London Plane trees (Platanus x acerifolia) before, during and after a heatwave: Losing leaves means less cooling benefit

https://doi.org/10.1016/j.ufug.2020.126746Get rights and content

Highlights

  • Up to half of P. acerifolia foliar canopy loss over a heatwave.

  • This, decreased microclimate and pedestrian thermal comfort benefits.

  • During heatwave, “extreme heat stress” estimated under tree shade.

  • After heatwave, canopy loss due to heatwave, reduced some ecosystem service benefit.

  • However, P. acerifolia had milder microclimate compared to open street location.

Abstract

Platanus x acerifolia (London Plane) is a widely planted street tree throughout cities in temperate and Mediterranean climates. Heatwave intensity and frequency is likely to increase in these cities as a combined result of the urban heat island and climate change. High air temperature during heatwaves can lead to canopy leaf loss in vulnerable tree species, such as P. acerifolia. This rapid change in canopy cover may lead to subsequent reductions in ecosystem service benefits, such as shade cooling, human well-being, pollution interception and stormwater interception. This study investigates canopy leaf loss in two streets planted with P. acerifolia trees following a summer heatwave and the impacts upon street micrometeorological conditions and human thermal comfort. The plant area index of P. acerifolia, trees, and the micrometeorological conditions under and away from these trees was measured before and after a >43 °C heatwave in Melbourne, Australia. Physiological equivalent temperature was calculated from micrometeorological parameters to estimate human thermal comfort. Canopy loss was significant (30–50 %) and rapid and reduced thermal comfort benefits provided by canopy shade on two of the three warm summer days measured after the heatwave. However, the under-canopy areas of the street maintained a cooler micrometeorological conditions than areas in the open. This study suggests that as summer air temperature extremes and heatwave frequency increase in urban areas, more vulnerable urban tree species may experience sparse canopy cover throughout summer months reducing some ecosystem service benefits when they are needed most.

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)

  • J.-M. Robine et al.

    Death toll exceeded 70,000 in Europe during the summer of 2003

    C. R. Biol.

    (2008)
  • R. Sanusi et al.

    Microclimate benefits that different street tree species provide to sidewalk pedestrians relate to differences in Plant Area Index

    Landsc. Urban Plan.

    (2017)
  • L. Shashua-Bar et al.

    Integrated thermal effects of generic built forms and vegetation on the UCL microclimate

    Build. Environ.

    (2006)
  • H. Sjöman et al.

    Urban forest resilience through tree selection—variation in drought tolerance in Acer

    Urban For. Urban Green.

    (2015)
  • M.D. Baptista et al.

    Variation in leaf area density drives the rainfall storage capacity of individual urban tree species

    Hydrol. Process.

    (2018)
  • Bureau of Meteorology

    Special Climate Statement 48 – One of Southeast Australia’s Most Significant Heatwaves

    (2014)
  • A.M. Coutts et al.

    Temperature and human thermal comfort effects of street trees across three contrasting street canyon environments

    Theor. Appl. Climatol.

    (2015)
  • Department of Health

    The Health Impacts of the January 2014 Heatwave in Victoria

    (2014)
  • J.E. Drake et al.

    Trees tolerate an extreme heatwave via sustained transpirational cooling and increased leaf thermal tolerance

    Glob. Change Biol.

    (2018)
  • B. Filewod et al.

    Impacts of a spring heatwave on canopy processes in a northern hardwood forest

    Glob. Change Biol.

    (2014)
  • R. Garcia-Herrera et al.

    A review of the european summer heatwave of 2003

    Crit. Rev. Environ. Sci. Technol.

    (2010)
  • J.I. García-Plazaola et al.

    Photoprotective responses of Mediterranean and Atlantic trees to the extreme heat-wave of summer 2003 in Southwestern Europe

    Trees

    (2008)
  • Cited by (0)

    View full text