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

Nitrogen (N) and phosphorus (P) are pivotal nutrients that support species diversity in plant communities. However, in nutrient-poor alpine treeline ecosystems, the mechanisms through which interspecific interactions among treeline plants regulate nitrogen uptake patterns, competitive capacity, and survival strategies across functional groups remain poorly understood. Using four plant communities in the alpine treeline of Sygera Mountain (southeastern Tibet) as study subjects, we employed in situ 15 N labeling to quantify inorganic nitrogen uptake rates in the leaves, branches, and fine roots of the dominant species in these communities. We further clarified the survival strategies, directionality of interspecific interactions, and competitive strength of two key treeline plant species under conditions shaped primarily by interspecific interactions and mediated by natural environmental factors. The results revealed three key findings: (1) Water use efficiency (WUE) was higher in mixed communities than in monospecific communities. Both Juniperus saltuaria and Rhododendron nivale preferred nitrate nitrogen (NO 3 − -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.