Abstract
Plants modify the climate and provide natural cooling through transpiration. However, plant response is not only dependent on the atmospheric evaporative demand due to the combined effects of wind speed, air temperature, humidity, and solar radiation, but is also dependent on the water transport within the plant leaf-xylem-root system. These interactions result in a dynamic response of the plant where transpiration hysteresis can influence the cooling provided by the plant. Therefore, a detailed understanding of such dynamics is key to the development of appropriate mitigation strategies and numerical models. In this study, we unveil the diurnal dynamics of the microclimate of a Buxus sempervirens plant using multiple high-resolution non-intrusive imaging techniques. The wake flow field is measured using stereoscopic particle image velocimetry, the spatiotemporal leaf temperature history is obtained using infrared thermography, and additionally, the plant porosity is obtained using X-ray tomography. We find that the wake velocity statistics are not directly linked with the distribution of the porosity but depends mainly on the geometry of the plant foliage which generates the shear flow. The interaction between the shear regions and the upstream boundary layer profile is seen to have a dominant effect on the wake turbulent kinetic energy distribution. Furthermore, the leaf area density distribution has a direct impact on the short-wave radiative heat flux absorption inside the foliage where 50% of the radiation is absorbed in the top 20% of the foliage. This localized radiation absorption results in a high local leaf and air temperature. Furthermore, a comparison of the diurnal variation of leaf temperature and the net plant transpiration rate enabled us to quantify the diurnal hysteresis resulting from the stomatal response lag. The day of this plant is seen to comprise of four stages of climatic conditions: no-cooling, high-cooling, equilibrium, and decaying-cooling stages.
Highlights
The influence of plants on the microclimate in an urban environment is of growing interest due to the need of mitigating detrimental effects of urbanization and climate change on urban air temperature (Demuzere et al, 2014; Dimoudi and Nikolopoulou, 2003; Matthews et al, 2017; Shashua-Bar et al, 2009; Shashua-Bar and Hoffman, 2000; Yu and Hien, 2006)
Higher plant transpiration is required to compensate for the high solar radiation absorption (Manickathan et al, 2018b), and this can result in a lower air temperature under the foliage (Wong et al, 2003)
The goal of the present study was to unveil the diurnal changes in plant microclimate using multiple non-intrusive imaging techniques such as stereoscopic particle image velocimetry (SPIV) for the flow field, infrared thermography for the leaf temperature and X-ray tomography for the plant microstructure
Summary
The influence of plants on the microclimate in an urban environment is of growing interest due to the need of mitigating detrimental effects of urbanization and climate change on urban air temperature (Demuzere et al, 2014; Dimoudi and Nikolopoulou, 2003; Matthews et al, 2017; Shashua-Bar et al, 2009; Shashua-Bar and Hoffman, 2000; Yu and Hien, 2006). Foliage density is parameterized using the leaf area index (LAI) to describe the net area of leaves, and leaf area density (LAD) to describe the foliage distribution within the plant volume These parameters are typically measured using optical techniques (Cao et al, 2012; Grant and Nickling, 1998; Guan et al, 2003; Liu et al, 2018; Manickathan et al, 2018a; Phattaralerphong and Sinoquet, 2005) that may compromise on the spatial accuracy and destructive techniques such as defoliation of the plant (Jonckheere et al, 2004; O’Neal et al, 2002). Higher plant transpiration is required to compensate for the high solar radiation absorption (Manickathan et al, 2018b), and this can result in a lower air temperature under the foliage (Wong et al, 2003). Experimental observations are key to determining the diurnal variability in the transpirative cooling performance of vegetation
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