Native plant recruitment is frequently reduced when invasive plants are present, and while the mechanisms that underlie this pattern are not always known, enemy release and competition are plausible explanations. However, when invasive plants decline through natural or human-imposed control, native plant recruitment does not always return to pre-infestation levels. Two possible explanations for this sustained recruitment limitation are: (1) pathogen spillover; and (2) changes in the soil microbiotic community, both resulting from plant invasion. In this study, we investigated whether either of these factors influence the recruitment of the native plants of the northern Great Plains following invasion from the non-native plant leafy spurge (Euphorbia esula). In a factorial greenhouse experiment, sterilized soils were inoculated with native field soils of different spurge invasion histories, and the soils were subsequently conditioned by leafy spurge. The leafy spurge was then removed, soils were sown with eight native species to quantify establishment and growth rates, and we conducted TRFLP analyses on roots to identify microbial inhabitants. We also conducted a fungal pathogen assay, applying 15 generalist isolates to spurge and 12 native species to determine pathogen susceptibility of native plants to common pathogens of leafy spurge.

Results/Conclusions

Germination rates of some native plants, including Artemisia frigida and Pascopyrum smithii, were negatively impacted by leafy spurge conditioning, and were positively impacted by inoculation (compared to sterilized soils) for some species, including A. frigida and Bouteloua gracilis. Plant height and biomass were lower when grown in the soil microbial community found at currently invaded sites compared to sites dominated by natives. The fungal pathogen  Fusarium oxysporum was found in root tissues of natives that performed poorly in the experiment, including Vicia americana, while three species of arbuscular mycorrhizal fungi were found in roots of native species that performed well. In the pathogen assay, eight isolates (including F. oxysporum) were associated with higher rates of damage across all species than the uninfected control group. These results indicate that leafy spurge presence can alter soil conditions that reduce native plant germination, and some species benefited from microbiota in the “living” soils. However, there are soil microbiota at currently invaded sites that inhibit native plant growth, indicating that pathogen loads are higher and beneficial microbiota are reduced. Finally, the degree of damage exhibited by natives in the assay indicates that generalist pathogens can infect natives, lending support to the pathogen spillover hypothesis.