Real-scale tests on the cyclic response of trees under toppling loads: experimental investigation and geotechnical interpretative framework

Tree failures, increasingly intensified by climate change and extreme weather events, represent a major risk in urban environments. Assessing tree stability has therefore become a crucial aspect of modern risk management. From a geotechnical perspective, the overturning resistance of trees—especially when governed by root system uprooting—can be studied using principles similar to those applied to shallow foundations in civil engineering. In this analogy, the root plate acts as a shallow footing that provides mechanical support against external forces. This study presents an experimental campaign of pulling tests on mature trees at the Botanic Garden of the University of Dundee (Scotland, UK). Tests were performed in orthogonal directions and included load relaxation and cyclic loading up to complete uprooting. The experimental data were interpreted within a geotechnical framework, highlighting analogies with cyclic loading effects on shallow foundations subjected to rocking. Global mechanical parameters at the trunk collar scale, such as rotational stiffness and damping ratio, were derived and analyzed with respect to different loading paths. The results suggest the validity of modelling the root plate as a shallow foundation and offer new insights into the mechanical response and toppling behavior of trees under lateral loads.