An investigation of the below-ground properties of two Polygonum species: native Polygonum virginianum and non-native invasive Polygonum cespitosum

Background/Question/Methods

Invasive species can have potentially enormous negative impacts on their new environments, and are second only to habitat loss as a threat to global biodiversity.  Though focus on invasion ecology has increased in recent years, much of the current body of research is anecdotal, and there is limited understanding of what makes certain plant species more invasive than others. There is some research that suggests that negative below-ground effects may play a role in the success of invasive plants, and previous research has shown that Polygonum cuspidatum (Japanese knotweed) has some negative soil effects. However, P. cuspidatum is a large, fast-growing plant, and its success as an invader may be due to these traits rather than its allelopathic potential.  For this reason, our research focuses on the potential allelopathic effects of two small-bodied Polygonum congeners: non-native, invasive Polygonum cespitosum (Oriental lady?s  thumb) and native Polygonum virginianum (Jumpseed).  We collected soil samples from three areas with high density patches of either P. cespitosum or P. virginianum and from areas directly adjacent to these high density patches. Into these six soil types, we planted three types of native New England wildflowers (Aster lateriflorus, Monarda fistulosa, and Helenium flexuosum) and measured the potential allelopathic effects on the wildflower seedlings by collecting germination percentage, plant height, number of leaves, and biomass.

Results/Conclusions:

Analysis of the germination data reveals significantly higher wildflower germination in soil collected from under high density patches of Polygonum than from soil adjacent to these high density patches. We found particularly high germination in soil from beneath P. cespitosum.  This result suggests that P. cespitosum has unexpected positive effects on soil quality (e.g., attraction of mycorrhizal symbiots, increased nutrient pools, decreased pathogen communities) that might help to explain the species' invasion success. Plant growth and biomass data suggest similar positive soil effects from P. cespitosum. These soil quality effects, in combination with the high germination of P. cespitosum seedlings across every treatment, may lead to high fecundity, emergence, and survival.