Day–Night Asymmetric Effects of Forests on Land Surface Temperature and Their Optimized Regulation in Climatic Transition Zones: A Case Study of the Semi-Arid to Semi-Humid Region of Northern China

Forests influence local climate via biogeophysical processes, but the diurnal asymmetry of their temperature effects—and its underlying drivers—remain poorly quantified, especially in climate-sensitive transition zones. This knowledge gap hinders the climate-adaptive planning of ecological restoration programs. Using multi-source remote sensing data (2002–2023) and a pixel-pairing approach, we systematically assessed the impacts of forests on land surface temperature (LST) in the semi-arid to semi-humid transition zone of northern China. Results reveal a consistent “daytime cooling–nighttime warming” pattern: forests reduced daytime LST by –0.57 °C but increased nighttime LST by +0.43 °C, yielding a slight net daily cooling. Mechanistic analyses identified a clear diurnal driver-switch: daytime cooling was dominated by biophysiological processes (primarily enhanced evapotranspiration), whereas nighttime warming was governed by physical structural forcing, notably aerodynamic roughness-induced turbulent mixing. This was robustly evidenced by persistent nighttime warming (~ +1.4 °C) during the dormant winter when evapotranspiration was negligible. Furthermore, forest cooling capacity exhibited a nonlinear response along the aridity gradient, with an optimal climatic window (aridity index ≈ 1.3–1.6) identified in the semi-arid to semi-humid transition zone where cooling per unit water use was maximized. These findings indicate that large-scale afforestation in drier regions may face a “high water cost–low cooling” trade-off, whereas focusing restoration efforts within this climatic transition zone can optimize both climate regulation and water sustainability. Our study provides a biophysical basis for spatially optimized ecological engineering, particularly for the “Three-North Shelterbelt Program” and similar initiatives worldwide.