ORIGINAL PAPER Temperature and human thermal comfort effects of street trees across three contrasting street canyon environments Andrew M. Coutts & Emma C. White & Nigel J. Tapper & Jason Beringer & Stephen J. Livesley Received: 13 August 2014 /Accepted: 3 February 2015 # Springer-Verlag Wien 2015 Abstract Urban street trees provide many environmental, so- cial, and economic benefits for our cities. This research ex- plored the role of street trees in Melbourne, Australia, in cooling the urban microclimate and improving human thermal comfort (HTC). Three east–west (E–W) oriented streets were studied in two contrasting street canyon forms (deep and shal- low) and between contrasting tree canopy covers (high and low). These streets were instrumented with multiple microcli- mate monitoring stations to continuously measure air temper- ature, humidity, solar radiation, wind speed and mean radiant temperature so as to calculate the Universal Thermal Climate Index (UTCI) from May 2011 to June 2013, focusing on sum- mertime conditions and heat events. Street trees supported average daytime cooling during heat events in the shallow canyon by around 0.2 to 0.6 °C and up to 0.9 °C during mid-morning (9:00–10:00). Maximum daytime cooling reached 1.5 °C in the shallow canyon. The influence of street tree canopies in the deep canyon was masked by the shading effect of the tall buildings. Trees were very effective at reduc- ing daytime UTCI in summer largely through a reduction in mean radiant temperature from shade, lowering thermal stress from very strong (UTCI>38 °C) down to strong (UTCI> 32 °C). The influence of street trees on canyon air temperature and HTC was highly localized and variable, depending on tree cover, geometry, and prevailing meteorological conditions. The cooling benefit of street tree canopies increases as street canyon geometry shallows and broadens. This should be rec- ognized in the strategic placement, density of planting, and species selection of street trees. 1 Introduction Street trees are an important part of the urban landscape, with the potential to improve amenity, provide stormwater quantity and quality benefits, and reduce building energy use (McPherson et al. 2011). Street trees can also help reduce high urban temperature through key vegetative processes of shad- ing and transpiration (Bowler et al. 2010). Several studies suggest that an increase in vegetation can help mitigate the urban heat island (UHI) (Loughner et al. 2012), while others promote vegetation as a way of modifying urban microcli- mates and human thermal comfort (HTC) (Shashua-Bar et al. 2010b). However urban street trees face significant chal- lenges including development and infrastructure pressures, maintenance issues, and poor water availability at times that can compromise their ability to mitigate urban heat and im- prove HTC. In Melbourne, and across southeast Australia, an extended drought period was experienced from 1997 to 2009 leading to the implementation of water restrictions that result- ed in detrimental effects on Melbourne’ s tree health (May et al. 2013). The combination of drought, water restrictions, and rapid export of stormwater away from the urban environ- ment leaves the urban landscape water-starved and can con- strain tree transpiration (Coutts et al. 2013). Urban climatic conditions of higher temperature and vapor pressure deficit (Peters et al. 2010) resulting from urban development A. M. Coutts (*) : E. C. White : N. J. Tapper School of Earth, Atmosphere and Environment, Monash University, Building 28, Wellington Rd, Clayton, VIC 3800, Australia e-mail: Andrew.Coutts@monash.edu A. M. Coutts : N. J. Tapper CRC for Water Sensitive Cities, Clayton, VIC 3800, Australia J. Beringer School of Earth and Environment, The University of Western Australia, Crawley, VIC 6009, Australia S. J. Livesley Department of Resource Management and Geography, The University of Melbourne, Melbourne, VIC 3121, Australia Theor Appl Climatol DOI 10.1007/s00704-015-1409-y