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. Understanding the stoichiometric characteristics of plants to a substrate shift is a key step in the use of a stoichiometric framework to predict ecosystem responses to environmental change. However, the response and mechanism of stoichiometry to a substrate shift in facultative epiphytes remains unknown. The foliar stoichiometric (N, P, K, Ca, Mg, S, Mn, Na, and Fe concentrations) and isotopic characteristics (δ13C and δ15N) of nine facultative epiphyte species in Xishuangbanna karst forest in southwest China were determined. The stoichiometry and isotopy were compared between the epiphytic and lithophytic individuals in the facultative epiphytes, and the possible causes of changes in elemental concentrations to substrate shifts were studied. We found that the lithophytes were enriched in δ15N and Ca, but depleted in elements such as Fe, K, and Mn compared with the epiphytes. The δ15N was positively correlated with P, N, S, and K, while the δ13C was negatively correlated with δ15N, P, N, S, and Fe. Following a principal component analysis (PCA), the first axis loaded organically bound elements (P, N, and S), while the second axis loaded non-organically bound elements (Fe, Mn, and K). The variances of non-organically bound elements were mostly affected by the substrate-related factors than organically bound elements. These results revealed that the substrate factor has a strong partitioning effect on elements such as K, Ca, Fe, Mn, and δ15N. The differences of element concentration and isotopy between the two ecotypes suggested stoichiometric and isotopic flexibility, which enabled facultative epiphytes to exploit rock and bark interchangeably. [ABSTRACT FROM AUTHOR]