Some like it hot? Unequal provision of tree shading in Australian subtropical suburbs

ABSTRACT

This pilot study, set in Brisbane, Australia, provides answers to the following research questions: (1) Is there inter- or intra-suburb inequality in street tree provision, in particular along routes that lead to public transport stops? (2) Are further investments in street trees justifiable on heatwave harm reduction alone? The pilot targets three suburbs away from the urban core, which have different socio-economic levels. The ‘number of street trees per kilometre’ is used as a foundational measure and a labour-intensive (but quite granular and accurate) data collection method is adopted. Our findings point toward inequality in the provision of street trees, especially in the lower income suburb. In the two wealthier suburbs too, street trees are not regarded as a pedestrian transport infrastructure asset. At the same time, our threshold analysis shows that street tree planting is justifiable on heatwave harm reduction alone. In the future, a study of all Brisbane streets would provide more conclusive answers.

KEYWORDS:

• Street trees
• shading
• walkability
• public transport access
• Brisbane
• Australia

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 Each method has advantages and drawbacks. Not all street trees are equal: different species and maturities yield different shade and size profiles. Therefore, canopy cover is a good proxy for measuring shading. The problem is that, to measure the canopy cover of a suburb one needs geospatial analysis tools and spatial data sets, such as rasters. These are not readily available for every city and their use generally leads to inaccuracies due to misattribution (errors of commission), failure to attribute (errors of omission), or inaccuracy due to the size of the pixels used in the raster (Hare 2014; Litman 2017; Transport and Main Roads 2011). Tree counts, on the other hand, provide a measure of the potential of a suburb, and this is key in estimating the longer-term preparedness of an area to factors such as climate change. The tree count method also captures recent tree planting initiatives, which are on the rise due to global warming concerns. In addition, counts are a simpler, more accurate, and more reliable measure of the presence of street trees at a given point in time. The two methods, canopy cover and tree counts, are becoming less mutually exclusive thanks to advances in LiDAR technology (Wu et al. 2013; Zhang, Zhou, and Qiu 2015; Department of Prime Minister and Cabinet 2021).