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

Abstract Epiphytes are believed to exhibit minimal internal nitrogen (N) isotopic composition (δ 15 N) variation due to severe N limitation in canopy habitats. However, the lack of empirical evidence leaves this question open, complicating the interpretation of foliar δ 15 N in understanding the N economy of epiphytes. Here, the variation in δ 15 N between leaves and roots (∆ 15 N leaf–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. A small ∆ 15 N leaf−root (+0.43‰) was observed in epiphytes, which exhibited relatively consistent values across groups (−0.20‰ to 1.78‰) but markedly species‐specific values (−2.13‰ to 2.15‰), independently of phylogeny, reflecting the combined effects of external N sources and N acquisition strategies of epiphytes. In particular, positive ∆ 15 N leaf−root values were primarily correlated with leaf N and root carbon (C), while negative ∆ 15 N leaf−root values were associated with leaf C isotopic composition (δ 13 C), indicating that distinct 15 N fractionation progresses were at work. Synthesis . These results demonstrate for the first time that ∆ 15 N leaf−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 δ 15 N—when accounting for 15 N fractionation—can provide valuable insights into the N dynamics of tropical epiphytes.