OOS 11-3 - Monitoring tree sway signals as indicators of water-tree interactions in built environments

The sway period of a tree (the time it takes for a tree to sway back and forth and back again) is dependent on various biomechanical properties, such as height, diameter, mass, and stiffness, among others. Here, we show how water-tree interactions influence biomechanical properties of trees resulting in the potential to monitor continuous measurements of tree sway acceleration as indicators of processes like interception and water stress processes. To monitor interception, we developed a new field monitoring approach that leverages accelerometers on trees to quantify continuous, dynamic time series of canopy water storage during storms. Using this approach, we additionally observed a hysteretic interception response in tree canopies, suggesting that interpreting interception processes through tree sway signals requires consideration of changing water (i.e., mass) distribution during and following storms. To monitor water stress in trees, we explored how changing soil water availability conditions influence tree sway dynamics over a growing season. We found that changes in tree sway patterns associated with water stress resulted from more substantial changes in tree stiffness than mass changes within the tree. Combined, continuous monitoring of tree sway period offers a potential opportunity to quantify multiple biophysical functions of trees. This advancement in whole-tree monitoring may help improve our understanding of how ecohydrological processes like interception and water stress affect ecosystem water availability, productivity, and runoff dynamics that are important for quantifying ecosystem services in cities.