Wind can alter plant growth and cause extensive, irreversible damage in forested areas. To better understand how to mitigate the effects of wind action, we investigated the sensitivity of tree aerodynamic behavior to the material and geometrical factors characterizing the aerial system. The mechanical response of a 35-yr-old maritime pine (Pinus pinaster, Pinaceae) submitted to static and dynamic wind loads is simulated with a finite element model. The branching structure is represented in three dimensions. Factor effects are evaluated using a fractional experimental design. Results show that material properties play only a limited role in tree dynamics. In contrast, small morphological variations can produce extreme behaviors such as either very little or nearly critical dissipation of stem oscillations. Slender trees are shown to be relatively more vulnerable to stem breakage than uprooting. Dynamic loading leads to deflections and forces up to 20% higher near the base of the tree than those calculated for a static loading of similar magnitude. Effects of branch geometry on dynamic amplification are substantial yet not linear. The flexibility of the aerial system is found to be critical to reducing the resistance to the airflow and thus to minimizing the risk of failure.

A finite element model was developed to study the influence of aerial architecture on the structural dynamics of trees. The model combines a complete description of the axes of the aerial architecture of the plant with numerical techniques suitable for dynamic nonlinear analyses. Trees were modeled on the basis of morphological measurements that were previously made on three 4-year-old Pinus pinaster Ait. saplings originating from even-aged stands. Calculated and measured oscillations were compared to evaluate model behavior. The computations allowed the characteristics of the fundamental mode of vibration to be estimated with satisfactory accuracy. Inclusion of a topological description of the aerial system in a mechanical model provided insight into the effect of tree architecture on tree dynamic behavior. Simplifications of the branching pattern in the model led to overestimations of the natural swaying frequency of saplings by 10 to 20%. Inadequate values of stem and root anchorage stiffness resulted in errors of 10 to 20%. Modeling results indicated that aerodynamic drag of needles is responsible for 80% of the damping in the studied trees. Additionally, damping of stem movement is reduced by one half when branch oscillations are not considered. It appears that the efficiency of the dissipative mechanisms depends directly on crown topology.

The aim of this study was to investigate the influence of aerial architecture on the dynamic characteristics of young maritime pines (Pinus pinaster Ait.) using a mechanistic approach. For this purpose, three 4-year-old saplings with prominent differences in their branching patterns were submitted to free oscillation tests. The tests were carried out with different methods and directions of mechanical loading in order to initiate the movement of each sapling. The oscillations of the different architectural elements, i.e. stem and branches of different topological order, were measured with inclinometers and strain gauges fixed to saplings. Successive pruning of the architectural elements was carried out to evaluate their relative influence on the dynamic characteristics of the trees. The aerial systems were digitized before the mechanical tests in order to use 3D visualization techniques and to make architectural analyses of the crown structure. Two distinct modes of deformation were detected during free oscillations. The natural swaying frequency ranged from 0.6-0.8 Hz for the saplings tested at the same period of the year. The frequency variations were partly explained by the morphological differences of the experimental subjects. The motions of the axes were found to depend on their topology, i.e. the movement of the axes of a given branching order was forced by the movement of their respective bearing axis. The axes of third branching order had a significant and negative effect on the damping of the natural deformation mode. Results point out the major role played by foliage, qualitatively and quantitatively, on the damping of tree motions and on coupling the motions of the crown components.

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