Monitoring Tree Sway as an Indicator of Water Stress

Understanding and predicting future consequences of increasingly frequent and intense droughts requires improved monitoring of forest response. Over the course of a day, tree mass and stiffness respond dynamically to changing atmospheric and hydraulic conditions. By conducting a 24-hr experiment, we sought to disentangle the effects of changing mass and stiffness on tree sway period. We observed that tree mass and stiffness are influenced by changes in tree water content and that diurnal changes in tree sway period are chiefly driven by the loss and recovery of tree stiffness. Over a season-long time series in two Quercus rubra (red oak) trees, we observed more pronounced and substantially higher midday increases (+7%) in sway period during days with the driest soil moisture (<0.09) as compared to days when soils were wetter. These findings suggest that continuously monitoring tree sway period offers an innovative approach to detecting water stress in trees.