Right tree, right place, right time: A visual-functional design approach to select and place trees for optimal shade benefit to commuting pedestrians

Highlights

• A performance-based design approach to enable visual-functional decisions on urban treescapes.

• Simulated tree shade benefits of streetscape design scenarios.

• Spatially explicit modelling and visualisation of treescape design and ecosystem service.

• Optimised tree shade benefit to encourage active school journeys by school children.

Abstract

Australia tops the world’s charts in occurrence of skin cancer and intensity of heat waves, while concurrently achieving high childhood obesity levels, due in part to low rates of physical activity. These issues converge in the challenge of protecting school children from heat and ultra-violet light exposure whilst simultaneously encouraging them to select active modes of transport for school journeys.

This paper describes a new performance-based tree-scape design approach for quantifying shaded walking routes using the pedestrian accessibility modelling tool “PedestrianCatch”, combined with visual-functional tree-scape modelling for both solar impact analysis and qualitative aesthetic outcomes of different street tree-scape designs. We test this design approach on a precinct surrounding a school as a case study.

The study results demonstrate the potential for targeted strategic street tree selection and planting in proximity to schools, providing the co-benefits of improved thermal comfort and reduced solar and ultra-violet exposure of children walking home from school. This performance-based design approach offers local government, public health and education departments with a way to mediate multiple and divergent concerns for climate amelioration, transport choices and population health by planting the right tree, positioned to provide shade in the right place at the right time.

Introduction

Urban tree-scape design decision making, has traditionally been undertaken by landscape architects, often in conjunction with community groups in public participatory forums (PPD), with a focus on qualitative visual impact (Arnold, 1980). In recent decades, the functional potential of street trees to improve many emerging urban environmental and social problems has caused a shift in the perception of the role of trees in cities (Bolund & Hunhammar, 1999; Freeman, Herriges, & Kling, 2014). This perceptual shift is reflected in the development of new terminology describing urban vegetation, for instance, as a form of ‘green infrastructure’ (Bélanger, 2009; Benedict & McMahon, 2002) and the development of quantitative approaches, adapted from financial modelling to calculate functional tree benefits; known as ecosystem service science (Costanza et al., 1998; Livesley, McPherson, & Calfapietra, 2016). The traditional qualitative, visually focused design approach and the quantitative functional ecosystem services approach to tree decision-making share many objectives, such as providing shade for pedestrians or the increase in thermal comfort of urban areas, but their methods and focus differ. Traditional design approaches use methods which emphasise production of clear communication imagery while ecosystem service science emphasises the role of provisioning, supporting, regulating and cultural services of trees and their benefit to human or environmental health (Carpenter et al., 2009; Costanza et al., 1998). What is missing is a performance-based tree-scape design method, which blends these two approaches, and can be used to analyse both the visual, as well as the human and environmental performance (ecosystem services) of tree-scape decisions (Olander et al., 2017; Rosenthal et al., 2015; Vogt et al., 2017). Along with the targeted visual and ecosystem service benefits of any performance-based approach, there may also be additional ‘co-benefits’ or disservices (Pataki et al., 2011).

Customarily, trees have been planted in symmetrical rows on either side of a street, stemming from the popular influence of French formal design on urban compositional ideas during British colonial expansion in the 19^th century (Dover & Massengale, 2013; Lawrence, 2006). While this symmetrical layout of trees has many aesthetic benefits (Arnold, 1980), depending on the orientation of the street, the time of day, the season, geographic location of the city, and the pedestrian peak-use time, this symmetrical street verge planting design can leave pedestrians exposed to solar radiation and vulnerable to heat (Norton et al., 2015).

A peak time of pedestrian footpath use is the mid-afternoon homeward journey undertaken by many school children, at approximately 15.30. School children who use active transport modes, such as walking and cycling for this journey, are more likely to reach recommended daily levels of physical activity, become adults with healthier lifestyle habits and are less likely to suffer from health problems such as cardio vascular disease and obesity (Faulkner, Buliung, Flora, & Fusco, 2009; Larouche, Saunders, Faulkner, Colley, & Tremblay, 2014). Encouraging walking and cycling to and from school is therefore clearly important, but several barriers are contributing to the steady decline of active modes for this journey (Larsen, Buliung, & Faulkner, 2015). These barriers include poor perceptions of aesthetics, child safety, and comfort of the street environment and the risks of exposure to harmful levels of air pollution, Ultra Violet (UV) radiation and heat (Bertazzon & Shahid, 2017; Dirks, Salmond, & Talbot, 2018; Huang, Lin, & Lien, 2015; Kim, Lee, & Jun-Hyun, 2018; Lee, Zhu, Yoon, & Varni, 2013; Sweeney & Von Hagen, 2016). Minimising these barriers is therefore a critical issue and one which can be in part, addressed through development of performance-based, spatially explicit design methods for strategic placement and selection of trees.

Tree canopy cover provides protection from both heat stress and UV exposure (Grant, Heisler, Gao, & Jenks, 2003; Green, Wallingford, & McBride, 2011). Increasing tree canopy coverage in streetscapes which lead to and from schools has been shown to encourage active transport mode selections for school journeys due to better perceptions of safety, aesthetic value and thermal comfort (Handy, Boarnet, Ewing, & Killingsworth, 2002; Parisi & Turnbull, 2014). However, for tree canopy cover to provide these benefits, adjustments to traditional tree placements and species selections are required that consider street orientation, surrounding built form, season and geographic location (Rantzoudi & Georgi, 2017; Sanusi, Johnstone, May, & Livesley, 2016).

While tree shade is not the only factor which reduces urban heat and UV exposure, it is often the most important contributor. Tree shade has been shown to lower surface temperatures by up to 20 °C and intercept significant amounts of UV radiation (Ali-Toudert & Mayer, 2007; Grant et al., 2003; Mayer, Holst, Dostal, Imbery, & Schindler, 2008; Na, Heisler, Nowak, & Grant, 2014; Thom, Coutts, Broadbent, & Tapper, 2016). Tree shade may only lead to a relatively small reduction in air temperature beneath the canopy, but the change in other climate factors under tree shade, such as relative humidity, wind speed, and overall radiation loads mean that perceived human thermal comfort is greatly improved (Sanusi, Johnstone, May & Livesley, 2017).

The importance of tree shade as a component of human thermal comfort can be quantified in several ways, though in general for a simple ecosystem service benefit calculation, it can be measured directly (using shoulder attached dosimeters) as comparative periods of time in or out of shade over total time spent outdoors (Boldemann et al., 2006; Pagels, 2017). It is more difficult to model the impact of shade on human activity, as agent-based models which include complex human behaviours that react to changes in thermal conditions are required (Melnikov, Krzhizhanovskaya, & Sloot, 2017). A more common modelling approach for thermal comfort is to calculate the shade at one point in the day when the sun is at the highest point in the sky (Solar noon), and the tree canopy outline is most closely aligned with the shade cast below (Norton, Coutts, Livesley, & Williams, 2013; Sanusi et al., 2016).

Street tree planting is not simple to re-configure as several complex interactive constraints need to be considered in decision making. If the primary objective of planting trees in streets was to provide pedestrian shade, and there were no constraints as to what tree species could be selected or their planting positions, then this objective could be achieved without trepidation. There are however several constraints that obstruct change to current tree selection, planting location and management practices. Some of these constraints are spatial, such as the need to accommodate existing services within street easements either above or below ground. Some constraints are environmental, such as changing diurnal and seasonal sun positions, suitable tree rooting volumes and challenging urban soil conditions (Dobbs, Kendal, & Nitschke, 2014). Some constraints are visual, such as historical or community visual and cultural preferences (Gobster, Nassauer, Daniel, & Gary, 2007; Lange & Hehl-Lange, 2011). These competing and interacting constraints affect tree-scape performance, and though they may be discussed during decision-making, are rarely recognised or visualised together in a way which portrays the complexity and possible compromise solutions that can be made. As such, some visual or environmental performance criteria or opportunities can be overlooked during tree-scape planning against more regulatory demands of traffic flow or infrastructure service provision (Batty, 2013; Grêt-Regamey, Celio, Klein, & Hayek, 2013; Rosenthal et al., 2015).

Visual simulation of trees in digital modelling platforms is an area of research for many disciplines from the gaming industry, defence, hydrology catchment management to agroforestry (Prusinkiewicz & Lindenmayer, 1990). While the uses of digital tree models may differ, many are constructed from the recursive branching methods developed by Honda (1971). These visually and structurally realistic three-dimensional, polygon dense tree models, have been shown to have high accuracy for environmental simulation (Sadeghi & Mistrick, 2018) but have required considerable computing power. Similar can be said of microclimate performance simulation undertaken in the early stages of design projects. Early attempts to compute microclimatic performance of designs initially often took many hours to run, making them unfeasible for adoption in industry (He & Hoyano, 2009). However, recent advances in computer software and hardware have resulted in increased rendering speed and efficiency which now make it feasible to both import multiple digital polygon-dense trees into large 3-dimensional urban precinct simulation projects and calculate basic microclimatic impacts (Belok, Rabea, Hanafi, & Bastawissi, 2019; Gill, 2013; Jabi, 2016; White & Langenheim, 2018).

Can tree-scape design be undertaken in a strategic and flexible way so that designers can quantify ecosystem service performance of trees for footpath shading of school children in pedestrian walking catchments, whilst still communicating traditional design performance objectives such as visual impact?