The role of street trees in enhancing human thermal comfort and CO2 absorption: A case study in Jeju, Republic of Korea

Trees are vital to urban development, significantly reducing urban heat and capturing CO₂. As a crucial asset for enhancing human thermal comfort and improving CO₂ conditions in urban areas, the effectiveness of trees depends on factors like location, magnitude, and leaf area index (LAI). This study aims to: (1) analyze micrometeorological parameters - including air temperature, humidity, short- and long-wave radiation, wind speed, and wind direction - at pedestrian levels to quantify the cooling effects of trees from a human-biometeorological perspective; (2) examine CO₂ uptake through continuous measurement campaigns of two commonly used street trees, the evergreen broad-leaved Daphniphyllum macropodum and the broad-leaved Zelkova serrata, on the campus of Jeju National University in 2024-2025; and (3) evaluate the impact of street tree planting on reducing heat stress and CO₂ across various temporal scales (diurnal, seasonal, and annual) using the validated ENVI-met v5.7.1 model. Results from both measurements and simulations indicate that trees can reduce human heat stress, calculated by physiological equivalent temperature (PET), by up to 20 K under tree canopies in summer. While the evergreen broad-leaved tree demonstrates significant CO₂ absorption in both summer and winter, the broad-leaved tree compensates for reduced winter CO₂ uptake due to its leafless state, as evidenced by both measured and simulated outputs.