Strong host specificity of a root hemi-parasite (Santalum acuminatum) limits its local distribution: beggars can be choosers

Aims: Santalum acuminatum (quandong) is a root hemi-parasite with a very wide distribution across southern Australia. Despite its very wide distribution, along the Jurien Bay chronosequence, it only occurs on the young Quindalup dunes, and it is absent on older dunes. The soils and local vegetation community change across the 10 km chronosequence, with higher species diversity correlated with lower soil phosphorus (P) levels. Here, we aimed to test whether the distribution of quandong on the dune systems can be explained by different neighbours (potential hosts) or different soil P concentrations across the chronosequence. Methods: Quandongs were grown in pots with 18 potential hosts for a year at three P levels, reflecting conditions across the chronosequence. Hemi-parasite growth and neighbour response were measured through the assessment of biomass, root mass ratios, haustorial size and frequency, δ ¹⁵ N and δ ¹³ C isotope signatures, as well as amino acid composition of xylem sap. Results: Effects of neighbour species on the growth of quandongs were stronger when they were paired with Acacia saligna than when grown with other legumes and non-legumes, indicating strong host specificity. Quandong growth with all other species was significantly less than when grown with A. saligna or without a host, indicating strong competition with a conspecific neighbour. Soil P concentration had little effect on quandong growth. Conclusion: Host specificity and competition from non-hosts comprise main drivers for the distribution of quandong across the Jurien Bay chronosequence, rather than soil P availability. Our results show the importance of host specificity and how it may restrict the distribution of hemi-parasitic plants in different plant communities along a steep ecological gradient.