The Duality of Reforestation Impacts on Surface and Air Temperature

Evidence is mounting that temperate-zone reforestation cools surface temperature (Tsurf), mitigating deleterious effects of climate warming. While Tsurf drives many biophysical processes, air temperature (Ta) is an equally important target for climate mitigation and adaptation. Whether reductions in Tsurf translate to reductions in Ta 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 Ta is assessed by targeting temperature metrics that are less sensitive to local canopy effects. Specifically, we consider the aerodynamic temperature (Taero), 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 (Textrap). 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, Tsurf is 4–6 °C cooler, and Taero and near-surface Textrap are 2–3 °C cooler, in the forests relative to the grassland. During the dormant season, daytime differences are smaller but still substantial. At night, differences in Taero are small, and near-surface Textrap is warmer over forests than grasslands during the growing season (by 0.5 to 1 °C). Finally, the influence of land cover on Textrap at the interface between the surface and mixed layer is small. Overall, reforestation appears to provide a meaningful opportunity for adaption to warmer daytime Ta in the southeastern United States, especially during the growing season.

Duke Scholars

Author Gabriel G. Katul Civil and Environmental Engineering