To estimate the wind force that causes windthrow damage to a tree, the drag coefficients of actual-sized trees were evaluated by a field test method. In this method, the wind velocity and stem deflection were monitored simultaneously. The wind force acting on a tree crown was calculated from the stem deflection; the stem stiffness was evaluated by conducting tree-bending tests. The results of tests conducted on three poplar trees showed that the drag coefficients decreased with an increase in the wind velocity. Although the variation in the drag coefficients was large at low wind velocity because of the vibrating behavior of the stem subjected to variable wind force, the variation at wind velocities above 10 m/s was small. The average drag coefficient at a wind velocity of 30 m/s was estimated by the curve fitting of a power function to the wind velocity-drag coefficient relationship, and it was found to be 0.102, which was smaller than that of actual-sized conifers studied in previous wind tunnel experiments. The drag coefficients of these crown areas in the defoliation season were smaller than those measured in the leaved season

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