Master of Engineering Science
Civil and Environmental Engineering
Environment and Sustainability
Hangan, Horia M.
Trees grown in urban environments provide environmental, economic and psychological benefits to their surrounding communities. However, urban trees also pose significant risks since damaged trees can cause serious harm to people, housing, and infrastructure by falling on sidewalks, roads, houses or power lines. To better understand the risk posed to trees by wind, models have been developed that estimates the required wind speed needed to damage a tree or group of trees, and the likelihood that such a wind speed is met or exceeded annually. The importance of such models is rising each year as the associated risk grows as well, due to an increase in urbanization, frequency and intensity of wind storms increasing with global warming and growing evidence that elevated atmospheric CO2 concentrations, driven by climate change, cause trees to grow faster and larger, likely increasing their fragility to wind. In this thesis, a model was created to consider the impacts of climate change on trees’ risk using analysis of wind trends globally and locally in the Toronto region, and by considering the impact of the steadily increasing concentration of CO2 in the atmosphere. The CO2 increase impact on trees has been inferred based on meta-analysis data from 219 papers studying the impact of elevated CO2 growing conditions on 293 tree samples of varying age and species. The model functions by estimating the return period of wind storms that can damage an individual tree via trunk rupture or overturning. Meta-analysis data indicates that the density of leaves in tree crowns is likely to change with elevated CO2 concentrations. The aerodynamic impact of this change is currently not well understood. In an effort to improve the model further, experimental wind testing was conducted at the Wind Engineering, Energy and Environment Dome (WindEEE) at Western University, where a 9-year-old, 1.9 m tall red maple (Acer rubrum) was subjected to wind speeds from 6-12 m/s. The testing was repeated 5 times, between each repetition the crown was thinned by 25% to simulate varying crown leaf densities of the tree, and to analyze the relationship between the density of leaves in the crown and the drag coefficient.
Summary for Lay Audience
Trees grown in cities provide many benefits to their surrounding communities, but they can also be quite dangerous in the event of a storm. If a tree is damaged in a wind storm, it can cause serious harm to people, housing, and infrastructure by falling on sidewalks, roads, houses or power lines. To better understand the danger that trees present to cities, scientific models have been created to estimate the intensity of a wind storm that would be required to damage a tree, and the likelihood that such a storm will occur each year. The importance of this type of modelling is increasing each year due to rapidly rising urban populations, growing evidence that climate change is frequent and severe storms. Many studies also have shown that climate change is increasing the concentration of CO2 in the atmosphere which is causing trees to grow faster and larger, making them more likely to be damaged in wind storms. This research proposes a new scientific model that considers the impacts of climate change on trees’ risk to wind damage using data local to the Toronto region, as well global data about the frequency and intensity of wind storms occurring, and by considering the impact of the steadily rising concentration of CO2 in the atmosphere. The impact of increased CO2 is estimated from 219 other scientific studies where trees of varying species and age were grown in high CO2 environments to measure how they grew differently. The model works by estimating how often it is expected that a wind storm will occur that could damage the simulated tree, either through overturning or trunk breakage. The model found that the density of foliage in the tree crown is likely to change with elevated CO2. The impact of this change is not very well understood currently in the scientific community, so to improve on this, experimental wind testing was conducted at the Wind Engineering, Energy, and Environment Dome (WindEEE) at Western University. In this testing, a 9-year-old, 1.9 m tall red maple was subjected to wind speeds from 6-12 m/s (21-43 km/h). During this testing, approximately 25% of the leaves on the tree were removed at a time to simulate a change in the density of leaves in the tree crown, helping us learn more about how the leaf density impacts the wind forces experienced by the tree, and improving the model.
Woolsey, Samuel J., "Effects of Climate Change on the Probability of Urban Tree Failures from Wind Gusts" (2022). Electronic Thesis and Dissertation Repository. 9060.
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