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

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 (?13C) 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 ?13C of 4‰ occurred: semi-deciduous species (Cistus sp. L.) showed the highest and the gymnosperm (Juniperus sp. L.) the lowest ?13C (20.3 ± 0.5‰ and 16.2 ± 0.18‰, respectively). Across all studied species, ?13C was correlated with (i) phenology (length of growing period) and (ii) leaf structure (leaf mass and N per area). The correlation of ?13C 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 ?13C 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 ?13C for the classification of functional groups yielding similar results based on multiple leaf traits or solely on ?13C. Hence ?13C provides an ecological tracer of different functional types, integrating structural, functional and phenological attributes.