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 (∆¹⁵N_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. 3. A small ∆¹⁵N_leaf−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 ∆¹⁵N_leaf−root values were primarily correlated with leaf N and root carbon (C), while negative ∆¹⁵N_leaf−root values were associated with leaf C isotopic composition (δ¹³C), indicating that distinct ¹⁵N fractionation progresses were at work. 4. <i>Synthesis</i>. These results demonstrate for the first time that ∆¹⁵N_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 δ¹⁵N—when accounting for ¹⁵N fractionation—can provide valuable insights into the N dynamics of tropical epiphytes.