Humidify Feedback of Wetland Changes in the China Side of the Heilongjiang River Basin

Understanding how wetland changes affect near-surface humidity is essential for evaluating their climate-influencing functions, especially in mid- and high-latitude regions. Here, using multi-source remote sensing data, we investigated the impacts of wetland area changes and leaf area index (LAI) on 2 m specific humidity (2m SH) within the China side of the Heilongjiang River Basin (CHRB) from 2003 to 2020 across latitudinal gradients and seasonal scales. The results indicated that the wetland area initially decreased and then increased, while the LAI rose by 0.015/year. A significant positive correlation was identified between wetland coverage and 2m SH, with a threshold of 60%. A transition point at 50°N was observed in the response of humidity to wetland area changes, shifting from an increase to a decrease in humidity. The Wetland LAI Change Humidity Index increased with latitude from 2003 until 2010, significantly decreasing thereafter (R² = 0.634, p < 0.05). Seasonally, the humidifying effect strengthened with latitude in spring and autumn, with a strong negative correlation observed in autumn between 2003 and 2010 (R² = 0.789, p < 0.001). These findings deepen the understanding of wetland–humidity interactions and provide a scientific basis for wetland conservation and regional climate adaptation, supporting SDG13.