Reduced vigor of a clonal invader: Lack of evidence for soilborne pathogens

• Ramets of smooth brome were less vigorous at the center of invading clones.
• Fewer plant parasitic nematodes were found at the center of invading clones.
• Soil pasteurization had no significant effect on the biomass of seedlings.
• Higher available N, P, and soil moisture at the center of invading clones suggest their senescence.
• Persistent impacts of smooth brome on prairie communities will require active restoration.

Predicting the impacts of exotic species on native ecosystems requires an understanding of the ecological processes that regulate biological invasions. Despite accumulating evidence that belowground interactions play a significant role in determining the outcome of plant invasions, few studies have examined the role of soilborne pathogens in invasion dynamics. We tested the hypothesis that accumulating negative plant-soil feedbacks reduce the vigor of clones of smooth brome (Bromus inermis), invading northern fescue prairies in Manitoba, Canada. We examined the density, biomass, and height of ramets of smooth brome from the center of invading clones to their fringes and tested the negative plant–soil feedback hypothesis using plant parasitic nematodes and soilborne pathogens. Ramets of smooth brome were less vigorous inside the invading clones compared with those along the fringes. The observed loss of vigor inside the clones was not related to plant parasitic nematodes or soilborne pathogens. Fewer soilborne plant parasitic nematodes were found at the center of invading clones and pasteurization had no effect on the mass of seedlings raised in soil collected in this area. Higher available soil nitrogen, phosphorus, and soil moisture at the center of invading clones suggest senescence in the older areas of the clone. However, other hypotheses, including potential impacts of other groups of plant pathogens and autotoxic effects from the accumulating litter, remain unexplored. Despite the reduced vigor of smooth brome at the center of invading clones, plant communities in this area remained less diverse. Future restoration of native prairies invaded by smooth brome will require active methods to reverse these negative impacts.