Optical evaluation of the shading properties of climbing fern <em>Lygodium japonicum</em> used as thermal buffering green wall plant

Abstract

Recently, thermal properties of the landscaped rooftops and walls have attracted the interest of researchers because of the potential to minimize energy consumption in urban areas and to aid summer-time thermal control. For this reason the creation of a plant-based shade for walls or above buildings is highly important. In this paper we evaluate using Lygodium japonicum, one of the many ferns and fern allies traditionally used in Japanese gardening, as a component of thermal-buffering green walls. Lygodium japonicum, the only climbing fern species in Japan, is fast-growing, adheres easily to walls and has a climbing nature. A simple thermal analysis of the sun-shading effect of Lygodium canopy suggested that local surface temperature above the ceramic tiles placed on the rooftop of a building can be buffered (lowered in daytime and maintained relatively warm at night) by the presence of leafy climbing ferns covering the tiles, possibly due to the reflection and absorbance of solar radiation. Furthermore, the presence of the plants may also slow the night-time release of heat from the building surface. Because plants installed on tall walls or on the tops of buildings are not easily accessed for manual care, we performed a real-time routine monitoring and control of plant growth status using various optical sensors that could be automated and monitored remotely for large-scale applications. For this purpose, the optical properties of a Lygodium canopy under solar incident light have been determined. In order to evaluate the natural shading and growing properties of a green canopy, the incident solar radiation spectrum (J), leaf canopy-filtered light spectrum (transmittance, T) and leaf-reflectivity spectrum ® were measured. By reading the reflectivity spectrum, concomitant chlorophyll fluorescence signals (F) from Lygodium leaves were also detected at 760 nm, which corresponds to the O2-A Fraunhofer line. Our data suggests that the daily change in photosynthetic status (P) can be traced by monitoring the change in relative F in relation to the estimated heat loss (H) and measured J, R, and T using a series of practical equations designed to roughly estimate the gross photosynthetic response within the plant canopy. Using our equations, the photosynthetic capacity in the plant canopy structure could be simply simulated and predictable by optical sensors.