Optimization of Planting Trees Can Improve Thermal Comfort in Historic Districts

Under the dual pressures of global climate change and rapid urbanization, historic districts face the challenge of improving livability and adapting to climate conditions while preserving their historical fabric. While street greening is recognized as a key mitigation strategy, the lack of quantitative, spatially explicit guidelines often leads to indiscriminate planting and inefficient resource use in practice. Taking the historic districts of Nanjing—a representative city in China’s hot-summer and cold-winter region—as a case study, we systematically explored the comprehensive impacts of street orientation, height-to-width ratios (H/W), and spacing of street trees on the microclimate of the districts through empirical analysis and ENVI-met simulation. Then we constructed a typical street canyon model to simulate winter and summer conditions, and regression models were established to identify suitable SVF ranges for different street orientations. Results indicate that the recommended SVF ranges vary by street orientation: 0.3–0.5 for S–N, SE–NW, and NE–SW streets, and 0.4–0.6 for E–W streets. Crucially, denser planting does not always improve comfort. These evidence-based thresholds were applied to the renewal of Yongyuan Road. The study delivers spatially explicit guidelines in the form of quantitative planting thresholds to support climate-resilient street tree planning in historic districts, helping to enhance planting precision and resource efficiency.