Stable carbon and nitrogen isotope compositions change with leaf age in two mangrove ferns

•δ13C decreased with increasing leaf age in two Mangrove fern species.•δ15N increased with increasing leaf age in two Mangrove fern species.•Earlier studies showed that translocation of assimilates from older to younger leaves is a major reason for such isotopic patterns. After assimilation, plants often fractionate stable carbon and nitrogen isotopes among different organs depending on synthesis and transport of metabolites. We investigated stable carbon and nitrogen isotope compositions among leaves of different age (0 to 6 months) in two mangrove fern species (Acrostichum danaeifolium Langsd. & Fisch. and Acrostichum aureum L.) from Mexico. Leaves of all ages were analysed for δ13C, δ15N, carbon and nitrogen concentrations and gas exchange parameters. In both species, δ13C slightly decreased with leaf age. Leaf salt concentration increased with leaf age, and thus did not decrease 13C discrimination markedly. Enrichment in 13C in younger versus older leaves can be explained by stage of development: Carbon is assimilated and incorporated into autotrophic leaves, but also transported as 13C-enriched carbohydrates into still expanding and more heterotrophic younger leaves—indicated by lower rates of photosynthesis. Depletion in 13C in old autotrophic leaves, which export photosynthetic assimilates, could mainly be explained by respiratory fractionation releasing 13C-enriched CO2. In contrast, δ15N values in A. danaeifolium increased with leaf age. This pattern could be related to the transport of 15N-depleted amino acids into younger leaves, and leaf construction with these compounds. In conclusion, 13C depletion and 15N enrichment with leaf age were described for other plant species earlier and were explained by different mechanisms of carbon and nitrogen assimilation and the export of these assimilates from older to younger leaves. These stable isotope patterns were approved for two mangrove fern species in this study.