Diagnosing above- and below-canopy temperature impacts of forest in the Netherlands during heatwaves

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 has demonstrated the need of effective measures for adaptation to persistent hot temperature extremes and ambitious mitigation to limit further increases in heatwave severity.

At local scales, forest management could be a potential approach of modifying surface energy budget and in this way alleviating heatwave impacts. In this study,  open-site, below-canopy, and above-canopy climatic conditions from 4 different sites during the time period 1997-2020 in the Netherlands were compared to investigate canopy functions of affecting above-canopy macroclimate and as a thermal insulator to regulate understory microclimate and land surface ecology. Using high-resolution sub-daily data sets from Loobos, in which water vapor and heat fluxes were measured every half an hour by a combination of eddy covariance flux measurements and a profile system, we analysed temperatures at three levels of Loobos (23.5m, 7.5m, and soil litter layer) of the same profile and compared them with those measured at open sites in De bilt and Deleen.

Heatwave periods are defined as a sequence of at least five days during which the daily maximum temperature exceeds the climatological mean over the reference period 1997-2010 by at least 5 °C. During heatwave periods, the cooling effects of the canopy on surface temperatures are stronger compared to normal periods while the canopy may aggravate the temperature above it during certain hours. By contrast, temperature differences are higher during normal times than heatwave periods when considering temperature buffer effects of canopy on understory climate (7.5m).

Further study on heat fluxes, Bowen ratio, and canopy effects on heat stress during normal conditions and heatwaves will be conducted as well. Relative humidity will be incorporated in measuring heat stress to reflect real conditions living bodies experience.