Characterizing relationships between tree biomechanics and 3D tree crown structural characteristics and assessing effects of drought and management

Background/Question/Methods

In urban-exurban landscapes coupled forest-infrastructure systems along forest edges are susceptible to damage associated with disturbances such as pest and pathogen outbreaks, drought, and storm events, the frequency, severity, and extent of which are increasing under climate change. These stressors affect the health, stability, and biomechanical properties of trees and the ability to withstand severe wind events. Disturbances and management interventions also alter characteristics of tree structure and morphology (e.g., branch and crown structure), which may be associated with tree failure and associated infrastructure damage. This study assesses the relationship between 3D crown structure and tree biomechanics, and characterizes the effect of drought on tree sway dynamics. Terrestrial laser scanning (TLS) and Unmanned Aerial Vehicle LiDAR (UAV-LiDAR) have been fused to create high resolution quantitative structural models (QSMs) for individual trees from which measures of crown structure are calculated (i.e., crown asymmetry, crown area, and crown density). Tree biomechanical properties are assessed using accelerometers that capture roll and pitch displacement in degrees of tilt at a frequency of 10hz. Accelerometers were installed on trees along forest roadsides on the University of Connecticut’s campus and ongoing experimental drought study plots on Thompson Farm, on the University of New Hampshire.

Results/Conclusions

Using TLS and UAV-LiDAR, QSMs were created for 34 individuals from common tree genera in Southern New England (e.g., Quercus spp., Acer spp., and Carya spp.) across three roadside sites in Connecticut. QSMs provide high resolution information for determining structural crown metrics that affect tree sway dynamics. Crown asymmetry, crown area, and crown density varied among species and individuals sway responses. In the ongoing experimental drought study located at the University of New Hampshire, Durham in New Hampshire, United States, 10 trees are equipped with accelerometer-based data loggers to capture differences in tree sway dynamics for both isohydric and anisohydric species, including Pinus strobus, Acer spp., and Quercus spp. Sample trees included those exposed to approximately 50% throughfall removal over seven years and control plots without throughfall removal. We observed significant differences in biomechanical properties among species regardless of exposure to throughfall removal. Linking crown structure to tree biomechanics has provided an increased understanding of the effects stressors have on roadside tree structure/movement as droughts increase in frequency and severity under climate change. A next step is to experimentally alter structure through tree trimming.