Barbora Vojáčková

Cactus spines are highly variable structures that perform multiple ecological functions and have considerable taxonomic importance. However, their anatomical and mechanical properties remain poorly understood. A better understanding of spine structure may provide insights into the ecological function. In this study, we compared morphological, anatomical, and mechanical spine traits in three columnar cacti from Argentina (Cereus hankeanus, Stetsonia coryne, and Trichocereus terscheckii) and evaluated how these traits vary along a developmental gradient extending from the apical meristem. Spines were sampled from a single rib of 10 individuals per species. We measured spine length, fibre dimensions, cell wall thickness, and performed mechanical tests on oven-dried spines to calculate modulus of elasticity, modulus of rupture, and toughness. Significant differences in spine traits were detected among species. Stetsonia had the longest spines and exhibited the highest toughness, whereas Trichocereus showed lower mechanical resistance despite having thicker cell walls. Anatomical variation was also evident, with Stetsonia having smaller and more elliptical fibres than Trichocereus. In addition to interspecific differences, spine length in Stetsonia and Cereus increased with distance from the apex, indicating developmental changes along the stem. Among mechanical traits, only toughness varied along the stem, decreasing with increasing distance from the apex in all species, while anatomical traits remained unchanged. Together, these findings reveal substantial interspecific, intraspecific, and positional variation in cactus spine traits, highlighting the influence of developmental position along the stem and spine ageing, as well as species-specific differences in spine structure and mechanical performance.

Tree risk assessment requires mechanical response studies, but simplification of the shape, material, or boundary conditions is necessary when dealing with such complex structures. To observe overall tree response, sub-structuring to several levels of detail can be used, enabled by recent developments in numerical methods and three-dimensional laser scanning (3D scan). This study aimed to determine an appropriate level of geometry and loading simplification allowed for high-order branches at the crown border, which is useful for the mechanical analysis of structured tree models. Four higher-order branches were pruned and experimentally tested by single-point loading. Beam and solid finite-element models (FEMs) were created based on measured geometric parameters and detailed 3D scans, respectively. The FEMs were used to analyze seven loading scenarios with force applied at (a) the center of gravity, (b) the top of side branches, (c) key discrete points, and (d) uniformly to the whole volume (to each finite element). Force was distributed by ratios weighted according to the mass, area, and diameter of side branches; or according to the mass of each finite element. The results showed no significant difference between the beam model and 3D scan-based model. The scenarios with finite elements’ mass-based force distribution deviated significantly from those of the other scenarios. The most simplified single-point loading caused a deviation in the deflection curve. The deviation of single-point loading in the case of the bending moment was related to force distribution ratios given by the branches architecture. Therefore, such loading simplification is not considered always appropriate. Consistency between the bending moment and branch deflection provided a representative mechanical response, recommended for further modeling of trees by sub-structuring.

The application of frequency-resonance method (FRM) for precise log bending testing is limited by complex geometry; usually the cone with variable cross. This case study presents the relationship among non-destructively tested material parameters of sugar maples and the numerical analysis of effect of simplifications in FRM. Four standing stems were measured to find basic geometry parameters, 3D scanned for precise geometry description, tested by pulling test (PT) to obtain elastic parameters, and, finally, cut down to process the logs. Four logs were measured by stress wave propagation (SWP) using an acoustic tomography (AT) device to obtain longitudinal sound velocities and then evaluated by the FRM to obtain natural frequencies in bending and longitudinal vibrations. Comparison was made between the dynamic moduli of elasticity (MOE), calculated from SWP and the FRM, and the static MOE calculated from the PT. In-situ experimental evaluation was accompanied by modal analysis by finite element method (FEM) working at three levels of geometry simplification (beam model, simplified solid model, and scan-based solid model); the natural frequencies of bending and longitudinal mode shapes were analyzed. The influence of geometry precision on the resulting dynamic response of logs was found regarding comparison to the experimental values.