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 stemderived organs
(stolons and rhizomes). This confirms that plant lifespan and belowground
nonacquisitive organs must be considered as determinants for (fine) root
traits in herbs.