Worldwide, many rare plant species occur in shallow-soil, drought-prone environments. For most of these species we do not know why they are rare, but it is likely that the adaptations that are required to be successful in their own specific habitat, hamper establishment and success in others. In the biodiversity hotspot of Mediterranean climate SW Australia more than 20% of the flora occurs on and is often confined to shallow-soil communities such as granite outcrops. Here we review our work comparing rare SW Australian shallow-soil endemics with common congeners occurring on deeper soils in order to ascertain (1) which traits confer success on shallow soils, and (2) why these traits may be maladaptive on deeper soils. Most of the work has been confined to species within the genus Hakea (Proteaceae) that are confined to shallow-soil ironstone communities and involved whole plant growth analysis, detailed investigations in root foraging and allocation patterns (using custom-made transparent long containers) and responses to different P-supply and light levels in glasshouse studies. Also, a large reciprocal transplant experiment in the field with glasshouse germinated seedlings of all species was conducted. To investigate the generality of our findings for other shallow-soil communities and species in other plant families and genera we compared 4 granite outcrop endemics with a common congener (genera: Allocasuarina, Calothamnus, Eucalyptus, Hakea) using similar methodologies in a glasshouse study.
Results/Conclusions
The rare shallow-soil Hakea species only differed consistently from their common congeners in their spatial root placement. They invested more in deep laterals and consequently explored the bottom of the containers much more quickly. Similar results were obtained comparing granite outcrop endemics with common congeners. Interestingly, whereas the common Hakea species responded to increased P supply by increasing allocation to superficial roots, shallow-soil endemics increased allocation to deep roots. These results strongly suggest that shallow-soil endemics are specialists that have a conservative root system morphology that allows them to explore a large rock surface area, thereby presumably increasing their chance to locate cracks leading to water sources in the underlying rock. Indeed the reciprocal transplant experiment showed that the rare shallow-soil Hakea species had higher survival rates but only in their own habitat. We suggest that the success of shallow-soil endemics in their own habitat as well as their failure to establish on deeper soils may reflect a fundamental trade-off between water and nutrient acquisition functions.