Quantifying the effects of freezing on tree sway frequencies

Wind induced sway was measured for 18 conifers in an even aged, naturally regenerated evergreen forest in Howland, ME between August 23, 2009 and January 29, 2010. Tree sway was measured using biaxial clinometers and wind speed, wind direction and temperature were measured using sonic anemometers, with all measurements at 10 Hz. This study quantifies changes in wind-loaded tree-sway frequencies for frozen and unfrozen stems. A tree's sway frequency significantly changes as freezing conditions change its material properties by increasing flexural stiffness. This study found that sway frequencies of frozen trees are increased by 27.9%, but frozen trees loaded with snow only exhibit an increase of 9.4%. The effects of atmospheric stability were also considered and were found to have no statistically significant effect on tree sway frequency. Datasets (30 min in duration) were chosen when the hourly average of wind speed was >2 m/s above the canopy. Three conditions were considered in the analysis of the data, one with temperatures well above freezing (>5 °C) one with freezing temperatures (<−5 °C) and a snow load on the branches and one with freezing temperatures (<−5 °C) and no snow load on the branches. The trees are thought to be frozen and have altered material properties at temperatures <−5 °C. This study has implications for improving the accuracy of mechanistic tree sway models as well as improving our understanding of air flow in the lowest layer of the atmosphere.

► This study measured in situ tree sway under natural dynamic wind loading.
► Tree sway frequencies increased by 27.9% when trees were frozen.
► Frozen trees loaded with snow only exhibit an increase of 9.4%.
► Atmospheric stability was found to have no statistically significant effect on tree sway frequencies.