Effects of afforestation on soil and ambient air temperature in a pair of catchments on the Chinese Loess Plateau

Climate models and satellite remote sensing have been used to determine the effects of afforestation on soil temperature at regional and global scales. However, the coarse spatial resolution of both methods have made them insensitive to local topography, which is a controlling factor of the surface energy budget, especially in areas with complex topography. While typically applied only locally at representative sites, long-term field measurements can shed light on the role of topography on soil temperature after afforestation and provide direct evidence to verify and calibrate the results from modeling and satellite observation. In this study, a pair of neighboring catchments (one afforested and the other with natural regrowth of grasses) on the Chinese Loess Plateau (CLP) was selected to assess the effects of afforestation on soil and ambient air temperature at different slope positions with an entire year of continuous measurements collected every 10 min. The results showed that the uphill slope in both catchments experienced higher soil and air temperatures than the downhill gully, where less solar radiation was received at the ground surface due to canopy interception and topographic shading. For example, the annual average soil (10–100 cm depth) and air temperature in the uphill slope in the forestland catchment was 1.09 and 1.22 °C higher than in the downhill gully, respectively. The effects of topography on soil and air temperature varied at different times of the year, and these effects interacted with the growth status of vegetation. In winter and spring, topography significantly affected soil and air temperature due to varied solar radiation received at different slope positions. By contrast, in summer, the effect of vegetation increased along with higher evapotranspiration and more solar radiation interception by the canopy, especially in the forestland catchment, which showed cooler soils than the other catchment during both the days and nights at all investigated slope positions. However, compared with the grassland catchment, the uphill slope of the forestland catchment had cooler air temperatures during the day, but warmer temperatures at night. We have concluded that both topography and afforestation influence the spatial variation and temporal dynamics of soil and air temperature, and they thereby the surface energy balance of the CLP. More local studies are warranted in order to continue to calibrate regional models and remote sensing data.