Soilborne pathogens can contribute to the maintenance of local plant diversity by reducing differences in competitive ability between co-occurring plant species. It has been hypothesized that efficient phosphorus (P) acquisition by plants in P-impoverished ecosystems might trade off against resistance to root pathogens. This could help explain high plant diversity in severely nutrient-impoverished ecosystems. However, empirical evidence of such a trade-off remains scarce. In hyperdiverse shrublands in south-western Australia, non-mycorrhizal cluster-rooted Proteaceae are very efficient at acquiring P. However, Proteaceae co-occur with many other plant species using other P-acquisition strategies, such as ectomycorrhizal (ECM) associations. In a glasshouse experiment, we grew Proteaceae and ECM plant species from hyperdiverse shrublands alone and in competition with each other, and in the presence or absence of native soilborne pathogens (Phytophthora spp.). We hypothesized that native Phytophthora species are more detrimental to Proteaceae than co-occurring ECM plants, due to a trade-off between highly efficient P-acquisition and pathogen defence, and that this equalizes differences in competitive ability between these two plant groups. When seedlings were grown alone, biomass of non-mycorrhizal plants was reduced in the presence of Phytopthora, while ECM species were unaffected by this pathogen. When non-mycorrhizal and ECM species were planted together, ECM plants grew better in the presence of Phytophthora than in its absence, because Phytophthora reduced the growth of the non-mycorrhizal competitors. Growth of ECM plants was positively correlated with per cent root colonization by ECM fungi, but this was only significant when ECM plants were grown in the presence of Phytophthora. Synthesis. Our study shows that native soilborne pathogens equalized differences in competitive ability between seedlings of contrasting nutrient-acquisition strategies, thus supporting the hypothesis proposing a trade-off between highly efficient P-acquisition and resistance against root pathogens. We found that non-mycorrhizal cluster-rooted species may be the most efficient at acquiring the growth-limiting resource, but that co-occurring ECM species are better defended against root pathogens. Our results suggest that native soilborne pathogens and ECM contribute to the maintenance of the plant hyperdiversity in severely P-impoverished ecosystems.
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