Reforestation and surface cooling in temperate zones: Mechanisms and implications

• Published in: Global change biology 2020-06, Vol.26 (6), p.3384-3401
• Main Authors: Zhang, Quan, Barnes, Mallory, Benson, Michael, Burakowski, Elizabeth, Oishi, A. Christopher, Ouimette, Andrew, Sanders‐DeMott, Rebecca, Stoy, Paul C., Wenzel, Matt, Xiong, Lihua, Yi, Koong, Novick, Kimberly A.
• Format: Article
• Language: English
• Subjects: Adaptation; Albedo; Albedo (solar); Carbon cycle; Climate; Climate change; Climate change mitigation; Climate effects; Cooling; Cooling effects; Covariance; Deforestation; Emissivity; energy balance; Enthalpy; Environmental changes; Forests; Grasslands; Heat exchange; Heat flux; Heat transfer; Land cover; Land surface temperature; Land use; Mitigation; Offsets; Physical properties; Radiation; Reforestation; Sensible heat; Surface cooling; Surface properties; Surface temperature; Temperate zones; Temperature; Terrestrial environments; Tropical climate; turbulent fluxes
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.15069

Land‐use/cover change (LUCC) is an important driver of environmental change, occurring at the same time as, and often interacting with, global climate change. Reforestation and deforestation have been critical aspects of LUCC over the past two centuries and are widely studied for their potential to perturb the global carbon cycle. More recently, there has been keen interest in understanding the extent to which reforestation affects terrestrial energy cycling and thus surface temperature directly by altering surface physical properties (e.g., albedo and emissivity) and land–atmosphere energy exchange. The impacts of reforestation on land surface temperature and their mechanisms are relatively well understood in tropical and boreal climates, but the effects of reforestation on warming and/or cooling in temperate zones are less certain. This study is designed to elucidate the biophysical mechanisms that link land cover and surface temperature in temperate ecosystems. To achieve this goal, we used data from six paired eddy‐covariance towers over co‐located forests and grasslands in the temperate eastern United States, where radiation components, latent and sensible heat fluxes, and meteorological conditions were measured. The results show that, at the annual time scale, the surface of the forests is 1–2°C cooler than grasslands, indicating a substantial cooling effect of reforestation. The enhanced latent and sensible heat fluxes of forests have an average cooling effect of −2.5°C, which offsets the net warming effect (+1.5°C) of albedo warming (+2.3°C) and emissivity cooling effect (−0.8°C) associated with surface properties. Additional daytime cooling over forests is driven by local feedbacks to incoming radiation. We further show that the forest cooling effect is most pronounced when land surface temperature is higher, often exceeding −5°C. Our results contribute important observational evidence that reforestation in the temperate zone offers opportunities for local climate mitigation and adaptation. Reforestation is proposed to be an efficient way to mitigate global warming. However, it remains unclear if reforestation can cool the surface in the temperate zone. Using six paired forest–grassland towers spanning a wide range of latitudes in the eastern United States, we show the forest surface is cooler than nearby grasslands by 1–2°C. The cooling is driven by enhanced latent and sensible heat fluxes and enhanced emissivity, which together overwhelm albedo‐