[Altitudinal variations of stem and leaf anatomical structures in Juniperus tibetica]

Juniperus tibetica is a constructive species on the Qinghai-Xizang Plateau. To investigate its ecological adaptation to the alpine environments along altitude gradients, we measured the anatomical structural traits of two-year-old branches (stem and leaves) across six altitude sampling sites within its main distribution range (3600-4600 m). With increasing altitude, J. tibetica leaves enhanced stress resistance, photosynthetic capacity, and transport capacity by increasing upper epidermal cuticle thickness, palisade tissue thickness, palisade cell length, palisade cell length/width ratio, and vascular tissues (leaf xylem and phloem). With rising altitude, stems improved cold resistance and optimized anti-embolism characteristics and water transport efficiency by increasing xylem proportion (stem xylem/stem), pith proportion (pith/stem), and decreasing tracheid area while increasing tracheid density. There were significant correlations among stem tracheid characteristics (tracheid density, tracheid area, tracheid wall thickness), leaf palisade tissue thickness, and leaf epidermal cuticle thickness of J. tibetica at different altitudes, reflecting the functional connections among stem water transport, leaf photosynthesis, and protective functions. The most plastic anatomical traits in stems and leaves were equivalent pith diameter (0.51) and leaf resin canal thickness (0.35). J. tibetica developed a comprehensive adaptation pattern to high-altitude environments through plastic adjustment of anatomical structures and the coordination among water transport traits, photosynthetic traits, and protective traits.