Carbon isotope discrimination as a tracer of functional traits in a mediterranean macchia plant community

Abstract

Characterising functional plant groups with simple robust parameters of structural and functional traits is an important tool in ecological research. The reliability of carbon isotope discrimination (Δ13C) as an indicator of functional types was assessed in a highly diverse mediterranean macchia comprising drought semi-deciduous malacophylls, evergreen sclerophylls and a gymnosperm. Pronounced differences in Δ13C of 4‰ occurred: semi-deciduous species (Cistus sp. L.) showed the highest and the gymnosperm (Juniperus sp. L.) the lowest Δ13C (20.3 ± 0.5‰ and 16.2 ± 0.18‰, respectively). Across all studied species, Δ13C was correlated with (i) phenology (length of growing period) and (ii) leaf structure (leaf mass and N per area). The correlation of Δ13C with leaf water potentials, an indicator of drought stress, was species-specific and only 6 out of 11 species exhibited a significant relationship. Thus, leaf phenology governs seasonal responsiveness of Δ13C to drought, which constrains its applicability as an indicator of water use efficiency, particularly in evergreen species with short growing periods. Principal components analysis indicated the robustness of Δ13C for the classification of functional groups yielding similar results based on multiple leaf traits or solely on Δ13C. Hence Δ13C provides an ecological tracer of different functional types, integrating structural, functional and phenological attributes.