Association between leaf and root functional spectra in Mediterranean coastal dune communities

Plant traits can be used as proxies for functioning and performance, including survival, growth and resource capture. In general, there are limited data on root traits because analysis of below-ground structures is complex and time-consuming. We focused our research on coastal dune vegetation in northern Tuscany. Mediterranean coastal dunes are extreme environments featuring a strong environmental gradient from the shoreline to inland regions. The most important factors which have a critical effect on adult plant survival are salinity, water stress, substrate instability, sand burial, wind abrasion, high temperature and nutrient limitation. The main aim of this research was to explore the association between leaf and root functional spectra, while considering phylogenetic relationships between species. This allowed us to identify key plant vegetation strategies in herbaceous coastal dune species. We considered four continuous leaf traits (SLA, LDMC, limb thickness and adaxial cuticle thickness), as well as four continuous root traits (RDMC, vessel diameter and frequency and vessel wall thickness) which are linked to relative growth rate (RGR), and acquisitive versus conservative syndrome. To achieve our goals, we positioned three random transects perpendicular to the shoreline ranging from upper beach to fixed dunes. In each transect, we identified three habitats as follows: frontal dunes, back dunes and slack areas where water temporarily accumulates. Multivariate analysis showed one trade-off aligned along the first axis between species with high SLA values and high root vessel frequencies, unlike plants which display conservative traits (thick leaves with high values of adaxial cuticle thickness, and large root vessels). Indeed, along the second axis, we found another leaf-root trade-off between high LDMC values and high values of root vessel wall thickness. Specifically, the front and the slack habitats showed no link between leaf and root spectra, while in the back habitat the two spectra were closely correlated (r=0.42, P<0.01), highlighting a covariance in root and leaf traits and potentially greater stability in edaphic conditions in this habitat. However, in the slack habitat, where water is temporarily available, we observed a phylogenetic clustering of both root and leaf traits driven by the presence of Poales species which have adapted to humid sites. Overall, in the ecosystem studied, we observed an evolutionary convergence of traits within phylogenetically distant species sharing similar functional traits at leaf and root level.