The hidden half of the fine root differentiation in herbs: nonacquisitive belowground organs determine fine-root traits

Plants rely on roots for absorption of nutrients from the soil. Differences in traits of fine roots and of the root system in general thus underlie differences among individual species in their ability to live in habitats differing in nutrient status and interactions with other species. Here we examine to what extent structure of the root system is determined by whole-plant parameters, namely the expected life span of the rooting units (either genetic individuals or ramets in clonally growing plants) and the type of belowground nonacquisitive organs such as rhizomes. By using phylogenetic comparative techniques we confirm the existence of two independent directions of variation in root traits, namely fast-slow continuum and cooperation continuum and show that the fast-slow continuum is associated with the increasing lifespan of the plant's rooting units (from annuals and stoloniferous species through rhizomatous species up to nonclonal perennial species). Lifespan of the rooting unit thus determines a range of root traits, namely root mass fraction (proportion of roots from the total biomass) and root tissue density. This shared continuum in root traits, life span of the rooting units and the type of belowground nonacquisitive organs is correlated with the environment where the species typically occur, with fast end (annual and stoloniferous species) occurring in productive and disturbed, and slow end (nonclonal perennials) in unproductive habitats. Further, clonal species have slightly shallower and thinner roots as their roots are relieved from the transport function which is served by horizontal stem-derived organs (stolons and rhizomes). This confirms that plant lifespan and belowground nonacquisitive organs must be considered as determinants for (fine) root traits in herbs.