Effect of plant traits and substrate moisture on the thermal performance of different plant species in vertical greenery systems

doi:10.1016/j.buildenv.2020.106815

Building and Environment

Volume 175, 15 May 2020, 106815

https://doi.org/10.1016/j.buildenv.2020.106815 Get rights and content

Highlights

•
Vertical greenery systems (VGSs) significantly reduced canopy air temperature.
•
Leaf area index and coverage regulated canopy temperature reduction on sunny days.
•
Substrate moisture strongly affected substrate temperature reduction.

Abstract

This study evaluated the effects of plant traits and substrate moisture on the thermal performance of four herbs and four shrubs, which are the most commonly used species in vertical greenery systems (VGSs), in humid subtropical Hong Kong over a one-year period. The canopy temperature reduction on sunny days was significantly correlated with canopy coverage and leaf area index (LAI), but not with daily evapotranspiration (ET). This indicated that the shading effect of VGSs, which is related to canopy coverage and LAI, was more prominent than ET cooling. The lack of significant correlation between substrate moisture, ET and canopy temperature indicated that substrate moisture and ET did not significantly enhance the canopy cooling of VGSs. Substrate moisture notably cooled the substrate on sunny days, and warmed the substrate on rainy days, which significantly affected substrate thermal behavior, but had less effect on canopy air temperature. The use of VGSs with eight common plant species on building envelopes reduced steady-state heat conduction by 18.7–39.8%, with Ficus elastica (rubber fig) causing the greatest canopy cooling.

Introduction

The modification of the thermal conductivity, heat capacity and land surface emissivity of urban areas due to land use/land cover change (LUCC) has led to the aggravation of the urban heat island (UHI) effect. To address this problem in the built environment, the use of vertical greenery systems (VGSs) has become increasingly popular [1,2]. VGSs can be categorized according to the position of the growing media into two major types: green façades (in which the growing media remain on the ground) and living walls (in which the growing media stand vertically in front of vertical building surfaces) [3]. VGSs not only improve the aesthetics and biodiversity of urban environments, but also enhance the energy efficiency and sustainability of buildings, especially via energy saving through heat insulation and UHI mitigation [[1], [2], [3], [4]]. The vertical greening of building envelopes can reduce canopy, wall and ambient temperatures through shading effects [[5], [6], [7], [8]], the insulating capacity of plants and substrates [6,9,10] and evapotranspiration [7,11].

Plant species vary in their thermal behaviors due to a range of intrinsic characteristics, such as canopy cover, foliar thickness, leaf number, leaf angle and leaf area index (LAI). Previous studies have suggested that the LAI of vegetation is associated with its shading effect [[12], [13], [14]] and the insulating properties of vertical greenery [6,15]. Susorova et al. [16] suggested that VGSs with high LAI and leaves parallel to the wall (which generates high attenuation coefficients) performed best at decreasing the wall surface temperature and heat flux, due to the favorable range of leaf angles. Koyama et al. [17] found that among various plant-based parameters in VGSs, the percentage cover of vegetation had the most profound effect on wall surface temperature reduction. Charoenkit and Yiemwattana [7] suggested that VGSs with plant species with smaller leaf sizes had better cooling effects. Certainly, vigorously growing plants can achieve full cover and provide substantial ecological benefits [18], making them the key to successful establishment of VGSs. Hence, it is essential to select plants with high LAI for VGSs, as these will give high coverage under prolonged elevated outdoor temperatures and possible water stress, thereby providing multiple ecological benefits for building envelopes and microclimates, such as ambient air temperature reduction.

The relationship between the thermal behavior of VGSs and plant type implies that substrate–moisture interactions may possibly influence evapotranspiration. For green roofs, substrate moisture content has been found to consistently increase thermal conductivity across a range of substrates and heat capacities [19], affecting downward heat transmission and the fluctuation of substrate temperature. Furthermore, substrate moisture regulates the availability of water for substrate transpiration and vegetative evaporation [20], which contributes to the latent cooling of building environments and accounts for a considerable proportion of the cooling effect. However, few studies have explored the effects of soil in VGSs. Although previous studies suggested that plant characteristics affect the thermal performance of VGSs, there is a lack of knowledge on the substrate thermal behavior and water balance of VGSs. It is also unclear which plant species that are widely used in VGSs have the most favorable plant characteristics and thermal performance. The role of substrate moisture in the thermal performance of VGSs has yet to be examined.

This study was performed to assess the growth and traits of the most commonly planted herbs and shrubs in VGSs, and to assess their thermal performance under typical weather conditions for one year in humid subtropical Hong Kong. The correlation between thermal properties and plant traits was evaluated. In addition, the substrate temperature regime and water balance of the VGSs were investigated. These resulting findings on canopy air temperature reduction and substrate thermal performance in VGSs will deepen our understanding of the role of plant traits and substrate moisture in thermal performance of VGSs, and inform plant selection in the design of VGSs to maximize their environmental benefits.

Section snippets

Study area

The experimental site was located on the main campus of The Chinese University of Hong Kong, located in the New Territories, Hong Kong (N 22° 25′ 10″, E 114° 12′ 24″). The annual precipitation is 2300 mm and the relative humidity is 78%. The annual wind speed is 11.0 km/h, mainly from the east. According to the climate data provided by the Hong Kong Observatory, the mean temperature between May and October in 2014 was 26.2–29.8 °C [21]. In the hottest months (June to September) the monthly

Plant traits

The traits of the eight plant species at the beginning of the experiment are shown in Fig. 2. F. elastica had the largest values of height, cover and leaf area. The leaf number of D. repens was significantly greater than those of P. claviformis, S. trifasciata, C. variegatum and F. elastica. The LAI of P. claviformis and S. trifasciata was significantly greater than those of C. blumei, C. variegatum and D. repens. The vertical LAI of P. claviformis was significantly greater than those of the

Conclusions

This study provides valuable information on the effects of plant traits and substrate moisture on the thermal performance of VGSs in subtropical Hong Kong. It was found that vegetated VGSs with different plant species varied significantly in their capability for reduction in temperature and heat transfer beneath the plant canopy under different weather conditions in summer. The use of plant species with higher canopy cover and LAI in the VGSs ensured greater reductions in canopy temperature.

The

Declaration of competing interest

We declare that we have no conflicts of interest to this work.

Acknowledgements

We thank Mr Ben Yeung for their technical assistance, Everplant Technology Limited for the construction of the VGSs on the testing room, and the Department of Geography and Resource Management (CUHK) for their provision of venue for the experimental setup. The study was supported by the Environment and Conservation Fund of the Government of the Hong Kong Special Administrative Region [grant number ECF Project 24/2010].

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Effect of plant traits and substrate moisture on the thermal performance of different plant species in vertical greenery systems

Highlights

Abstract

Introduction

Section snippets

Study area

Plant traits

Conclusions

Declaration of competing interest

Acknowledgements

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