Help from under ground: soil biota facilitate knotweed invasion

Soil biota can be important drivers of plant community structure. Depending on the balance between antagonistic and mutualistic interactions, they can limit or promote the success of plant species. This is particularly important in the context of exotic plant invasions where soil biota can either increase the biotic resistance of habitats, or they can shift the balance between exotic and native plants towards the exotics and thereby greatly contribute to their dominance. Here, we explored the role of soil biota in the invasion success of exotic knotweed (Fallopia × bohemica), one of the world's most noxious invasive plants. We created artificial native plant communities that were experimentally invaded by knotweed, using a range of substrates where we manipulated different fractions of soil biota. We found that invasive knotweed benefited more from the overall presence of soil biota than any of the six native species. In particular the presence of the full natural soil biota strongly shifted the competitive balance in favor of knotweed. Soil biota promoted both regeneration and growth of the invader, which suggests that soil organisms may be important both in the early establishment of knotweed and possibly its later dominance of native communities. Addition of activated carbon to the soil made the advantage of knotweed disappear, which suggests that the mechanisms underlying the positive soil biota effects are chemically mediated. Our study demonstrates that soil organisms play a key role in the invasion success of exotic knotweed.

Hybridization increases invasive knotweed success

Hybridization is one of the fundamental mechanisms by which rapid evolution can occur in exotic species. If hybrids show increased vigour, this could significantly contribute to invasion success. Here, we compared the success of the two invasive knotweeds, Fallopia japonica and F.sachalinensis, and their hybrid, F.x bohemica, in competing against experimental communities of native plants. Using plant material from multiple clones of each taxon collected across a latitudinal gradient in Central Europe, we found that knotweed hybrids performed significantly better in competition with a native community and that they more strongly reduced the growth of the native plants. One of the parental species, F.sachalinensis, regenerated significantly less well from rhizomes, and this difference disappeared if activated carbon was added to the substrate, which suggests allelopathic inhibition of F.sachalinensis regeneration by native plants. We found substantial within-taxon variation in competitive success in all knotweed taxa, but variation was generally greatest in the hybrid. Interestingly, there was also significant variation within the genetically uniform F.japonica, possibly reflecting epigenetic differences. Our study shows that invasive knotweed hybrids are indeed more competitive than their parents and that hybridization increased the invasiveness of the exotic knotweed complex.