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Gravity caused approximately one quarter of the critical bending moments. The weakest points along the trunks of the three largest trees resisted mean above-canopy wind speeds ranging from 10.2 to 12.7 m s −1 (3.3-fold in the strongest gust), but the two smallest were well protected by a dense layer of leaves from the bending tops of larger trees, and could have resisted stronger winds. We then focused on forces working on storm-bent (maximally bent) trees caused by gravity and the strongest gust in a 1-h simulation with a large-eddy simulation model. We felled and measured five Norway spruces ( Picea abies) in an unthinned monoculture in southeastern Finland planted 67 years earlier. However, advancements using this biomechanical approach have been delayed by difficulties in modelling bending of trunks and wind gusts. Realistic calculations on minimum trunk diameters needed to resist bending moments caused by wind and gravity would be a significant step forward. However, this understanding is fragmented into isolated schools of thought and has been far from complete. Understanding why trunks (tree stems) are the size that they are is important. Spruce trunk tapering corresponds closely to tapering required to resist bending forces caused by wind and gravity.