Should leaves be removed on the east or the west? Diurnal patterns of cluster-zone microclimate and leaf photosynthesis in hedgerow grapevines during an extraordinarily hot summer in Hokkaido in 2023

Leaf removal is a commonly applied canopy management in vineyards as it has a significant impact on the microclimate around grape clusters and canopy photosynthetic efficiency. This effect is observed within the complex radiative and thermal dynamics associated with hedgerow environments. We evaluated the effects of leaf removal on cluster-zone microclimate during an extraordinarily hot 2023 summer in central Hokkaido. The surface temperature of east-facing clusters was up to 10°C higher than ambient temperatures when the clusters were exposed to sunlight by leaf removal. The temperature increase of east-facing clusters covered by leaves (i.e., without leaf removal) was significantly suppressed. Such radiation-dependent heating was hardly observed in west-facing clusters during the afternoon when wind speeds were highest. Effective cumulative cluster temperatures above 25°C were higher for east-facing clusters with leaf removal than east-facing clusters without leaf removal and west-facing clusters with and without leaf removal. Although the relative humidity in the middle of canopy decreased slightly faster in the morning when the east-side leaves were removed, no marked difference was observed under high humidity conditions. Further, diurnal patterns in net photosynthetic rates for leaves from the upper, middle, and lower canopy were measured on both sides of hedges to assess their contributions to canopy photosynthetic gain on two hot and sunny days. Despite record high air temperatures, the photosynthetic rates for leaves on both sides of hedgerows did not decrease at midday. East-side leaves in the upper canopy tended to show higher photosynthetic rates, likely due to increased light interception and atmospheric CO2 concentrations during the morning. Our results suggest that removing east-side leaves, a conventional practice aimed at suppressing cluster temperature, is not always effective for this purpose, and that canopy photosynthetic gain does not differ markedly between the sides of hedgerows and within the canopy, even during heatwaves.