Tree Performance and Cooling Benefits in Multi-layered Urban Forests

Urban forests are vital for climate adaptation, mitigating heat stress and enhancing thermal comfort in cities. However, among many influencing factors, their effectiveness depends on vegetation structure and composition. While trees provide cooling benefits, their functioning is influenced by complex interactions with ground surface characteristics and understory vegetation. These interactions pose a management challenge: understory vegetation may compete with trees for water and nutrients, while also influencing the local microclimate through enhanced transpiration and modified surface energy balance. This dynamic remains underexplored in urban forest planning, where selecting optimal vegetation combinations to balance thermal comfort and water use needs careful consideration. Our research examines these relationships by studying two tree species with contrasting morphological and physiological traits—Robinia pseudoacacia and Acer platanoides—under three configurations: tree-impervious, tree-grass, and tree-shrub-grass. Through monitoring in summer in Munich (2023-2024), we employed weather stations, soil moisture sensors, sap flow measurements, and gas analyzers to assess how different vegetation combinations affect tree water use, transpirational cooling, and human thermal comfort. Preliminary results show 20% variation in tree water use across understory configurations. R.pseudoacacia exhibited 27% lower transpiration in tree-shrub-grass sites compared to tree-grass-only sites, while A.platanoides showed no significant differences between combinations. Cooling effects, measured as Physiological Equivalent Temperature reductions, varied by up to 4.2°C, with maximum cooling in tree-shrub-grass configurations. These findings demonstrate understory vegetation's significant influence on tree performance and cooling benefits, providing urban planners with insights on optimizing urban forest structure to maximize ecosystem services and improve urban microclimates.