Responding to Future Climatic Conditions of Heat and Flood: A Design Decision Support System for Street Trees

Visual Benefits of Trees

Trees are often an integral part of urban design in new or renewed cities of the 19^th century, profoundly influencing the quality of streetscapes. Traditional urban street tree planting is arranged in evenly spaced symmetrical rows, reflecting the influence of French Formal compositional ideas on urban design during the expansion of the British Colonial Empire (Lawrence, 2008). These historic, formal tree species, placement and spacing choices were driven by visual and symbolic principles of the time, rather than environmental concerns. Visually, formal street tree arrangements ‘provide a sense of streetscape enclosure’, ‘define the street as an outdoor room’, and ‘distinguish a boundary between different street uses’ (Arnold, 1980; Ewing & Handy, 2009). However, over the past century, urban trees have become increasingly valuable to cities for reasons beyond the visual, expanding to include their functional attributes in moderating environmental problems related to the impacts of climate change.

Environmental Benefits of Trees

City streets are complicated, spatially competitive, and climatically diverse environments. Trees, as opposed to low shrubs, groundcover and lawn, are particularly important for spatially constrained city streets, as they require comparatively small amounts of high value land for substantial environmental and human health returns or a high ‘green plot ratio’ (Ong, 2003; Taylor et al., 2015). The environmental returns of urban trees are increasingly understood to be extensive, including noise attenuation and air pollution filtration through particulate interception (Maher et al., 2013), urban heat moderation through surface shading (Lindberg & Grimmond, 2011), air temperature cooling through evapotranspiration (Shashua-Bar et al., 2010), reduction of pedestrian UV exposure (Parisi et al., 2000, 2001), and storm water management, through water absorption or the ‘sponge’ actions of the root system, and canopy rainfall interception (Carlyle-Moses et al., 2020; Szota et al., 2018).

Melbourne Context

Melbourne, Australia, is an illustrative example of a heat and flood prone city, seeking to harness urban forestry to improve climatic resilience. Melbourne and its local government areas (LGAs) have large proportions of low diversity, over mature tree stock planted during colonial settlement, that struggled to withstand the impacts of urban expansion, development intensity, and climate change, particularly the prolonged heat waves and flash flooding of the past two decades (BOM, 2013; Rizmal, 2019). Extensive amounts of these historically significant existing trees now require urgent replacement.

With this major tree-stock renewal challenge, comes a unique opportunity to reconsider not only street tree pallets, more suitable for the local climate, but also the design and decision-making processes for tree selection to respond to spatial and temporal conditions and implications (Dobbs et al., 2014; Langenheim et al., 2020; White & Langenheim, 2018b). For instance, shading the impermeable, dark asphalt and bluestone surfaces of Melbourne’s wide exposed streets with strategically placed trees would substantially contribute to heat mitigation, particularly in areas of low and mid-rise density (Coutts, 2014; Mohd Sanusi, 2015), while the speed of storm water flow in flash flood events could be reduced through increased used of dense canopied trees to intercept rainfall (Carlyle-Moses et al., 2020).