Stoichiometric and isotopic flexibility: facultative epiphytes exploit rock and bark interchangeably

•Facultative epiphytes exploit rock and bark interchangeably in a Chinese karst forest.•Lithophytes were enriched in δ15N and Ca, and depleted in Fe, K, and Mn.•Stoichiometry differences between two ecotypes suggested the flexibility of non-organically bound elements.•A substrate shift led to foliar stoichiometry differences between two ecotypes.