Carbon allocation traits are coordinated across plant organs, yet dry matter content is decoupled in three Mediterranean woody species

Background and aims: Trait-based functional ecology often assumes that plant organs (e.g. leaf, stem, root) having dense tissues (high dry matter content; DMC) are carbon (C)-expensive. However, this assumption remains largely untested. Here, we examine whether 1) C-allocation traits co-vary forming coordinated functional strategies, 2) isotopic composition of leaf carbon (δ13C) can be effectively traced across organs and can estimate C present in upper soil layers thereby forming a plant-soil continuum, and 3) easy-to-collect traits can predict key soil ecosystem functions (C stock, nutrient status). Methods: We focused on three widely distributed Mediterranean woody species (Cistus salviifolius, Erica arborea, Quercus ilex), measuring plant traits and soil parameters from three sites in Italy. We applied a Bayesian analytical framework considering the co-varying effects of biotic (species, developmental stage) and abiotic (site, fire disturbance) factors. Results: Trait co-variation indicates integrated functional strategies for C allocation, especially for δ13C across organs. However, DMC of different organs is decoupled from %C and δ13C of the relative organ. We find no evidence for the occurrence of a δ13C plant-soil continuum, nor for easy-to-collect traits to predict soil functions. Conclusion: The widely assumed positive relationship between DMC and C investment does not emerge in the three focal Mediterranean woody species, suggesting within-organ water content regulation as a key morpho-physiological adjustment in our model system. While the focal species are representative of the phylogenetic and functional variation in the Mediterranean woody flora, the generality of our findings should be tested in other regions hosting mediterranean-type ecosystems.