Root system topology and diameter distribution of species from habitats differing in inundation frequency

1. We compared the root systems of seven halophytic species that occur at different elevations on a salt marsh, in order to (i) test the hypothesis that variations in root system architecture reflect adaptation to inundation frequency or nitrogen limitation, and (ii) verify the theoretically predicted relationships between root diameter, link magnitude and root topology. Diameters and lengths of individual laterals were determined along root axes, and branching patterns were quantified by calculating a topological index (TI). 2. Chenopodiaceae (annual dicots) showed that with increasing elevation, the branch density and length of individual first-order laterals tended to increase, so that the relative length of the main axes decreased. Root branching of the Chenopodiaceae at lower elevations was herringbone-like, whereas species from higher elevations had smaller TIs because their branching patterns were more complex. 3. The Gramineae, too, showed a tendency to increased length of individual laterals with increasing elevation. However, TI was not related to elevation, did not indicate a herringbone structure for all species, and was within the same range of that of the Chenopodiaceae. 4. As root topology of the Chenopodiaceae is related to elevation, but that of the grasses is not, topology is not necessarily an important adaptive trait in all plant families that inhabit the salt marsh. Short first- and second-order laterals may represent a more general architectural adaptation to frequent inundation, with longer first-order laterals being favourable to competition for nutrients. 5. Diameters at the root base tended to decrease if root branching was herringbone-like (TI close to 1). Roots of first-order laterals were approximately one-third of the diameter of the main axes; second-order laterals were approximately half the diameter of the first-order laterals. These ratios illustrate the value of using the developmental segment-ordering system in describing roots. The theoretically predicted relationship between root diameter and link magnitude was not present within individual orders of roots, whereas diameter did slowly increase with magnitude when combining different root orders. 6. In the absence of a clear relationship between root diameter and link magnitude, the predicted high carbon costs associated with herringbone root systems disappear, whereas the advantage of minimized inter-root competition remains. Consequently, herringbone root systems will be most efficient in terms of nutrients gained per carbon invested. However, dichotomous root systems offer a greater potential for exploring the soil, which contributes to the potential competitiveness of plants growing in nutrient limited habitats.