Tree diversity and mycorrhizal associations jointly regulate seasonal canopy cooling in forests

The increasing frequency and intensity of heatwaves under climate change highlight the need to understand how biodiversity regulates forest canopy thermal dynamics. Although tree diversity can buffer the microclimate, its effects on canopy temperature and the role of mycorrhizal symbioses remain unclear. We addressed this question in the MyDiv tree diversity experiment in Germany, where tree species richness (1-, 2-, and 4-species mixtures) and mycorrhizal types (arbuscular, ectomycorrhizal, and mixed) are factorially manipulated. During the 2024 growing season, we conducted nine uncrewed aerial vehicle (UAV) surveys using integrated thermal and LiDAR sensors to quantify canopy temperature and structural complexity, together with measurements of leaf water content, specific leaf area, soil moisture, and vapour pressure deficit (VPD). Increasing tree diversity generally reduced canopy temperature, although the strength of this relationship varied seasonally and among mycorrhizal types. Cooling effects were strongest during peak summer heat and were more pronounced in arbuscular mycorrhizal (AM) communities than in ectomycorrhizal (EM) and mixed (AM+EM) communities. In contrast, EM communities exhibited greater canopy structural complexity, whereas AM communities maintained higher soil and leaf water content. Structural complexity increased with tree diversity but did not necessarily result in greater canopy cooling. Structural equation modelling revealed that forest thermal buffering emerged through complementary structural and hydraulic pathways, whose relative importance shifted seasonally, with hydraulic regulation becoming increasingly important under hotter and drier conditions. By linking canopy temperature, canopy structure, and plant water relations, our study provides mechanistic insights for understanding how multiple facets of biodiversity regulate forest thermal buffering under climate warming.