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