Climate warming has significantly changed the near-surface soil freeze state, significantly impacting terrestrial ecosystems and regional agroforestry production. As Northeast China (NEC) is highly sensitive to climate change, this study introduces the concept of velocity to analyze the spatial pattern of frozen days (FDAY), onset date of soil freeze (FON), offset date of soil freeze (FOFF), and number of soil freeze/thaw cycles in spring (FTC) in NEC from 1979 to 2020. We observed that the velocity changes of FDAY, FON, and FTC in croplands were significantly higher than those in forests (difference >1 km yr-1), with the fastest velocity changes found in the cropland of the Songnen Plain. The highest velocity of FOFF was found in the forests of the Greater Khingan Range. In most study areas (>60%), the isoline of FDAY/FON/FOFF/FTC showed a northward movement. The isoline of FDAY/FON/FOFF/FTC moved in the cold direction in each cropland region (Sanjiang, Songnen, and Liaohe River Plains) and forest regions (Greater Khingan and Lesser Khingan Ranges, and the Changbai Mountains). The results of the quantitative analysis indicate that air temperature (TA) had a more significant effect on the velocity change of FDAY and FON in cropland, whereas snowpack is the dominant factor in forests. In both forests and croplands, the main factor affecting the velocity of FOFF was snowpack, and TA mainly affected the FTC. This study is significant for formulating regional climate change countermeasures and maintaining ecological security in cold regions.
Keywords: Cropland; Forest; Near-surface soil freeze state; Northeast China; Velocity.
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