Asymmetric microclimate effects of afforestation across vertical stratification: a five-year field observation on the Loess Plateau

Abstract Afforestation is a key strategy to mitigate climate warming, yet its direct biophysical impacts on microclimate remain contentious in temperate regions. To address this, we conducted a 5-year in situ study (2019–2023) in paired catchments (afforested vs. natural grassland) on the Chinese Loess Plateau, monitoring temperature-moisture dynamics from 1 m belowground to 2 m above ground. The results indicate that afforestation induces divergent microclimate regulation across vertical strata. At 2 m aboveground, afforestation resulted in a significant increase in mean annual air temperature by 0.10℃ (P = 0.041) and a decrease in mean annual relative humidity by 1.69% (P = 0.003), whereas at 1 m it led to a significant decrease in air temperature by 0.35℃ (P = 0.004) and an increase in relative humidity by 0.59% (P = 0.038). In soils, afforestation overall resulted in a decrease in temperature by 0.54℃ (P < 0.001) and an increase in moisture by 0.74% (P = 0.019). Seasonally, the strongest effects of afforestation were observed in summer and winter, with effects particularly pronounced in winter, when afforestation resulted in a significant increase in air temperature at 2 m (0.157℃, P = 0.003) but a pronounced decrease in soil temperature, especially in the 10–40 cm layers (1.3–1.5℃, P < 0.01), highlighting contrasting responses between air and soil. Analysis of stand attributes indicated that leaf area index was the strongest regulator across vertical layers, exerting pronounced cooling effects on air and soil temperatures; canopy gap fraction was consistently associated with the afforestation-induced warming effect, particularly affecting soil minimum temperature. Synthetically, our results demonstrate pronounced vertical and seasonal variability in afforestation-induced biophysical climate regulation. Notably, under ongoing global warming, the microclimate regulation effect of afforestation via biophysical processes may weaken in semi-arid and arid ecosystems.