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The science of urban greening

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Greener and denser cities - The Australian School of Urban Forestry, Part II

Sustainable cities mean denser cities, with a concentrated population to minimize travel times, energy use, and vehicle emissions. But with more high density housing and mixed-used buildings crowded into urban areas, where will all the trees go?


Like many Central Business Districts around the world, Melbourne's CBD is booming. The city is awash in half-built skyscrapers and construction cranes, and a glance at the municipal plan shows an urban core that will be completely transformed by new high rises in just a few short years. And yet, as I mentioned in my first post about the Australian School of Urban Forestry (ASUF) at the University of Melbourne, the city also has an ambitious plan to increase tree cover and greenspace. This is a fundamental paradox of urban sustainability that all cities are facing: go dense or go green?


The green roof option


One increasingly common way for cities to have their cake and eat it too is to make use of rooftops. While many new buildings have insulating greenroofs or private rooftop gardens for occupants, Melbourne is experimenting with public elevated parks, including Sky Park - a public square suspended 12 meters above a busy city street. While at ASUF, we visited its first phase, which also serves as the entrance to the Melbourne offices of the Arup engineering company.

Sky Park in Melbourne above Collins St.

Where space is at a premium, there's hope that greenroofs could serve double, triple, or even quadruple duty - for public park space, building insulation, stormwater capture, and habitat for pollinators and other desirable species. Research on whether and how greenroofs can serve all of these functions at once is ongoing, with the University of Melbourne and other local Australian universities at the forefront of experimental designs and monitoring of greenroof performance and green infrastructure research. This is one of the reasons I'll be in residence at the Burnley campus of the School of Ecosystem and Forest Sciences for the next month, so more on these experiments is coming soon.


The infrastructure option


Stormwater green infrastructure is another double-duty function of urban trees, which can be planted for aesthetic and recreational greenspace as well as conduits for absorbing excess water running off of roofs, streets, and other impervious surfaces. In the U.S., bioswales, rain gardens, and stormwater retention basins are a common type of green infrastructure, often referred to as Low Impact Development (LID), or Best Management Practices (BMPs) when applied to Clean Water Act regulatory compliance. In Australia, stormwater capture is part of Water Sensitive Urban Design (WSUD) intended to capture and absorb excess stormwater. Regardless of the terminology, the goals of these measures are to replace pipes and canals with living, plant-based systems including tree plantings, gardens, nature strips, or wetlands.


All modern cities have stormwater infrastructure of some kind, lining streets and directing roof runoff away from flood zones. Replacing storm gutters and concrete canals with greenspace provides more locations to weave greenspace into crowded cities. We visited a number of WSUD projects around Melbourne that use stormwater runoff for passive irrigation of tree plantings, which is a promising way to support urban tree growth without using potable drinking water for irrigation. In fact, as we heard from Chris Szota, stormwater runoff is so abundant around passively irrigated trees that small plantings have to be carefully engineered to avoid waterlogging the soil. These living experiments are showing that stormwater is a valuable resource that can be used to support new greenspace and shouldn't simply be discharged as urban waste.

Passive irrigation of tree plantings with stormwater

This is another area of expertise at the University of Melbourne, Burnley, which includes the Waterway Ecosystem Research Group.


The healthcare option


One of my favorite field trips at ASUF was a visit to the Royal Park Nature Play playground. This recent design intervention combined many functions of urban greenspace by blending existing open space with extensive community engagement, indigenous knowledge and culture, expertise in child development, and therapeutic benefits of outdoor space in a single redevelopment project. Located at the interface between the CBD, the relatively wild landscape of Royal Park, and the Royal Children's Hospital, the project is a playground with natural elements that's widely used by the public, but is also a therapeutic space for patients.


A view of Royal Park Nature Play from its designed, artifical hilltop

Therapeutic and healing gardens are an increasingly popular feature of hospitals and healthcare facilities, but I haven't encountered one as carefully woven into the larger urban fabric as Royal Park Nature Play. Broadly speaking, outcomes of various hospital and healing gardens have been positive, but as a researcher I'm also struck by how little attention has been given to clinical and post-occupancy studies that would inform exactly how they should be designed for different uses and outcomes. Yet the resources devoted to healthcare are enormous, especially in the U.S. where expenditures are the highest in the world - about 17% of the Gross Domestic Product. Given this investment, it's entirely feasible to study the outcomes of therapeutic greenspaces with the same attention that's given to other health interventions. And it's equally urgent to find effective and efficient options for livable, affordable, and equitable cities. For urban dwellers these issues are closely related, so why not coordinate healthcare investments, the built environment, and urban outcomes?


Nature inspired play space

What's striking about greenspace projects in Melbourne is how much space there really is to design urban nature in any number of combinations. However, retrofitting urban spaces in which nature was an afterthought in the original design is far more costly and challenging than integrated approaches. Unfortunately, there are still disconnects and a lack of coordination between the many departments, agencies, and stakeholders that shape the built environment and its relationship to greenspace. In this study of Melbourne-area municipal managers, Camilo Ordóñez and co-authors conclude that coordinated decision-making is as important as budgets and financial resources for effective planning and management of urban forests.


Coordination and integrated ecological planning and design are possible, and in fact, key ingredients for going greener while going denser. It can be done, but it takes a very different approach than what most cities have done in the past.


Many thanks to Sara Barron for all her work on ASUF, and to Steve Livesley for generously hosting my entire visit.

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