Abstract: Aims Competition has been shown to modify the niche breadth of coexisting species, but within-species interactions have received little attention. Establishing small juvenile individuals and established, larger, sexually reproducing adult individuals represent two life-history stages within species. We investigated the nitrogen and carbon resource use of adult and juvenile individuals and similarity of symbiotic fungal community composition in these two plant life stages. We used the plant Solidago virgaurea growing in a simplified system in the low Arctic as model species.
Methods Isotopic signatures (foliar δ 15 N and foliar δ 13 C) were analysed to characterize nitrogen acquisition and water-use efficiency of the plants. Symbiotic root fungal community composition was estimated by cloning and sequencing small subunit ribosomal RNA gene.
Important findings The isotopic signatures differed significantly between the life stages, indicating that the establishing juvenile cohort used relatively more amino acids or gained N through mycorrhizal symbiosis in comparison to the established adult plants. Symbiotic fungal communities did not differ between the two plant cohorts suggesting a possibility that the plants shared the same mycorrhizal network. We conclude that competition-mediated differences in plant resource use may create niche differentiation between the two life-history stages and enable them to coexist.

Key words: competition, low Arctic, niche, resource use, stable isotope natural abundance