Increased leaf temperature reduces photosynthetic capacity of top-of-canopy leaves in the wet tropical forest of Costa Rica

Warming temperatures, high vapor pressure deficit (VPD), and excess light during the middle of the day can reduce CO₂ assimilation and cause stomatal closure, a phenomenon known as midday depression of photosynthesis. However, the role of light, temperature and VPD in driving the diurnal cycle of photosynthesis remain poorly studied in tropical biomes. Here we use quantum efficiency of photosystem II in the light (ϕPSII) as indicator of photosynthetic efficiency for top-of-canopy leaves for six tree species with distinct leaf morphology, across eight sampling campaigns over two years. We find midday decreases in ϕPSII when temperature, solar radiation and VPD were higher than normal. Interestingly, the difference between leaf temperature and air temperature is the most important factor driving changes in ϕPSII, while light is less prominent. We also estimated canopy temperature using outgoing longwave irradiance and found that canopy temperature deviates from air temperature at air temperatures of around 27-28 °C, likely indicating a thermal threshold for photochemistry at the canopy level. These measurements can be combined with state-of-the-art satellite remote sensing (e.g. solar-induced chlorophyll fluorescence and land surface temperatures) to better understand temperature thresholds to photosynthesis and transpiration across scales.