From Aesthetics to Green Infrastructure
Street trees can have a tremendous impact on the quality of public spaces relative to cost (Moore, 2009) and the choice of street tree species, their arrangement and spacing are an integral part of street design in many cities built or renewed during the 19th century (Nadel, Oberlander, & Bohm, 1977). Trees have been used in urban street design to provide symbolic definition between streetscape uses, visual markers for specific human activities and streetscape enclosure (Lawrence, 2008). Urban street trees have also become a valuable visual and physical connection for urban residents with nature (Beatley, 2011).
While these visual and symbolic uses are still important design considerations, in recent years there has also been a growing recognition of the ecosystem services trees contribute to urban environments (Ely, 2009; Young, 2010). These functional contributions include mitigation of the Urban Heat Island Effect through surface shading (Lindberg & Grimmond, 2011), air temperature cooling through evapotranspiration (Shashua-Bar, Tsiros, & Hoffman, 2010), grey water filtration (Gómez-Baggethun & Barton, 2013), pollution filtration (Maher, Ahmed, Davison, Karloukovski, & Clarke, 2013), noise attenuation, reduction of pedestrian UV exposure and thermal comfort (Parisi, Kimlin, Wong, & Wilson, 2001, 2000).
This shift in the perception of urban trees from a purely aesthetic consideration to both aesthetic and infrastructural or structural consideration, strengthens the argument for their continued inclusion in cities (Mullaney, Lucke, & Trueman, 2015). For heavily constrained urban sites, trees provide the highest ‘green plot’ ratio of all vegetation types taking up a comparatively small amount of valuable urban real-estate (compared with lawn for example), for a high return in heat mitigation, water sensitive urban design (WSUD), aesthetics and ecology, making them ideal for inclusion in urban systems (Ong 2003). However, the complicated and competitive spatial and climatic environment of more heavily developed sites increases the need to model, quantify and advocate for the microclimatic benefits provided by trees (Oke, 1988). Urban densification is associated with increasing percentages of impervious surfaces, exacerbating the impact of heat in cities (Mullaney et al., 2015).
Melbourne, Australia, for example, is getting bigger, denser and hotter (Metropolitan Planning Authority, 2014). Melbourne’s heat issues are intensified in urban streets with large areas of unshaded, dark, impermeable pavements such as the most frequently found asphalt and bluestone (Nakayama & Fujita, 2010). The shading of these surfaces is possibly the most substantial contribution trees could make to heat mitigation particularly areas of low and mid-rise density (Coutts, 2014; Mohd Sanusi, 2015).
In addition, Melbourne is experiencing increased extreme weather events including prolonged and intense heat waves (Bureau of Meteorology, 2013) and altered precipitation patterns (Akompab et al., 2013) which are affecting the range of plants which can be successfully grown as street trees (Mitchell, O’Grady, Hayes, & Pinkard, 2014). In Melbourne, a large proportion of the current tree stock has reached the end of its useful life expectancy due partially to environmental damage and partially to consistent planting age, requiring a substantial renewal over the coming decade (Shears, 2009).
With this major tree-stock renewal challenge, comes a unique opportunity to reconsider not only the kinds of street tree pallets that may be more suitable to the local climate, but also reconsider design and decision-making processes involving trees to increase engagement with spatial and temporal implications. There is potential to redesign the structure and layout of some streets to maximise the ecosystem services and heat mitigating functions trees can provide within the street environment if they are strategically located and afforded with optimal growing conditions (Bolund & Hunhammar, 1999; Dobbs, Kendal, & Nitschke, 2014; Escobedo, Kroeger, & Wagner, 2011).
In order to avoid unsatisfactory or status quo tree species and positioning during this renewal period, sophisticated decision support and visualisation tools capable of garnering public support for tree decisions will be required (Dobbs et al., 2014). Traditional 2D modelling (plan, section and elevation drawings), usually employed for communication of street tree choices, lack integration of science based information and will not be adequate for the challenging and complex decision making required (Escobedo et al., 2011).
This paper presents a review of current methods of tree selection modelling in urban design and touches on relevant techniques and technological advances in tree modelling in a range of other disciplines including ecology and computational botany, defence and entertainment industries, and illumination and architectural sciences, which can be adopted by the design profession to better inform decision making.
Based on these findings, the authors then put forward an approach that brings together disparate data sources, recent advances in software and hardware, with digital tree modelling techniques which enables a clearer, more quantifiable method of assessing the spatio-temporal and ecosystem service implications of design decision making involving trees. Based on the classic definition of a decision support system: an architecture that accommodates ‘a system of systems’ facilitating customisation and flexibility fit for a specific purpose (Batty, 2016; Geertman & Stillwell, 2009), the authors outline a prototype for an urban street tree decision support system (DSS).
The prototype DSS utilises computational ‘geometric’ plant modelling, traditionally focused on the architectural form of plants and developed around the needs of agronomy and forestry, (De Reffye & Hu, 2003) and defence (Weber & Penn, 1995), together with “parametric proxy-object” modelling developed for the entertainment industry (Rebollo, Remolar, Chover, Gumbau, & Ripollés, 2007), with photometric daylight analysis modelling (Kota, Haberl, Clayton, & Yan, 2014). The decision support system is explored through two case studies in the north-west of Melbourne. The authors use the approach to output both informative and compelling visualisations which demonstrate the visual impact of tree renewal strategies, in both the short and long terms coupled with measurable ecosystem services, particularly quantifying the provision of shade.