Assessing tree canopy cooling efficiency in different local climate zones: A cost-benefit analysis

Abstract

Specific mechanisms by which tree canopy coverage (TCC) influences cooling and heat alleviation across different local climate zones (LCZs) are not yet fully understood, particularly in terms of nurturing costs and the threshold of TCC. This study, using six typical LCZs in Xi'an as a case study, employed a combination of field measurements and numerical simulations to establish the mathematical relationships between TCC, cooling intensity, and heat alleviation at the LCZ scale. A cooling benefit evaluation system, incorporating nurturing costs, was developed to precisely define the TCC threshold for each LCZ. The findings indicated that: 1) Tree cooling effects were influenced by wind direction and building shadow, with low-rise LCZs showing heightened sensitivity to TCC variations; 2) Low-rise LCZs had greater potential for thermal comfort improvement, with the most significant enhancements in the middle of streets in midrise and low-rise LCZs as greenery increased, and LCZ2 exhibiting optimal thermal comfort on the downwind side of streets; 3) A linear regression relationship existed between cooling intensity and TCC across six LCZs, while a logarithmic regression relationship was observed between heat alleviation and TCC in all LCZs except LCZ6; 4) TCC had a quadratic regression relationship with cooling efficiency, with high and compact LCZs reaching the TCC threshold sooner. The TCC thresholds for the LCZs were 24.08 % (LCZ1), 33.18 % (LCZ2), 45.35 % (LCZ3), 32.49 % (LCZ4), 44.89 % (LCZ5) and 47.12 % (LCZ6). Our research elucidated the cooling and heat alleviation effects of TCC at LCZ scales and offered a quantitative basis for optimizing tree planting management by balancing cooling benefits and nurturing costs.