Sway frequency analysis of wind-loaded trees under freezing conditions

Tree sway frequencies are thought to significantly change as freezing conditions change their material properties and increase flexural stiffness. There is a strong coupling between plant motion and wind. Seasonal freezing temperature will increase the stiffness of the canopy and should alter the coherent gust structure within the roughness sub-layer. This study has implications for improving understanding of flows in the lowest layer of the atmosphere by quantifying any change in sway tree sway frequencies at temperatures <-10º C. Beginning October 2009, the sway of select trees in Howland, ME were measured using biaxial clinometers. Wind speed, wind direction and temperature were measured using sonic anemometers, with all measurements at 10 Hz. Datasets (2-3 hours in duration) were chosen when the hourly average of wind speed was approximately 2 m/s above the canopy. Two conditions were considered in the analysis of the data, one with temperatures well above freezing (11º and 15º C) and the other with cold temperatures (-18 º and -10º C) where the trees are thought to be frozen and have altered material properties. Wind directions were consistent between datasets with standard deviation of less than 7 degrees. Clinometer data was converted from rotation angles to displacement distance. Tree sway displacements were transformed into the frequency domain using a Fourier analysis. A preliminary analysis shows a 9-12% increase in sway frequency for cold trees compared to warmer trees.