Group-consistent but species-specific nitrogen isotope fractionation between leaves and roots in tropical vascular epiphytes

1. Epiphytes are believed to exhibit minimal internal nitrogen (N) isotopic composition (δ15N) variation due to severe N limitation in canopy habitats. However, the lack of empirical evidence leaves this question open, complicating the interpretation of foliar δ15N in understanding the N economy of epiphytes. 2. Here, the variation in δ15N between leaves and roots (∆15Nleaf−root) of 24 epiphyte species from the tropical Xishuangbanna region of southwest China was calculated. Subsequent linear regression and variance decomposition were performed to identify the underlying driving factors. 3. A small ∆15Nleaf−root (+0.43‰) was observed in epiphytes, which exhibited relatively consistent values across groups (−0.20~1.78‰) but markedly species-specific values (−2.13~2.15‰), independently of phylogeny, reflecting the combined effects of external N sources and N acquisition strategies of epiphytes. In particular, positive ∆15Nleaf−root values were primarily correlated with leaf N and root carbon (C), while negative ∆15Nleaf−root values were associated with leaf C isotopic composition (δ13C), indicating that distinct 15N fractionation progresses were at work. 4. Synthesis. These results demonstrate for the first time that ∆15Nleaf−root is minimal yet noteworthy within epiphytes compared with soil-rooted plants, reflecting the more severe N stress in the canopy habitats, and suggest that foliar δ15N—when accounting for 15N fractionation—can provide valuable insights into the N dynamics of tropical epiphytes.