Park et al. (2026) Disentangling the relative importance of tree-cast shadows and transpiration in mitigating urban extreme heat

Identification

• Journal: Scientific Reports
• Year: 2026
• Date: 2026-09-04
• Authors: Kyeongjoo Park, Jong‐Jin Baik
• DOI: 10.1038/s41598-026-68979-5

Research Groups

• School of Earth and Environmental Sciences, Seoul National University, South Korea.

Short Summary

This study uses numerical simulations to quantify the cooling effects of street trees, finding that transpiration is significantly more effective than tree-cast shadows in mitigating urban extreme heat during the daytime.

Objective

• To disentangle and compare the relative importance of tree-cast shadows and transpiration in reducing urban canyon air temperatures during extreme heat events.

Study Configuration

• Spatial Scale: Urban canyon level (simulated street trees with a crown radius of 1 m).
• Temporal Scale: Diurnal cycle (daytime and nighttime) during an extreme heat event.

Methodology and Data

• Models used: Weather Research and Forecasting (WRF) model coupled with an urban canopy model and a factor-separation method.
• Data sources: Online numerical simulations.

Main Results

• Daytime Cooling: Transpiration is the dominant cooling mechanism, reducing air temperature by $1.15 \pm 0.09\text{ °C}$, whereas tree-cast shadows reduce it by $0.43 \pm 0.06\text{ °C}$.
• Nighttime Cooling: Both effects are minimal and comparable, with transpiration reducing temperature by $0.15 \pm 0.03\text{ °C}$ and shadows by $0.09 \pm 0.02\text{ °C}$.
• Radiation Balance: The warming caused by the interception of direct shortwave radiation by trees exceeds the cooling provided by tree-cast shadows during the day.
• Consistency: The relative importance of transpiration versus shading remains similar regardless of variations in urban geometry or tree-crown size.

Contributions

• Provides quantitative evidence that transpiration is the primary driver of tree-induced cooling in urban environments, emphasizing that managing soil moisture is critical to sustaining these cooling benefits.

Funding

• National Research Foundation of Korea (NRF), grant RS-2025-00562044.

Citation

@article{Park2026Disentangling,
  author = {Park, Kyeongjoo and Baik, Jong‐Jin},
  title = {Disentangling the relative importance of tree-cast shadows and transpiration in mitigating urban extreme heat},
  journal = {Scientific Reports},
  year = {2026},
  doi = {10.1038/s41598-026-68979-5},
  url = {https://doi.org/10.1038/s41598-026-68979-5}
}