Assessing Cooling Effect Dynamics of Ficus nitida Under Different Irrigation Regimes in a Mediterranean City

Abstract

Urbanization and climate change have exacerbated the Urban Heat Island (UHI) effect, particularly in semi-arid cities, posing significant challenges to urban livability. Using urban trees is one approach to cool cities. In semi-arid contexts and also due to climate change, water availability for wide spread irrigation to increase urban trees is currently low and will be exasperated in the future effecting the cooling capacity of urban tree canopies. Therefore, finding a balance between irrigation demand and effective cooling capacity of urban trees is key. This thesis explores the cooling dynamics of Ficus nitida, a resilient and widely used tree species in Mediterranean cities, under varying irrigation regimes. The study aims to identify an irrigation threshold that balances optimal cooling performance with water conservation by analyzing temperature variations at three levels: near the leaf, at human height, and near the soil. The research was conducted as a controlled experiment using 16 Ficus nitida trees divided into four replications of each irrigation treatment: 100%, 80%, 60%, and 40% of optimal water needs. Temperature data were collected over three summer months (July, August, and September) in 2024, using sensors placed at the three levels. This approach enabled a detailed assessment of the trees' cooling effects under deficit irrigation conditions. The findings revealed that the best cooling effects at the leaf and human height levels were achieved with 40% irrigation, demonstrating that significant microclimatic amelioration can occur with reduced water use. Conversely, the most substantial cooling effect at the soil level was observed with 100% irrigation. However, this soil-level cooling did not significantly influence human-perceived cooling at higher levels. Daily analyses showed that cooling effects were most pronounced during the hottest periods of the day, with temperature reductions of up to 7.0°C under the 100% regime and 3.5°C under the 40% regime. Nighttime cooling effects were less pronounced but still evident, with temperature reductions of 2.5°C and 1.0°C for the 100% and 40% regimes, respectively. Seasonal analysis indicated that cooling peaked during the hottest month, with reductions compared to unshaded conditions reaching 3.79°C in July, 4.07°C in August, and 3.61°C in September. These findings highlight the importance of tailoring irrigation schedules to maximize cooling during critical periods, thereby enhancing urban comfort while conserving water resources. This research underscores the value of Ficus nitida as a nature-based solution for mitigating urban heat and improving climate resilience. Future studies could investigate the effects of lower irrigation levels, include larger sample sizes, and examine other tree species to provide further insights into sustainable urban landscaping practices.