Negative plant soil feedback influences plant community development in grassland restoration

Restoration of degraded lands is primarily guided by plant community ecology. However, many seed-based restorations fail or result in unpredictable outcomes, which indicates a need to better understand what determines community trajectory and stability. Here we surveyed two ongoing restoration projects, examined data from an eight-year-old field experiment, and conducted several greenhouse experiments to determine if an observed sudden boom-and-bust pattern of western yarrow (Achillea millefolium) was due to an accumulation of soil pathogens, and if those could spill over and influence other native species commonly used in restoration of semi-arid grasslands. We show the lower yarrow cover in the older restoration project coincided with a higher relative abundance of putative fungal pathogens, in particular a Paraphoma sp., which is known to cause crown rot. This pathogen also increased over time in the field experiment while yarrow declined. The onset of this decline was density-dependent and occurred faster in monocultures with high yarrow density than in mixed plots where yarrow density was lower. In the greenhouse, yarrow grew better with fungicides in live, but not sterile soil, suggesting suppressive effects of fungal pathogens. However, other native species grew better in yarrow-trained soil than soil conditioned by another native plant, indicating no pathogen spill-over after yarrow but instead a potential to facilitate soil conditions for succeeding natives. The rapid establishment and competitive nature of yarrow followed by pathogen-mediated declines make it an interesting potential bridge species in restoration of degraded semi-arid grasslands. We demonstrate that plant soil feedback can be a strong driver of plant community development during restoration. Incorporating it into foundational theories of restoration ecology, which are currently based largey on plant-plant interactions may improve predictability in restoration projects.