The biomechanical consequences of the internode biomass inequality in Phyllostachys culms.

PREMISE: Node-internode modular units characterize the construction of many herbaceous stems such as those of bamboo culms, providing hydraulic conductance and mechanical support such that internode biomass inequality observed within a culm significantly influences growth and structural stability. However, this inequality has not been studied. Traditional metrics, such as the coefficient of variation (CV), and the Gini coefficient (GC), can obscure sources of variation. METHODS: To address this analytical gap, we used the performance equation (PE) and its generalized version (GPE) to fit the rotated and right-shifted Lorenz curve (RRLC) of the internode mass distribution to quantify the inequality of each of 835 culms of 14 Phyllostachys species. The adjusted root mean square error and corrected Akaike information criterion were used to compare contending equations. Based on the better model fit, the Lorenz asymmetry coefficient (LAC) was calculated to determine whether many large (or a few large) internodes contribute to the within-culm inequality of the internode mass distribution. RESULTS: The GPE was significantly better than the PE in describing the RRLC. The mean LAC was 0.6436 (±0.0273 SD), which was significantly greater than 0.5, indicating that the inequality in internode mass is primarily driven by many rather than a few large internodes. CONCLUSIONS: These findings highlight a mechanical reinforcement strategy in bamboo whereby biomass is preferentially allocated to lower internodes to enhance flexural stiffness to deal with gravitational and wind-induced dynamic loads. The RRLC-LAC framework is shown to be effective in elucidating the biomass allocation strategies that balance growth with structural support.