Assessing thermo-physiological effects of different tree species within the inner regions of the urban canyon; confronting in-situ extreme heat stress in Istanbul during the El-Niño summer of 2023.

This study investigates the impact of street morphology and tree species on thermal comfort in Istanbul during the July 2023 El Niño event, focusing on worst-case scenarios. Field measurements were conducted in the most common street morphologies and compared with data obtained from meteorological stations (MS). Subsequently, the influence of tree presence were evaluated for the measured streets, and PET assessments were conducted by incorporating fisheye photographs of the most common tree species in the region into the SVF calculations. The results indicate that EW-oriented streets, particularly the Left Lateral, experience beyond extreme heat stress due to extended sun exposure. PET results from MS were inconsistent with local conditions. The analysis of Platanus orientalis, Populus canadensis, and Robinia pseudoacacia on PET in different street orientations showed reductions of 5-6 °C, particularly for the first two species during morning and midday. This equates to a PET reduction from Beyond extreme heat stress (I) to Extreme heat stress, for heat stress beyond 41 °C. Although this reduction is significant, tree shade had limited impact under such extreme heat. The study found that trees on the left side were more effective when placed on one side, while the right side provided stronger cooling when trees were on both sides in both E-W and N-S streets. Additionally, during the El Niño period, the influence of street morphology on thermal comfort in 'Csa' climates begins to reflect the conditions of 'BWh' climates, with higher levels of heat stress. As climate change continues to intensify, these extreme heat conditions may become typical in the future.