Impacts of Reflective Heat Gain on Substrate Temperatures and Plant Growth Heterogeneity of a Constructed Green Roof

Green roofs have grown in importance due to their various advantages, such as aiding in stormwater management, conserving energy, and providing ecosystem services. In densely populated urban environments, many of these roofs experience reflective heat from adjacent buildings, creating severe microclimates that can stress plants. Despite these challenging conditions, their impact on plant growth across different areas of green roofs has been minimally studied and documented. Our research focused on uncovering and measuring the uneven distribution of plant growth and extreme substrate conditions on green roofs through field observations and statistical methods. We conducted our study on a green roof subject to intense sunlight reflection off the building's exterior wall. Over a year, we used soil sensors to record substrate conditions every 15 min, and plant growth metrics were collected manually every month. We developed a scoring system to evaluate plant growth. Our statistical analysis revealed significant differences in substrate temperature, moisture content, and plant growth across the green roof. The field observations recorded a maximum substrate temperature of 53 degrees C on the year's hottest day, with a maximum spatial temperature variation of approximately 6 degrees C. We found a strong link between substrate temperature and plant growth through regression analysis. This study highlights how reflective heat can lead to suboptimal plant growth in certain areas of green roofs, suggesting that building architects and green roof designers should consider microclimate factors in their designs, implement adaptive green roof design (e.g., no planting zone, adaptive irrigation), select suitable plants, or incorporate shade structures when necessary.