Response of streamflow components and evapotranspiration to changes in tree species composition in a subboreal permafrost watershed in the Greater Khingan Mountains of Northeastern China

• Calibration of the Soil and Water Assessment Tool model using streamflow, baseflow and evapotranspiration data. • The transition from Betula platyphylla to Larix gmelinii increases streamflow by reducing evapotranspiration. • Restoring Larix gmelinii is critical for sustaining baseflow under future climate scenarios. Changes in watershed water resources are often linked to land use changes, but the influence of forest structure, especially the composition of tree species, plays a crucial role in hydrological processes. This impact is particularly pronounced in the Greater Khingan Mountains, where forest cover has remained consistently high over time. Understanding and quantifying how variations in tree species composition affect watershed hydrology is essential for effective management and conservation efforts in this region. This study utilized daily runoff, baseflow separation, and various remote sensing evapotranspiration data to develop an accurate SWAT model for the Tahe River Basin, a typical forest watershed in the Greater Khingan Mountains. We assessed how the proportions of the dominant tree species, Larix gmelinii and Betula platyphylla , influence streamflow components and evapotranspiration. The results show that increased proportion of Larix gmelinii significantly enhances the hydrological function. Specifically, a 10% increase in Larix gmelinii leads to a 0.64 mm decrease in evapotranspiration, and increases runoff and baseflow by 0.66 mm and 4.98 mm, respectively. The effects of the conversion between Larix gmelinii and Betula platyphylla on these hydrological components become more pronounced under wetter conditions. Simulations based on two CMIP6 future climate scenarios indicate a trend towards warmer and wetter conditions in the Tahe River Basin, leading to further increases in runoff and evapotranspiration. Larix gmelinii is crucial for maintaining future baseflow, to stabilize baseflow and manage extreme drought events, Larix gmelinii proportions of 30% under SSP245 and 90% under SSP585 are recommended. The findings reveal the sensitivity of subboreal permafrost hydrology to forest composition changes and offer critical insights for climate adaptation strategies. Additionally, they emphasize the importance of restoring Larix gmelinii forests to enhance the hydrological function.