Urban tree architectural modifications over the growing season and water restriction significantly contribute to variations in climate services

Street trees have gained attention for improving thermal comfort in cities, yet seasonal changes in tree services in relation to the evolution of tree architecture are not well understood. This study hypothesizes that over a growing season and for sunny days, changes in climate services primarily rely on tree architecture influence on cast-shadow, especially during summer droughts. Two alignments of ornamental apple trees were grown in a reduced-scale street canyon, with a non-vegetated control zone. All trees were well watered until July 5th, 2022, after which one alignment experienced moderate water restriction while the other remained well-watered. The street microclimate was characterized, the universal thermal climate index (UTCI) calculated, and tree architecture at the organ and crown scales was measured. On sunny days, well-watered trees improved thermal comfort from 5.8°C UTCI in mid-May to 7.9°C in late August, while water restriction reduced this benefit by up to 2.7°C UTCI after 8 weeks. The changes in thermal comfort were primarily linked to tree architectural development and cast-shadow, although these effects were moderate due to leaf area index high enough in May to ensure 87.5 % of interception of radiation. Increases in crown projected area and volume enhanced tree services, with larger effects on the side than just below the tree canopy. Water restriction diminished thermal comfort by inhibiting stem growth and accelerating leaf fall, thus reducing light interception. This study highlights the significant role of architectural plasticity in tree climate services under water restriction, warranting further research in other species.