Assessing the Effectiveness of Urban Heat Reduction From Trees and Plants

Many cities and regions across the world have now committed to expanded areas dedicated to greenspace to mitigate rising temperatures and the heat-trapping effects of the ‘urban heat island’.

The definitions of ‘increased urban green space’ include wider use of street trees, maintenance of remnant urban forests, green roofs and green walls, and other plantings that replace heat-absorbing surfaces such as concrete, glass and steel.

However, questions remain: how much area is needed to provide cooling? Which plants are most suited to reducing urban heat especially as both heating and moisture demands increase?

 

Some scientists have attempted to assess urban cooling by comparing the air temperature in parks with that of nearby streets, a simple measure that would assess an urban region side by side. One analysis from a range of other studies showed that there was comparatively little benefit, just 1.0 degree Celsius of cooling during the day and less at night (Bowler et al 2010) between parks and adjacent streets.

Two studies in Hong Kong and Manchester found that street trees offered greater cooling benefits than green roofs, and that mature street tree plantings with around five per cent density were sufficient to reduce the pavement temperature by one degree (and five per cent density from saplings was enough for half a degree reduction). As another point of comparison, replacing all vegetation with asphalt raised the surface temperature by 3.2 degrees Celsius, demonstrating the definite benefit from plantings (Skelhorn et al 2014).

Thermal image on the effects of trees on urban heat

Thermal image on the effects of trees on urban heat

Researchers at the University of Hull showed that the best measure of a tree’s ability to provide cooling could be determined by its water flow, which is measured using sap flow meters that assess evapotranspiration as water is moved from the roots to the leaves and outwards to the atmosphere.

A healthy, well-watered Callery Pear (Pyrus calleryana, a widely-used urban street tree in Australia and Europe) of around four metres in height could absorb around 60 per cent of the incoming solar radiation and offer a cooling benefit around six kilowatts (about the same as a mid-sized home air conditioning unit) (Ennos 2015).

Simple assessments of urban planting benefits provide mixed answers – the challenge for urban planners, landscape professionals and others in the quest for more greenspace is to account for the right area, type, selection and maintenance of urban greenspace that keeps its cooling benefits active and functioning.

This will require modelling and analysis across the engineering, urban planning, horticultural and other disciplines to provide a better assessment of the right planting mix for a given region.

References:

  • Bowler D, Buyung-Ali L, Knight T M and Pullin A S (2012), ‘Urban greening to cool towns and cities: A systematic review of the empirical evidence’, Landscape and Urban Planning, Volume 97, Issue 3, 15 September 2010, Pages 147-155.
  • Skelhorn C, Lindley S and Levermore G (2014), ‘The impact of vegetation types on air and surface temperatures in a temperate city: A fine scale assessment in Manchester, UK’, Landscape and Urban Planning, Volume 121, January 2014, Pages 129-140.
  • Ennos, R (2015), ‘Can trees really cool our cities down?’, accessed on 19 April 2020, <https://theconversation.com/can-trees-really-cool-our-cities-down-44099>

 

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