Contrasting below- and above-canopy climate regulation services of a temperate forest during heatwaves

• First study of above- and below- canopy effects with continuous ground observations. • Temperate forests have simultaneous warming and cooling services during heatwaves. • Forests can mediate impacts from compound heatwaves on near-surface environment. Heatwaves have significant effects on ecosystems and human populations. Human habitability is impacted severely as human exposure to heatwaves is projected to increase. Future risk of heatwaves requires effective measures for adaptation to persistent hot temperature extremes and ambitious mitigation to limit further increases in heatwave severity. At local scales, afforestation and reforestation could be a potential approach of modifying the (near-)surface energy budget and temperature, in this way alleviating heatwave impacts. In this study, thermal characteristics and energy fluxes across open-site, below-canopy, and above-canopy environments are analysed and compared, to investigate canopy's dual functions in affecting above-canopy macroclimate and acting as a thermal insulator that regulates understory microclimate and litter layer environment. Using high-resolution sub-daily datasets from the Loobos flux tower site in the Netherlands, complemented by routine weather data from 3 nearby meteorological stations, we analysed temperatures at three levels of Loobos (23.5 m, 7.5 m, and litter layer) along the same vertical profile and compared them with those measured at nearby open sites. During heatwave periods, the cooling effects of the canopy on litter layer temperature are up to 12.5 K while the canopy may also amplify the temperature above it by up to 5 K between 15 and 23 pm accompanied with increasing sensible heat. In the conditions of daytime, the site-average canopy effects increase quasi-linearly (R 2 > 0.78) with the rising open-site temperature. This research reveals the ability of the forest in providing contrasting climate regulation ecosystem services on both below-canopy and above-canopy environments, in which the canopy's potential in accommodating the temperature of near-surface environments during both day and nocturnal times to alleviate impacts from compound heatwaves is highlighted.