The reactions of trees to wind, rockfall, and snow and debris flow depend largely on how strong and deformable their anchorage in the soil is. Here, the resistive turning moment M of the root-soil system as a function of the rotation φ at the stem base plays the major role. M(φ) describes the behavior of the root-soil system when subject to rotational moment, with the maximum M(φ) indicating the anchorage strength M_a of the tree. We assessed M(φ) of 66 Norway spruce (Picea abies L. Karst) by pulling them over with a winch. These 45- to 170-year-old trees grew at sites of low and high elevation, with a diameter at breast height DBH=14-69 cm and a height H=9-42 m. M(φ) displayed a strong nonlinear behavior. M_a was reached at a lower φ for large trees than for small trees. Thus overhanging tree weight contributed less to M_a for the large trees. Overturning also occurred at a lower φ for the large trees. These observations show that the rotational ductility of the root-soil system is higher for small trees. M_a could be described by four monovariate linear regression equations of tree weight, stem weight, stem volume and DBH²·H (0.80<R²<0.95), and φ at M_a, φ a, by a power law of DBH²·H (R²=0.85). We found significantly higher M_a for the low-elevation spruces than for the high-elevation spruces, which were more shallowly anchored, but no significant difference in φ_a. The 66 curves of M(φ), normalized (_n) by M_a in M-direction and by φ_a in φ-direction, yielded one characteristic average curve: M̄_n(φ_n). Using M̄_n(φ_n) and the predictions of M_a and φ_a, it is shown that M(φ) and the curves associated with M(φ) can be predicted with a relative standard error ≤25%. The parameterization of M(φ) by tree size and weight is novel and provides useful information for predicting with finite-element computer models how trees will react to natural hazards