Coordination between leaf and root traits in Mediterranean coastal dune plants

The global spectrum of plant form and function allows to understand plant performance, species distributions, and ecosystem processes worldwide. Whilst most research has focused on aboveground vegetative traits (e.g., leaf economic spectrum, LES), contrasting evidence on any coordination between the LES and RES (root economic spectrum) has been reported, depending on the study system, considered traits or phylogenetic constraints (1). As traits covariation under strong environmental pressures could exhibit multidimensional pattern, studying how different functions (and related traits) vary along environmental gradients and accounting for species’ phylogenetic relatedness may help to elucidate the strength of coordination between above- and belowground trait variation. Here, we tested for a coordination between leaf and root traits within a phylogenetic comparative framework, by focusing on 39 angiosperm species living on coastal dunes of central Mediterranean. Plants were sampled in 3 distinct habitats aligned along the shoreline-inland gradient: (i) “front-habitat”, located between the shoreline and the foredune crest, exhibiting the most extreme environmental conditions (substrate instability, salinity and water stress); (ii) “back-habitat”, located behind the foredune crest in the interdunal grasslands and the fixed dunes, characterized by higher soil stability, low levels of salinity, but subjected to water stress; (iii) “slack-habitat”, located behind the foredune crest in the dune slacks that were occasionally flooded, characterized by higher substrate stability, and low levels of salinity (2). Three-quarter of trait variation is captured in two-dimensional spectra with low phylogenetic signals, indicating coordination and trade-off between leaf and root traits only moderately influenced by the species’ phylogenetic relatedness. Specifically, each habitat spectrum is shaped by the interaction of the carbon economic strategy and the resistance to several environmental conditions that change along the ecological gradient. Indeed, the low phylogenetic contribution in each habitat is likely driven by the strong environmental pressures acting as trait-filtering. Moreover, aboveground traits support the LES in all habitats, while belowground traits are consistent with the RES in the back-habitat only. A coordination between leaf and root traits was found in the back-habitat, which is characterized by less environmental pressures, supporting the whole-plant spectrum (PES). No coordination was found in the other two habitats, facing more extreme environmental conditions (front-habitat) or strong fluctuation of water availability during the year (slack-habitat). This study confirms the complexity to search for a PES in ecosystems characterized by multiple environmental pressures as those investigated here. Exploring functional diversity of both above- and belowground traits at the community level and accounting for regeneration traits (3) within a phylogenetic framework may be a future perspective to disentangle functional (co)variation in our study system.