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

1 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. 2 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. 3 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. 4 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.