The Duality of Reforestation Impacts on Surface and Air Temperature

Evidence is mounting that temperate-zone reforestation cools surface temperature (T-surf), mitigating deleterious effects of climate warming. While T-surf drives many biophysical processes, air temperature (T-a) is an equally important target for climate mitigation and adaptation. Whether reductions in T-surf translate to reductions in T-a remains complex, fraught by several nonlinear and intertwined processes. In particular, forest canopy structure strongly affects near-surface temperature gradients, complicating cross-site comparison. Here the influence of reforestation on T-a is assessed by targeting temperature metrics that are less sensitive to local canopy effects. Specifically, we consider the aerodynamic temperature (T-aero), estimated using a novel procedure that does not rely on the assumptions of Monin-Obukhov similarity theory, as well as the extrapolated temperature into the surface layer (T-extrap). The approach is tested with flux tower data from a grass field, pine plantation, and mature hardwood stand co-located in the Duke Forest (North Carolina, USA). During growing season daytime periods, T-surf is 4-6 degrees C cooler, and T-aero and near-surface T-extrap are 2-3 degrees C cooler, in the forests relative to the grassland. During the dormant season, daytime differences are smaller but still substantial. At night, differences in T-aero are small, and near-surface T-extrap is warmer over forests than grasslands during the growing season (by 0.5 to 1 degrees C). Finally, the influence of land cover on T-extrap at the interface between the surface and mixed layer is small. Overall, reforestation appears to provide a meaningful opportunity for adaption to warmer daytime T-a in the southeastern United States, especially during the growing season. Plain Language Summary Reforestation-the process of reestablishing trees where they once dominated-has long been viewed as a strategy to remove CO2 from the atmosphere. Recently, attention has focused on understanding if reforestation also offers a direct temperature cooling benefit. By using more water (a cooling process) and increasing the transfer of heat energy away from the surface, forests may offer a meaningful opportunity for local climate mitigation and adaptation. Evidence is mounting that indeed, in the temperature and tropical zones, the surface of forests is cooler than grasslands and croplands. However, due to confounding effects of forest canopies on wind and temperature profiles near the surface, it has previously been hard to assess if forests also cool the air. Here we present a new approach that accounts for canopy effects, allowing for a more direct assessment of the potential for reforestation to cool near-surface air temperature. Using a case study from the North Carolina Piedmont, we find that while the air cooling effect of forests is not a large as the surface cooling effect, it is still on the order of 2-3 degrees C during summer daytime periods-times when the need for climate adaptation strategies are