Shallow-soil endemics: adaptive advantages and constraints of a specialized root-system morphology

Worldwide, many rare plant species occur in shallow-soil, drought-prone environments. For most of these species, the adaptations required to be successful in their own habitats, as well as their possible consequences for establishment and persistence in others, are unknown. Here, two rare Hakea (Proteaceae) species confined to shallow-soil communities in mediterranean-climate south-western Australia were compared with four congeners commonly occurring on deeper soils. Seedlings were grown for 7 months in a glasshouse in individual 1.8 x 0.2-m containers, to allow for unconstrained root development. In addition, a reciprocal transplant experiment was carried out. The rare Hakea species differed consistently from their common congeners in their spatial root placement. They invested more in deep roots and explored the bottom of the containers much more quickly. In the reciprocal transplant experiment they showed increased survival in their own habitat, but not in others. This research suggests that shallow-soil endemics have a specialized root system that allows them to explore a large rock surface area, thereby presumably increasing their chance to locate cracks in the underlying rock. However, this root-system morphology may be maladaptive on deeper soils, providing a possible explanation for the restricted distribution of many shallow-soil endemics.