Dynamics of swaying of Picea sitchensis.

Six 26-year-old Sitka spruce (Picea sitchensis (Bong.) Carr.) trees growing in a Scottish plantation were swayed manually to determine their mechanical dynamics. The natural frequency of sway of the intact trees (mean height 14.2 m and mean stem diameter 14.5 cm at 1.3 m) was on average 0.35 Hz. The variation of this frequency with tree size was found to be well described by engineering mechanics theory. In particular, shape parameters could be defined for both intact and branchless trees, which, along with stem size, density and elasticity, could predict the natural sway frequency using a simple formula. The damping of sway was found to consist of three components, (1) interference of branches with those of neighbors, (2) aerodynamic drag on foliage, and (3) damping in the stem. For the sample of six trees, which spanned the diameter range at the experimental site, the importance of these three components to overall damping was in the ratio 5/4/1 for the median sized tree. Interference between neighbors depended on the distance to neighbors, as well as on the sizes of the chosen tree and its neighbors. Aerodynamic damping was larger for larger trees and the energy lost to this force was similar in magnitude to that calculated to be lost using drag coefficients from published wind tunnel and other studies. The amount of damping from the stem was linearly related to stem diameter.