Interspecific interactions alter survival strategies of tree-shrub plants and hinder treeline ascent at alpine treelines in Southeastern Tibet: Evidence from¹³C and¹⁵N.

-N), and this preference increased in mixed communities for both species, with the increase in R. nivale substantially greater than that in J. saltuaria. (2) Within mixed communities, the inorganic nitrogen uptake rates of leaves, branches, and fine roots in R. nivale were significantly higher than those in J. saltuaria. Interspecific interactions exerted a facilitative effect on R. nivale but an inhibitory effect on J. saltuaria, and soil total phosphorus (TP) was identified as the key nutrient supporting their growth and survival. (3) Compared with pure J. saltuaria stands, J. saltuaria-dominated mixed communities allocated a higher proportion of nutrients to aboveground organs and therefore adopted an acquisitive nutrient strategy. In contrast, relative to pure R. nivale communities, mixed communities dominated by R. nivale preferentially translocated nutrients to root systems, reflecting a defensive and conservative nutrient allocation pattern. Overall, this study confirms that the interspecific relationships and nutrient strategies of J. saltuaria and R. nivale in the Sygera Mountain alpine treeline are consistent with the Competitive Exclusion Hypothesis, Facilitation Hypothesis, and Productivity-Nutrient Allocation Hypothesis. From physiological and ecological perspectives, this study elucidates the regulatory mechanisms shaping interspecific relationships among alpine treeline plants. The findings advance our understanding of how biological factors, particularly interspecific interactions, modulate treeline migration under climate warming and provide critical theoretical support for endangered species protection and ecological restoration in this high-altitude region.