Abstract: Changes in vegetation types regulate regional land surface temperature through biophysical processes. In this study, the Weather Research and Forecasting (WRF) mesoscale model was used to simulate conditions across Northeast China in 2018. Two numerical experiments were designed, including one simulation (2018_ctr) retaining all original land surface types over the study domain and another (2018_grass) where all forests within the domain were converted to grassland. We quantified the disparities in albedo, leaf area index (LAI), shortwave and longwave radiation, and diverse surface heat fluxes between forest and grassland grids to explore the biophysical effects of afforestation and their impacts on land surface temperature. The WRF outputs were validated against interpolated ERA reanalysis data, and the model well reproduced the spatial pattern of land surface temperature with a correlation coefficient of 0.9258. The simulated temperature exhibits an overall warm but acceptable bias, confirming the model’s applicability for regional analysis. The results showed that afforestation markedly increases LAI and reduces winter surface albedo, which further modulates land surface temperature. In the vegetation replacement zones of northern Northeast China, land surface temperature of forest grids exceeds that of converted grassland grids by more than 0.8 K in winter daytime, with a nocturnal warming magnitude of 1 K. During summer daytime, forest surfaces in these zones are over 0.8 K cooler than grassland surfaces, areas without vegetation conversion undergo a 0.8 K warming, and forest surfaces stay 0.4 K warmer than grassland surfaces at night. Spring and autumn follow similar diurnal patterns, featuring warming in the northeastern part and cooling in the southeastern part of the study region. Diagnoses of surface energy balance combined with the Decomposition of Temperature Metric (DTM) reveal that daytime warming is primarily driven by enhanced absorption of surface shortwave radiation, while sensible heat flux mitigates such warming. Daytime cooling induced by afforestation arises from both radiative and nonradiative pathways. Reduced shortwave radiation absorption dominates cooling in summer and autumn, while strengthened sensible and latent heat release from forests to the atmosphere in winter and spring accelerates surface heat loss and generates nonradiative cooling. Nocturnal warming from spring to autumn is predominantly controlled by longwave radiation, with the most pronounced signal in summer, and soil heat flux exerts a substantial warming effect on land surfaces in winter.

Key words: Northeast China, surface temperature, biophysical effect, direct decomposed temperature metric, afforestation