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Tree Canopy Interception

FLASHBACK TO 2012: “Given the huge knowledge bases that the sciences have built up around the hydrology of urban watersheds, it can come as a surprise when we realize how little is known about some of the basics. The urban tree canopy is an example,” stated (the late) Dr. Charles Rowney, Scientific Authority, when the Partnership for Water Sustainability announced the addition of a Tree Canopy Module to the Water Balance Model


“Tree canopy interception of rainfall is a technical area where the fundamentals are well understood, but the empirical basis, the availability of actual observations, is still in its infancy. When it comes to the urban canopy, we just don’t have a lot to go on. Considering the importance of urban trees, we’re not sure why this knowledge gap has persisted. But when we began our research, it quickly became clear that there is a lot to learn about some of things that are important in dealing with the tree canopy. And it became just as clear that we had to improve the science,” stated the late Dr. Charles Rowney.

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URBAN TREES AS A RAINWATER MANAGEMENT TOOL: “We found that individually planted trees capture, store and release stormwater back into the atmosphere—a process called transpiration—at a rate three times that of trees in a forest,” stated doctoral candidate Sarah Ponte, Department of Environmental Science and Technology at the University of Maryland (November 2021)


“We explored how trees function in different urban contexts, from streets to small patches of forest. This is knowledge that can help support the management of green infrastructure,” stated Sarah Ponte. “Understanding how different management contexts affect urban ecohydrologic fluxes, such as transpiration, can aid the development of policy on the application and effectiveness of urban tree canopy as a tool for stormwater runoff reduction at watershed and city scales,”

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HOW TREES COULD SAVE THE CLIMATE: “Our study shows clearly that forest restoration is the best climate change solution available today, and it provides hard evidence to justify the investment,” stated Professor Thomas Crowther, Swiss Federal Institute of Technology (July 2019)


Around 0.9 billion hectares of land worldwide would be suitable for reforestation, which could ultimately capture two thirds of human-made carbon emissions. This would be the most effective method to combat climate change. “We all knew that restoring forests could play a part in tackling climate change, but we didn’t really know how big the impact would be. But we must act quickly, as new forests will take decades to mature and achieve their full potential as a source of natural carbon storage,” stated Thomas Crowther.

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URBAN TREE CANOPY: “A new science of valuing nature will shape our urban projects of the future,” says Bonnie Keeler – her area of expertise at the University of Minnesota is natural capital and the value of ecosystems


Bonnie Keeler reviewed 1,200 scientific studies on increasingly popular green infrastructures such as urban forests, parks, rain gardens, and wetlands and found in a recent paper that it’s unclear how well any of them stack up against “gray” solutions like concrete storm sewers and air conditioning. “There is a huge interest in expanding funding for green infrastructures,” she said. “But we don’t have a tool to understand their value.” Keeler’s review found that most evaluations of urban trees focused on two benefits: filtering air and sequestering carbon. Few considered the costs of maintenance, replacement, or public health..

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URBAN TREE CANOPY: “What the economy really needs is more trees” – Ross Gittins, the Sydney Morning Herald’s economics editor


“Planting trees in parks, gardens or streets has many benefits, helping to cool cities, slowing stormwater run-off, filtering air pollution, providing habitat for some animals, making people happier and encouraging walking,” wrote Ross Gittins. “Shading from strategically placed street trees can lower surrounding temperatures by up to 6 degrees – or up to 20 degrees over roads. Green roofs and walls can naturally cool buildings, substantially lowering demand for air-conditioning.”

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USE OF ‘MACHINE LEARNING’ TO DISTINGUISH BETWEEN TREE CANOPY AND TREES: “Trees are pretty hard to map. So, what’s the solution if we want to map tree canopies in places with complex geographies? How do we fill in the gaps between official street tree census and trees in parks and on private property?” – Tim Wallace, geographer for the New York Times


“(Tree) surveys are expensive to conduct, difficult to maintain, and provide an incomplete picture of the entire extent of the urban tree canopy. Both the San Francisco inventory and the New York City TreesCount! do an impeccable job mapping the location, size and health of street trees, but exclude large chunks within the cities, like parks,” wrote Tim Wallace. “This data gap is neither accidental nor purposeful. The trees they mapped were a product of bureaucratic choices and limitations.”

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‘Green roof’ bus shelter incorporates the concept of forest architecture: “The prototype and research will help justify whether a larger investment into such an idea would be worth it,” said Tabinda Shah, UBC student


The roof or shelter would be made of treated wood that can withstand the elements and host a layer of plants that are hardy and succulent, and can thrive in not just the rain but the dry months too. The excess water from the roof would run off into the ground to recharge the water table. “We’re hoping to have the prototype constructed along Wesbrook Mall at the University of British Columbia, but in an ideal world, we would want these all over the city street networks of Vancouver,” Tabinda Shah said.

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Make Cities Greener and Cooler: Australia to set goals for increasing Urban Tree Canopy


“We will work with Australian cities to set decade by decade goals out to 2050 for increased overall tree coverage,” stated Acting Minister for Cities Greg Hunt. “Green cities — cities with high levels of trees, foliage and green spaces — provide enormous benefits to their residents. Increasing urban canopy coverage decreases heat, which improves health and quality of life.” He emphasised the susceptibility to extreme heat of people living in large cities. Urban development pressures can lead to treeless streets that amplify the “heat island” effect on hot days.

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“We have been investigating the environmental benefits of trees in urban areas, and their role in climate-proofing our cities," reports Professor Roland Ennos, University of Hull


“To determine whether the humble tree really can provide such robust defences, we first need to understand the role they play in soaking up excess rain water. All floods, whether fluvial (when rivers burst their banks) or pluvial (when rainfall overwhelms drainage systems before it reaches rivers), are caused because the rain cannot soak into the soil fast enough. And while climate change is causing bigger and bigger storms, our alterations to the environment – especially to the ground surface – have been one of the major causes of the increased frequency of flooding events in modern times,” wrote Roland Ennos.

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"We often take trees and green spaces for granted, but we shouldn’t," writes David Suzuki


“Urban forests contribute greatly to reducing greenhouse gas emissions. Urban trees also help us adapt to and cope with climate change impacts by shading communities during periods of extreme heat. The unique, multi-purpose benefits of living, green infrastructure make it an incredibly valuable tool for cities and towns to improve resiliency in the face of climate change,” says David Suzuki. “Recognizing urban trees as infrastructure assets opens up new ways to assess their value and justify investment in their maintenance.. Living, green infrastructure increases in value over time, unlike grey infrastructure.”

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