Canopy niche diversity and complementarity impact the forest vertical thermal environment in an urban area

Species with diversity and complementary architectural traits can optimize crown packing to enhance the efficiency of light use within forest communities. However, direct empirical evidence for further aspects of influencing factors on canopy interactions and their cooling potentials in different species levels has been lacking. Here, unmanned aerial vehicle LiDAR was used to measure the diversity and complementarity of canopy niches and analyse their impacts on the vertical microclimate of urban forests. We show that: (1) The maximum vertical temperature differences in urban forests across different species levels are observed at 12:00, with fluctuations ranging from 9.49°C to 10.6°C. The average rate of change varies between −5.08°C and 10.6°C. Forest stands with 2–3 species exhibit the greatest fluctuation and highest average rate of change, reaching 15.58°C. (2) At the single-species level, the strength of the variable relationship with vertical temperature differences is highest for canopy niche diversity (strucPCA1), canopy niche complementarity (GSCI), and leaf area index, with R² values between 0.77 and 0.94 (p < 0.001). The univariate relationship strength of canopy niche diversity (strucPCA1 and strucPCA2) with vertical temperature differences is higher in stands with richer species diversity, indicated by R² values of 0.24–0.5 (p < 0.05). (3) In stands at the 2–3 species level, the vertical temperature differences are significantly affected by canopy niche diversity (strucPCA1 and strucPCA2). At the 4–5 species level, the influence of canopy niche diversity (strucPCA1 and strucPCA2), complementarity (GSI and GSCI), and the leaf area index on vertical temperature differences is more pronounced. This study could be helpful for optimizing canopy structure and functional strategies for forest ecological management at different species levels to optimize vertical microclimates in urban forests.