London Plane trees (Platanus x acerifolia) before, during and after a heatwave: Losing leaves means less cooling benefit
Introduction
A heatwave is a period of three or more days with exceptionally hot weather that is unusual for the local climate (Nairn and Fawcett, 2011). Heatwaves can lead to human health problems in vulnerable people, especially the elderly and people with pre-existing health problems (Campbell et al., 2018). The 2003 European heatwave led to a ‘mortality increase’ of between 40,000 (Garcia-Herrera et al., 2010) and 70,000 people (Robine et al., 2008). Heatwaves will impact large cities more severely because they interact with the urban heat island (Campbell et al., 2018). In addition, the frequency and intensity of heatwave events are predicted to increase in coming decades due to climate change (Stone et al., 2014; Schiermeier, 2019). As such, cities in the warmer parts of Central and South America, Central and Mediterranean Europe and Southeast Asia are predicted to experience large increases in heat-related morbidity and mortality under future climate change scenarios (Gasparrini et al., 2017).
Urban climate adaptation strategies are being developed in many global cities and countries to reduce population vulnerability to high air temperature extremes (Stone et al., 2014). These strategies revolve around education, ‘early warning’ communication systems, community cool refuges and increased urban vegetation cover and impervious surface albedo (Stone et al., 2014). Increasing urban tree canopy cover has been modelled to provide a significant cooling benefit at the city- (Ng et al., 2012) and street-scale (Shashua-Bar et al., 2006), has significant human thermal comfort benefits have been measured (Coutts et al., 2015; Sanusi et al., 2016; de Abreu-Harbich et al., 2015). However, the leaves of some trees are more vulnerable to high air temperature than others. These more vulnerable urban trees survive a heatwave, and associated high evaporative demand, through a strategy of ‘avoidance’ by simply dropping their leaves to reduce their transpiring canopy surface (Teskey 2015). Within days of a heatwave, extensive leaf shedding can occur (Filewod and Thomas, 2014) reducing micrometeorological condition and pedestrian thermal comfort benefits for the rest of summer.
This study quantifies Platanus x acerifolia leaf loss in response to a four-day heatwave where daily maximum air temperature was >41 °C. The changes in street micrometeorological conditions and human thermal comfort before, during and after the heatwave were also measured. This study addresses two specific research questions:
- 1
What level of canopy loss do streets with P. acerifolia trees experience after a heatwave?
- 2
What are the micrometeorological and human thermal comfort benefits under P. acerifolia before, during and after a heatwave?
Section snippets
Study location
Melbourne typically has a warm to hot summer with average daytime air temperature maximum of 23.9–25.9 °C and relative humidity of 47%–68%. In January 2014, Melbourne experienced a heatwave event of four consecutive daytime maximum air temperature >41 °C, and two consecutive night maximum air temperature >27 °C (Bureau of Meteorology, 2014; Department of Health, 2014). Heatwaves in Melbourne are characterised by a hot air mass moving south from central Australia, such that daytime air
Changes in plant area index
In both streets there was a significant reduction in PAI during the 11 days after the heatwave (p ≤ 0.05) (Table 1; Supplementary Fig. 1). In Clifton Street, the tree canopy PAI reduced by almost 50 %, as compared to Elm Grove that experienced a PAI reduction of almost 30 % (Table 1). Tree canopy PAI did not reduce much more between 11 and 65 days after the heatwave (Table 1).
Diurnal patterns in micrometeorological and PET before, during and after the heatwave
In Clifton Street, diurnal patterns of Ta under P. acerifolia canopies are very similar to those measured in the open
Tree canopy changes after a heatwave
In our study, four consecutive days of high maximum Ta; ranging from 41.7 °C to 43.9 °C, may have pushed the P. acerifolia leaves beyond their Ta thresholds thus forcing the trees to drop a large proportion (30–50 %) of their canopy (Supplementary Fig. 1). The significant PAI reduction of these P. acerifolia trees may be explained by the high Ta stress over consecutive days, but this stress may have been exacerbated the low water availability in these streets. Leaf loss is technically a plant
Conclusion
Platanus x acerifolia trees are vulnerable to heatwave conditions and can lose up to half of their foliar canopy over a matter of days. This reduction in P. acerifolia canopy cover decreases their ability to provide micrometeorological and pedestrian thermal comfort benefit from solar radiation interception and transpiration for the rest of the summer.
During the heatwave, the micrometeorological benefits under P. acerifolia canopies were less than those measured on a warm sunny day, such that
CRediT authorship contribution statement
Ruzana Sanusi: Conceptualization, Methodology, Visualization, Formal analysis, Project administration, Investigation, Writing - original draft, Writing - review & editing. Stephen J. Livesley: Conceptualization, Methodology, Visualization, Investigation, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no conflict of interest
Acknowledgements
Financial support from the City of Melbourne Research is gratefully acknowledged. The authors wish to thank the City of Yarra for allowing us to conduct this research in Richmond. We are also grateful for the assistance in the data analysis Statistical Consulting Centre of the University of Melbourne.
References (45)
- et al.
Effects of asymmetry, galleries, overhanging facades and vegetation on thermal comfort in urban street canyons
Sol. Energy
(2007) - et al.
Heatwave and health impact research: a global review
Health Place
(2018) - et al.
Effect of tree planting design and tree species on human thermal comfort in the tropics
Landsc. Urban Plan.
(2015) - et al.
The contribution of urban green spaces to the improvement of environment in cities: case study of Chania, Greece
Build. Environ.
(2010) - et al.
Mapping leaf area of urban greenery using aerial LiDAR and ground-based measurements in Gothenburg, Sweden
Urban For. Urban Green.
(2017) - et al.
Maximum extent of human heat stress reduction on building areas due to urban greening
Urban For. Urban Green.
(2018) - et al.
Contribution of trees and grasslands to the mitigation of human heat stress in a residential district of Freiburg, Southwest Germany
Landsc. Urban Plan.
(2016) - et al.
Tree surface temperature in an urban environment
Agric. For. Meteorol.
(2010) - et al.
Atmospheric net particle accumulation on 96 plant species with contrasting morphological and anatomical leaf characteristics in a common garden experiment
Atmos. Environ.
(2019) - et al.
A study on the cooling effects of greening in a high-density city: an experience from Hong Kong
Build. Environ.
(2012)