COS 98-1 - Does pathogen accumulation make western yarrow (Achillea millefolium) a good bridge species in restoration?

Background/Question/Methods

Ecological restoration of degraded grasslands often fails due to pervasive seed banks and early competitive dynamics. Rapid establishment of native plants is imperative to success, but most native species are weak competitors at the seedling stage, tending to grow and establish slowly. One exception is western yarrow (Achillea millefolium L.) which develops high-density stands quickly. However, at our study site in western Montana we observed dense stands of yarrow declined rapidly just four to five years after establishment. We explored the role of fungal pathogens in the decline of yarrow using soil and root samples collected from two restoration areas of different ages and an ongoing field experiment. In the field experiment, yarrow was planted in monoculture or a mixture with other native forbs and half of the plots were treated with fungicide. We surveyed pathogen abundance and used molecular methods to characterize fungal pathogens in soil and yarrow roots. Next, we asked whether pathogen accumulation associated with yarrow could interfere with the establishment of other native forbs. To do that, we grew three native forbs commonly used in restoration in yarrow and non-yarrow trained soil with and without fungicides in the greenhouse.

Results/Conclusions

Abundance of soil fungal pathogens was significantly greater in the older restoration area, and indicator species analysis revealed that a root rot fungal pathogen in the genus Paraphoma was more associated with yarrow roots in the older restoration area. In the experimental plots, yarrow cover declined in all plot types and treatments over time, but cover was significantly higher where fungicides had been added, suggesting that at least part of the decline was due to fungal pathogens. Indeed, fungal pathogen communities differed between control and fungicided plots and Paraphoma was more associated with soil collected from control plots. There also appeared to be density-dependent effect because the decline of yarrow started earlier in plots where yarrow cover was highest. In the greenhouse, yarrow trained soil did not cause growth suppression in other native forbs, suggesting that whatever suppressed yarrow was species-specific. Overall, our findings suggest that a naturally occurring fungal pathogen may accumulate in a density-dependent fashion and limit yarrow abundance. This self-regulation may allow yarrow to serve as a bridge species in restoration by quickly capturing open niche space and indirectly facilitating restoration species that germinate or arrive naturally.