Data from: Native soil-borne pathogens equalise differences in competitive ability between plants of contrasting nutrient-acquisition strategies

Soil-borne pathogens can contribute to the maintenance of local plant diversity by reducing differences in competitive ability between co-occurring plant species. It has been hypothesised 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 nutrient-poor ecosystems. However, empirical evidence of such trade-off remains scarce.
2. In hyperdiverse shrublands from south-western Australia, non-mycorrhizal cluster-rooted Proteaceae are most efficient in acquiring P. Still, Proteaceae coexist locally with many other plant species using other P-acquisition strategies, such as ectomycorrhizal (ECM) associations.
3. In a glasshouse experiment, we grew Proteaceae and ECM plant species from these shrublands alone and in competition with each other, and in the presence or absence of native soil-borne pathogens (<i>Phytophthora</i> spp.). We hypothesised that native <i>Phytophthora</i> species would be more detrimental to Proteaceae than co-occurring ECM plants, due to a trade-off between efficient P-acquisition and pathogen defence, and that this would reduce differences in competitiveness between these two plant groups.
4. When seedlings where grown alone, biomass of non-mycorrhizal plants was reduced in presence of <i>Phytophthora</i>, while ECM species were not affected by the presence of this pathogen. When non-mycorrhizal and ECM species were planted in competition with each other, ECM plants grew better in presence of Phytophthora than in its absence, because <i>Phytophthora</i> reduced the growth of the stronger non-mycorrhizal plant competitors.
5. Growth of ECM plants was positively correlated with percent root colonisation by ECM fungi, but this positive correlation was only significant when ECM plants were grown in presence of <i>Phytophthora</i>.
Synthesis. Our study shows that native soil-borne pathogens can equalise differences in competitive ability between plants of contrasting nutrient-acquisition strategies, thus supporting the hypothesis proposing a trade-off between efficient P-acquisition and resistance against root pathogens. Indeed, we found that non-mycorrhizal cluster-rooted species may be 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 soil-borne pathogens could contribute to the maintenance of the high local plant diversity found in these P-impoverished ecosystems.