Aims <p>Variability in the capacity of tree root anchorage, driven by differences in root system architecture (RSA), is closely associated with root geometrical traits, yet their role in root–soil interaction during overturning remains poorly understood. This study aims to (1) examine correlations between root geometrical traits and RSA’s overturning performance; (2) investigate how distinct root components contribute to anchorage; and (3) identify sources of variability in root anchorage and propose a combined trait to explain this variability. </p> Methods <p>An advanced root anchorage model was employed to realistically simulate root–soil interaction and assess root anchorage capacity. A total of 90 statistically different RSAs of <i>Pinus pinaster</i> trees were generated using a calibrated root growth model. Spearman’s correlation (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\rho\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>ρ</mi> </math></EquationSource> </InlineEquation>) was used to identify key geometrical traits influencing maximum overturning resistance (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({M}_{p,max}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>M</mi> <mrow> <mi>p</mi> <mo>,</mo> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>).</p> Results <p>Substantial variabilities in root overturning resistance were observed. RSAs that mobilised greater <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\({M}_{p,max}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>M</mi> <mrow> <mi>p</mi> <mo>,</mo> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </math></EquationSource> </InlineEquation> maintained greater residual resistance at larger rotations. Windward lateral roots played a critical role in anchorage, with their surface area, volume and cross-sectional area of broken root segments showing strong correlations with <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({M}_{p,max}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>M</mi> <mrow> <mi>p</mi> <mo>,</mo> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>. However, no single traits explained the variabilities in <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\({M}_{p,max}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>M</mi> <mrow> <mi>p</mi> <mo>,</mo> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>.</p> Conclusions <p>Investigation of root–soil load transfer processes suggested that proximal lateral roots experienced axial pull-out, whilst distal lateral roots resisted overturning through bending. The deformation pattern of vertical root components (i.e. taproot and sinker) was closely associated with root aspect ratio. A new fourth-order combined trait, incorporating root volume, diameter, and embedded depth, was proposed to effectively capture the <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({M}_{p,max}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>M</mi> <mrow> <mi>p</mi> <mo>,</mo> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </math></EquationSource> </InlineEquation> variabilities.</p>