The mechanical role of bark

Area of Science:

• Plant biomechanics
• Wood science
• Forestry

Background:

• The mechanical role of tree bark in stem structural integrity is not fully understood.
• Bark's contribution to resisting bending forces may vary across species and with age.

Purpose of the Study:

• To quantify the contribution of bark to the bending resistance of young stems in three tree species.
• To investigate the age-dependency and species-specificity of bark's mechanical role.
• To develop a model predicting bark's mechanical importance based on thickness and stiffness.

Main Methods:

• Bending tests were performed on stem segments of Acer saccharum, Fraxinus americana, and Quercus robur, both with and without bark.
• Mechanical properties (stiffness, second moment of area) of bark and wood were analyzed.
• A mathematical model was developed to relate bark properties to its contribution to flexural rigidity.

Main Results:

• Bark significantly increased stem bending resistance in all tested species, with contributions varying by age and species.
• For F. americana and Q. robur, wood became more important than bark in older segments (≥ 6 years).
• A. saccharum bark was as important as wood in 7-year-old segments, but not significantly stiffening in younger or older stems.
• Bark stiffness averaged 50% of wood stiffness, but its geometric contribution (second moment of area) offset this.

Conclusions:

• The bark's geometric properties play a crucial role in its mechanical contribution to stem stiffness, often offsetting its lower material stiffness compared to wood.
• A model indicates bark is as important as wood when its radial thickness is 32% of wood thickness and stiffness is 50%.
• The mechanical role of bark is critical for understanding the behavior of juvenile woody stems under bending stress and should not be neglected.