While reforestation has been widely heralded as a means of sequestering carbon into the soil, there is growing evidence that it also serves to directly cool the land surface. But forests’ impacts on air temperature (measured over forests rather than within them) have been difficult to assess because of the confounding impacts of forest canopies on wind and temperature profiles near the surface. This study was implemented to create a new method for assessing to what degree forests also cool the air.
Most studies measure surface temperature, which “represents the aggregated temperature of solid canopy and soil elements,” and is measured at a midway point between the ground and the top of the canopy. Air temperature, on the other hand, is measured above the vegetation canopy (whether grasslands or forest). The study site, located in the Piedmont region near Durham North Carolina, consists of an old-field grassland, a pine forest, and an unevenly aged oak hickory forest, all within close proximity to each other. The study assessed temperatures at various heights in and above the grasslands and the two forests.
The authors found that surface temperatures are much lower in forests than in grassland; this difference often exceeds 5°C at midday during the growing season. Furthermore, the air is cooler over forests than over grasslands, though to a lesser degree than the surface temperature difference. The annual average air temperature difference of forests compared to grassland is 0.5°C to 1°C, while the difference reaches 2°C to 3°C during daytime growing season periods.
“Making the connection between land cover, surface temperature, and air temperature is becoming necessary for obtaining a complete picture of the climate mitigation and adaptation potential of managed land cover changes, including reforestation,” the authors conclude. “This energy balance perspective on the climate mitigation and adaptation potential of reforestation is especially relevant right now” given a recent global surge of interest in reforestation to sequester carbon [Novik & Katul 2020: 13].
Novick, Kimberly A. & Gabriel G. Katul, 2020, The Duality of Reforestation Impacts on Surface and Air Temperature, Journal of Geophysical Research: Biogeosciences 124, https://doi.org/10.1029/2019JG005543.