© 1991 Heron Publishing—Victoria, Canada
Dynamics of swaying of Picea sitchensis
R. Milne
Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian EH26 0QB, Scotland / Received October 1, 1990
Summary
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.
