Dependence of epiphytic community on autochthonous and allochthonous sources of nitrogen in three forest habitats of southern Vietnam

Aims Experimental data concerning possible trophic relationships between phorophytes and epiphytes are contradictory. Several studies have indirectly shown that epiphytes affect the nitrogen cycle in tropical forests, but the degree of interaction between nutrition cycles of epiphytes and phorophytes remains unclear. We hypothesised that, in the case of preferential usage of autochthonous sources, a correlation between isotopic composition of epiphytes and phorophytes would be observed, whereas no such correlation suggests mainly external (atmospheric) N sources. Methods To identify the trophic relationship between epiphytes and phorophytes, the isotopic composition of nitrogen was studied in soils, phorophytes, epiphytes, and canopy plants having access to autochthonous nitrogen, such as parasites, ground-rooted climbers, or carnivorous plants. The study was conducted in three habitats in southern Vietnam: a savanna-like forest on Phu Quoc Island, a montane forest in Bidoup and a lowland forest in Cat Tien. Results Isotopic composition of nitrogen was similar in different taxonomic and morphological groups of epiphytes. Indicating the dependence on wet atmospheric deposits, all epiphytes were 2-4 parts per thousand depleted in N-15 relative to phorophytes and canopy plants having access to autochthonous nitrogen. A correlation in nitrogen isotopic composition between phorophytes and epiphytes was observed in two of the three studied habitats (R-2 = 0.47 and 0.74 for Phu Quoc and Bidoup, respectively). Conclusions At the ecosystem level, the epiphytic community can have different degree of dependence on autochthonous and external nitrogen sources. Both, bulk delta N-15 values and the correlation between N-15 content of epiphytes and phorophytes can indicate a relative contribution of autochthonous and allochthonous nitrogen in the nutrition of epiphytic plants.