysis shows that the ranking of species according to bark thickness for a given tree size is usually maintained throughout the life of the tree

Summary

Introduction

Bark is a multifunctional tissue (Angyalossy et al, 2016; Rosell, 2019). Its functions include transport of photosynthates (Evert, 2006), storage of carbohydrates and water (Rosell and Olson, 2014), defense against physical and biological aggression (Pausas, 2015), and mechanical support (Niklas, 1999; Rosell and Olson, 2014). The mechanical function was long thought to be limited to contributing to stem stiffness. Recent works reported the involvement of inner bark (i.e. secondary phloem), hereafter bark, in the active control of stem orientation (Clair et al, 2019). Bark fulfills this function by generating mechanical stress during stem secondary growth. Combined with a source of asymmetry (here eccentric growth), this active generation of mechanical stress results in a bending moment capable of altering stem curvature and orientation (Clair et al, 2019)

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Conclusion