Swaying of trees in response to wind and thinning in a stand of Scots pine

Observations have been made of the windspeed, wind direction, and tree movement at the edge and 20 m within a stand of Scots pine (Pinus sylvestris L.) close to 11 m in height. The spectra of windspeed near canopy top, together with the output of accelerometers and video observations of tree movement at mid-crown, were compared in the same stand prior and two years after first thinning. Furthermore, the transfer of wind energy into tree movement was investigated by calculating the mechanical transfer function (H-m(2)) between the wind spectrum (S-uu) and the tree's response (S-yy), i.e. H-m(2) = S-yy/S-uu. Trees were found to behave like damped harmonic oscillators. They reacted to sudden increases in windspeed, reached their greatest displacement during the first cycle, and then returned to their rest position under the influence of damping. The spectral peak frequencies in S,, and in H-m(2) coincided with the estimated natural sway frequency of trees. Response in the second mode was, however, also evident, especially within the unthinned stand. The periodogram plots showed a consistent trend of a marked decrease in the response of the tree to increase in frequency. Almost no difference in the wind energy transfer, i.e. peak frequencies and peak width, and damping of the system was found between Scot pine at 2700 and 1500 stems per hectare. However, along the stand edge tree movement was greater than within the stand indicating greater wind energy transfer and damping of the system along the stand edge than within the stand.