Background <p>Climate-sensitive boreal peatlands have experienced tree expansion, yet the consequences of expanded tree&#xa0;litter for species interactions during decomposition of litter mixtures remain poorly understood, particularly in the late stage.</p> Method <p>Using a 6-year field experiment in a boreal peatland of Northeast China, we&#xa0;measured litter mass loss of two range expanded tree species (<i>Alnus sibirica</i> and <i>Betula platyphylla</i>) and three native species (<i>Betula fruticosa</i>, <i>Vaccinium uliginosum</i>, and <i>Sphagnum palustre</i>) under single-species decomposition, and investigated temporal variations of species interactions during decomposition of litter mixtures comprising both expanded trees and native species.</p> Results <p>Litter mass loss of expanded tree species and native vascular plants did not differ after 5 and 6&#xa0;years of decomposition (72.8% ~ 76.5% and 78.7% ~ 81.8%, respectively), whereas <i>S. palustre</i> litter consistently decomposed more slowly than vascular plant litter. The correlation coefficient between litter mass loss and initial chemical properties showed the shift in key factors from N availability in the early stage to lignin and total phenolic contents in the late stage. Across all litter mixtures, species interactions shifted from synergistic to additive effects as decomposition progressed. During the initial two years, expanded tree litter accelerated the mass loss of nutrient-poor native plant litter, while decomposition of the tree litter itself was unaffected by the neighbor litter.</p> Conclusion <p>These results indicate that long-term decomposition dynamics of litter mixtures can be reliably inferred from single-species decomposition patterns and suggest that expanded tree litter could not alter species interactions in the late stage of litter mixture decomposition in climate-sensitive boreal peatlands.</p>