Abstract:Natural variation in nitrogen (N) isotopic composition (δ15N) of leaves has been used to study N acquisition and utilization strategies in vascular epiphytes. However, the 15N fractionation during plant N uptake, translocation, and assimilation can affect the reliability of relevant studies based on the natural abundance of 15N. Nevertheless, whether internal 15N fractionation occurs in epiphytes, leading to differences in δ15N values between various organs, and whether leaf δ15N can serve as a reliable proxy for whole-plant δ15N, remain unclear. This study investigated epiphytes in the subtropical evergreen broadleaf forests of the Ailao Mountains, analyzing the variation in δ15N values across the whole plant and individual organs, as well as the relationships between inter-organ δ15N differences and other traits. The results show that the δ15N values of epiphytic leaves (?3.48‰), stems (?3.47‰), and roots (?2.25‰) were significantly negative, suggesting a primary reliance on atmospheric deposition of 15N-depleted N sources. With the exception of Oreocharis longifolia, all other studied epiphytes (including Cautleya gracilis, Lepisorus scolopendrium, and Pleione hookeriana) exhibited significant N isotope fractionation effects among their organs, with differences in leaf-root (?15Nleaf?root: ?1.23‰), leaf-stem (?15Nleaf?stem: ?0.62‰), and stem-root (?15Nstem?root: ?1.10‰), which were generally lower than those of terrestrial plants. Additionally, the leaf-root 15N fractionation values showed significant negative correlations with leaf (r = ? 0.57; p <0.01)and root δ13C (r = ? 0.57, ?0.50; p <0.05), potentially indicating a high sensitivity to water deficiency. The leaf δ15N values of epiphytes were significantly correlated with whole-plant δ15N (R2adj. = 0.526, P < 0.01), confirming that leaf δ15N can reliably serve as a reliable proxy for whole-plant δ15N. These findings provide important insights into the N acquisition strategies and ecological adaptation mechanisms of vascular epiphytes, highlighting the utility of leaf δ15N as a research tool.
