Land-use legacy and the persistence of invasive Avena barbata on abandoned farmland

1. Exotic species invasion onto abandoned farmlands has been linked to an increase in the availability of soil nutrients after cultivation. Avena barbata Pott. (hereafter Avena) is an exotic annual grass that invades old-fields in south-western Australia and persists for decades after abandonment. We hypothesized that the competitive ability of Avena against native woody seedlings is increased on old-fields affected by P-fertilizer residues.2. We tested this hypothesis in a pot study by growing single native seedlings in competition with increasing densities of Avena, at old-field and pre-agricultural levels of soil P. Then, we planted the same four species, Acacia acuminata Benth., Allocasuarina campestris (Diels) L.A.S.Johnson, Eucalyptus loxophleba Benth. subsp. loxophleba and Hakea recurva Meisn., into plots within an old-field affected by P-fertilizer residues, and removed Avena from half of the plots. Also, as water availability limits seedling establishment, particularly in the presence of Avena, we compared the establishment of seedlings planted into microcatchments to improve water availability with that of seedlings planted into level sites.3. We found that P enrichment did not substantially enhance the ability of Avena to compete with any of the native species that we tested in pots. Avena was the superior competitor against all four native species at both P levels.4. In the field, the chances of native seedling establishment were markedly improved by the removal of Avena. Microcatchments improved the survival of E. loxophleba subsp. loxophleba in competition with Avena, but not that of the three other native species. In the absence of Avena, microcatchments made inconsistent or little difference to the survival and biomass of the planted native seedlings.5. Synthesis and applications. Reducing soil P will not be sufficient to promote species coexistence on old-fields because Avena remains competitive at low soil P. The combination of the land-use legacies, reduced native seed supply and the introduction of an invasive species that is a superior competitor regardless of P availability, all contribute to the persistence of this invasive species on old-fields in south-western Australia. The restoration of the historic ecosystem will require intense effort and there is a risk that planted communities will be prone to re-invasion.