Tracking seasonal variation in modal properties of an open-grown green ash (Fraxinus pennsylvanica)

Authors

Affiliations

Daniel C. Burcham

Department of Horticulture and Landscape Architecture, Colorado State University

Zuo Zhu

Vibration Engineering Section, Faculty of Environment, Science, and Economy, University of Exeter

Siu-Kui Au

School of Civil and Environmental Engineering, Nanyang Technological University

Abstract

Despite longstanding interest and varied applications, the vibration behavior of trees remains poorly understood, largely due to methodological limitations. To more effectively identify and partition sources of natural variation in the modal properties of trees, there is a need for more reliable estimates derived from long-term observations. Using improved Bayesian methods, the modal properties of one green ash tree were identified over an entire growing season with simultaneous measurements of nearby weather conditions and phenological observations. The analysis confirmed many features of tree vibration observed in an earlier related study, including two prevalent close, nearly orthogonal modes and amplitude dependence for modal properties, but the more extensive and detailed measurements also revealed new characteristics, including changes in the arrangement of partial mode shapes and sizeable variation in modal properties over multiple time scales. For the leafless tree, the two modes were consistently oriented in directions aligned with the planting layout of surrounding trees, but their orientation varied erratically over time, while remaining roughly perpendicular to one another, during periods with leaves. Though some of the modal property variation closely paralleled other measurements, including weather conditions and vegetative phenophases, in physically reasonable ways, the reasons for other changes were not clear. Using a detailed assessment of the governing factors for estimate uncertainty, the cause of low-quality estimates was attributed to very close or nearly identical modes, and the causes and implications of close modes for tree vibration monitoring should be a main priority for future studies.

Illustrative figure

Figure: Power spectral density (PSD) and singular value (SV) spectra computed from a one-hour time history of ambient vibration of a green ash (Fraxinus pennsylvanica ‘Sherwood Glen’). Although the PSD spectrum shows dynamic amplification around 0.8 Hz for all three axes, the SV spectrum indicates the existence of two close modes around the same frequency.

Materials

• Paper

BibTeX citation

@article{BurchamZhuAu:2025,
    Author = {Daniel C. Burcham, Zuo Zhu, Siu-Kui Au},
    Doi = {10.1101/2025.10.06.680820},
    Journal = {},
    Month = {},
    Pages = {},
    Title = {Tracking seasonal variation in modal properties of an open-grown green ash (Fraxinus pennsylvanica)},
    Volume = {},
    Year = {2025}}