Plant ecological indicator values as predictors of fine-root trait variations

Fine roots play key roles in the capacity of plants to face environmental constraints and their traits reflect adaptations to the environment, including soil structure, resource availability and climate. However, the inaccuracy of global soil and climate databases to account for the large environmental variation occurring at small spatial scale prevents accurate estimations of the linkages between environmental variables and fine-root strategies. Here, using two global databases on fine-root traits (Rhizopolis-db) and species phylogenetic relatedness, and a regional database of species ecological indicator values (Baseflor), we quantified the predictive value of ecological indicator values, as an alternative to classical coarse soil and climate indicators, on the variation in four major fine-root traits. A strong phylogenetic signal was found among species for fine-root mean diameter, specific root length (SRL) and root tissue density (RTD), but less so for root nitrogen concentration (RNC). After accounting for this relatedness, ecological indicators still explained a large part of trait variation in our dataset for SRL, RTD and RNC. Multi-indicators best modelR(2)reached .40 for SRL and RTD, and .44 for RNC, whereas it was only 0.10 for diameter. Ecological indicators of nutrient availability and soil texture were those that most strongly related to SRL, RTD and RNC. Specifically, plant fast resources use strategies characterized by high SRL, RNC and low RTD occurred more frequently in nutrient-rich soils and in soils with light sandy textures. Additionally, light availability and atmospheric temperature were negatively related with SRL and continentality negatively influenced RNC. With respect to both nutrient and water availability ecological indicator values, opposite adaptations were observed between growth forms, particularly between woody and herbaceous species, limiting our ability to define simple, widely applicable patterns of trait-environment relationships. Synthesis. Our analysis demonstrates that species ecological indicator values are valuable predictors of plant below-ground strategies. It provides original evidence that herbaceous species with fine-root traits representative of fast resource use strategies typically occur in more favourable soil habitats (high nutrient and water availability); meanwhile, woody species may show the opposite trend. Other important environmental parameters concomitantly influence fine-root trait variation in contrasting ways.