Exotic plants have the ability to modify abiotic and biotic soil conditions, but little is known about the legacy of invasion once an exotic plant has been controlled. Leafy spurge (Euphorbia esula), a nonnative invasive plant of the northern Great Plains USA, has been successfully controlled in many areas, but plant communities typically have one of two fates following its control: a slow return of native plants, or a secondary invasion of other exotic plants. We hypothesize that leafy spurge has negative effects on subsequent plant establishment via two distinct mechanisms. First, we hypothesize that leafy spurge root exudates may directly inhibit native plant seed germination. Second, we consider the potential for exudates to stimulate soilborne pathogen population growth, contributing to the accumulation of pathogens of native plant species and reduced establishment of those plants. We examined the effect of leafy spurge root exudates at varying concentrations on the germination of native prairie seeds in a growth chamber. We also investigated the degree to which fungal pathogen growth on agar varies with leafy spurge and native plant root exudates, and the combined effects of leafy spurge root exudates and fungal pathogens on native plant germination and growth in a glasshouse.
Results/Conclusions
Leafy spurge root exudates inhibited the germination of most native species tested, an effect which increased with exudate concentrations, indicating a potential for reducing native plant establishment. The fungal pathogen growth response to root exudates varied among fungal isolates and plant species. Leafy spurge root exudates did not elicit strong fungal growth stimulation or inhibition in most cases, but root exudates of some native plant species consistently enhanced pathogen growth, while others inhibited pathogen growth. This suggests that soilborne pathogenic fungi may accumulate in the soils of some native plant species, potentially limiting plant establishment, while pathogen suppression in the soils of other native species may enhance plant establishment. Native plant germination and seedling biomass were generally reduced when exposed to fungal pathogens for one month, but enhanced when exposed to pathogens and leafy spurge root exudates combined. This may be a temporary effect from fungi metabolizing leafy spurge exudates instead of attacking seeds or seedlings, indicating that pathogen accumulation via spurge may not occur in the field. However, variation in native plant responses to spurge exudates and in soilborne pathogen response to native root exudates may prove valuable in selecting plant species for restoration of previously spurge-infested lands.