Understanding the anchorage of a complex root system architecture (RSA) in soil upon tree overturning is vital to evaluate tree stability under lateral loads. Empirical correlations between root anchorage capacity and root morphological traits have been established, but the role of soil in these correlations has been ignored. This study developed and validated a threedimensional finite element model and then used it to investigate the underlying mechanisms of root-soil load transfer mechanisms in terms of the evolution of soil stress states and root strength mobilisation during overturning. Two root traits--radial distance and embedded depth--influenced root anchorage capacity remarkably. The pattern of soil stress state evolution near taproots and laterals was remarkably different. Roots that displaced in the direction more aligned with the soil's major principal stress were more effective to mobilise their strength to resist against overturning. The failure envelope defined by the normalised peak moment capacity in the x- and y-direction of the asymmetric RSA was elliptic, displaying anisotropic overturning under combined load conditions.

Background and aimsUrban trees in coastal cities like Hong Kong may suffer from an uprooting failure when subjected to extreme winds. A proper numerical model for tree uprooting simulation can help to select tree species or soil types that better resist uprooting failure. However, modeling tree uprooting is challenging as it is a cross-disciplinary problem involving complex root system architectures (RSAs) and large deformation of both roots and soils. This study aims to develop a hybrid numerical model that combines truss elements and material point method (MPM) to simulate the entire large-deformation uprooting process of trees with complex RSAs.MethodsThe tree uprooting model is developed by coupling truss elements in finite element method (FEM) with MPM. Laboratory pull-out tests using artificial roots and real root cuttings are adopted to validate the developed model. A comparative study is performed to investigate the difference between using complex and simplified RSAs in tree uprooting simulations.ResultsThe developed model provides consistent predictions of peak load, critical displacement and failure mode when compared with results from laboratory tests. Moreover, the comparative study shows that the uprooting resistance obtained with a complex RSA is higher than that with a simplified RSA. The difference varies with the soil and root mechanical properties.ConclusionThe developed hybrid model offers a novel way for simulating an entire tree uprooting process involving large deformations and complex RSAs. The study shows that using a simplified RSA to approximate the complex RSA might result in misleading failure modes.

Artificial pulling tests are the most practical method of assessing the maximum resistance of trees to lateral forces (e.g., from the wind), particularly in relation to their anchoring capacity in the ground. The traditional method is to pull the tree monotonically until failure. However, there are still many uncertainties regarding the possibility of mimicking wind gusts in such a tree pulling test. More specifically, it is supposed that a succession of wind gusts during a windstorm may cause fatigue to the root system, leading to a propagation of damage at the rootsoil interface which will eventually lead to the collapse of the tree. This work aims to provide initial insights into the biomechanical response of shallow-rooted Norway spruce (Picea abies (L.) Karst.) growing in mineral soils by repeatedly pulling to failure six trees with increasing load magnitude. The mechanical behaviour of the tested trees was first analysed using a classical equilibrium approach by calculating peak applied loads, stem base rotation, equivalent stiffness trend over subsequent cycles and residual rotations. Then, the biomechanics of the trees were analysed using an energetic approach, focusing on the energy absorbed and dissipated during either the single load cycle or the complete cyclic test, by applying consolidated procedures used in the field of mechanical engineering. Results show how small but measurable residual rotations were measured after each load repetition, indicating permanent damage even in seemingly undamaged trees. Additionally, loads producing base rotations about 0.3 - 0.4 times those corresponding to the peak resistance dissipate less than 1 % of the maximum dissipated energy calculated at the same peak point. Additionally, this peak energy is found to be strongly correlated to both the peak moment and a typical stem volume predictor such as diameter at breast height squared times height. All these outcomes are intended to provide a starting point for the development of a different characterisation of tree resistance as an alternative to the current methodologies, especially when it is important to consider the effects of repeated loading on trees.