Are trade-offs in allocation pattern and root morphology related to species abundance? A congeneric comparison between rare and common species in the south-western Australian flora

Many narrowly endemic species are restricted to distinctive edaphic environments. The adaptations that make these species successful in their restricted habitat may incur a cost, and decrease their success in more common habitats. We compared growth, biomass allocation and root morphology of two narrowly endemic Hakea species (Proteaceae) of Mediterranean south-western Australia with those of five more widespread congeners, in a glasshouse study. The rare Hakea species occur in endangered winter-wet shrublands that grow on skeletal (0–20 cm deep) soils overlying massive ironstone rock, whereas their common congeners occur nearby on deeper wetland and non-wetland soils. The ironstone endemics differed consistently from their widespread congeners in some important root characteristics. During early development they allocated relatively more biomass to their roots, and had a higher specific root length due to a lower average root diameter and a lower root mass density. Therefore, when compared at the same plant mass, the ironstone endemics had a considerably greater total root length. The ironstone endemics also favoured root growth in deeper layers of the substrate: they invested up to 64% of their root mass in the bottom 10 cm of 40-cm-deep pots, vs. 35% for common species. Only in the ironstone endemics did the extension of the main root axis continue at the same rate after reaching the bottom of the pot. We suggest that the observed differences are the consequence of evolutionary trade-offs, and represent specializations of the endemic species to increase the chances of getting access to water before the onset of severe summer drought in these habitats. However, while adaptive in a shallow-soil habitat, these traits may reduce success on deeper soils by compromising both below-ground and above-ground competitive abilities.