Water-Stressed Plants Do Not Cool: Leaf Surface Temperature of Living Wall Plants under Drought Stress

Michael Gräf,Markus Immitzer,Rosemarie Stangl,Peter Hietz

doi:10.3390/su13073910

Michael Gräf, Markus Immitzer + Show 2 more

Open Access

https://doi.org/10.3390/su13073910

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Abstract

Urban green infrastructures offer thermal regulation to mitigate urban heat island effects. To gain a better understanding of the cooling ability of transpiring plants at the leaf level, we developed a method to measure the time series of thermal data with a miniaturized, uncalibrated thermal infrared camera. We examined the canopy temperature of four characteristic living wall plants (Heuchera x cultorum, Bergenia cordifolia, Geranium sanguineum, and Brunnera macrophylla) under increasing drought stress and compared them with a well-watered control group. The method proved suitable to evaluate differences in canopy temperature between the different treatments. Leaf temperatures of water-stressed plants were 6 to 8 °C higher than those well-watered, with differences among species. In order to cool through transpiration, vegetation in green infrastructures must be sufficiently supplied with water. Thermal cameras were found to be useful to monitor vertical greening because leaf surface temperature is closely related to drought stress. The usage of thermal cameras mounted on unmanned aerial vehicles could be a rapid and easy monitoring system to cover large façades.

Highlights

Accepted: 30 March 2021In the 21st century, a major challenge for urban areas is to cope with the increasing ambient air temperature due to the urban heat imbalance [1]
We investigated the temperature differences between stressed and non-stressed plants to gain a better understanding of the cooling potential of transpiring plants
While the cooling effect of green infrastructures (GIs) is usually evaluated using the ambient temperature as an indicator, we focused on plant leaves [39,40]

Summary

Introduction

In the 21st century, a major challenge for urban areas is to cope with the increasing ambient air temperature due to the urban heat imbalance [1] This is largely caused by the increased absorption of shortwave radiation and the excessive release of anthropogenic heat, as well as the decreased latent heat transfer [2]. Adequate and sustainable urban planning that mitigates the consequences of global increases in temperatures and the associated negative effects on human health is necessary [7,8,9,10] This includes the use of materials such as thermochromic or radiative cooling structures to decrease the urban temperature or the intensive use of urban greenery and water for evaporative cooling [11].

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