Active layer thickening enhances environmental adaptability of larch and birch in northeast China’s permafrost region

Climate warming deepens active layer thickness (ALT), altering soil water availability and reshaping plant water-acquisition strategies. However, variations in water use and environmental adaptability remain poorly understood across tree species in permafrost regions. Here we selected larch ( Larix gmelinii ) and birch ( Betula platyphylla ) under different ALT as the study species. Using hydrogen and oxygen stable isotopes, we analyzed seasonal changes in uptake depth and quantified water uptake plasticity, and assessed the importance of species and soil properties in explaining water-source contributions. The results showed that during the growing season (May-September), both species exhibited a downward migration of their principal water uptake layer. Along the ALT gradient, both species accessed deeper soil water and exhibited greater water uptake plasticity under a thicker active layer. Under similar ALT conditions, B. platyphylla showed a more rapid shift toward deeper soil water, greater use of deeper soil water, and slightly higher water uptake plasticity than L. gmelinii . Species accounted for the greatest proportion of variation in water-source contributions at 30–60 cm, whereas species and soil property were the primary drivers for water uptake at 0–30 and 60–100 cm, particularly for B. platyphylla . Taken together, these results suggest that both species show environmental adaptability under deeper ALT, while B. platyphylla may have a greater potential to compete for deep soil water under permafrost degradation. These results enhance our understanding of water uptake of different tree species in permafrost regions in a warming climate.