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 = 14-69cm and a height H = 9-42m. 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 DBH 2 ·H (0.80 < R 2 < 0.95), and ϕ at M a, ϕ a, by a power law of DBH2·H (R 2 = 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: Mn(ϕn) . Using Mn(ϕn) 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 hazard