In the U.S., Potentilla recta is an invasive, exotic forb. Previous research suggests that the soil biota of native congeners may provide biotic resistance to P. recta, presumably because phylogenetically similar species may culture and be susceptible to similar pathogens. The purpose of our study was to investigate whether: (1) biotic resistance can be attributed to effects of the whole microbial community (<2 mm), or the small (<20µm) fraction that isolates potential pathogens from soil mutualists, such as arbuscular mycorrhizal fungi (AMF), and (2) if conspecific biotic resistance mediates the distribution of P. recta in an intermountain grassland in western Montana.
In the greenhouse, we grew P. recta with three soil microbial treatments [none, small fraction (<20 µm), whole community (<2 mm)] from the rhizospheres of either invading P. recta, congeners P. gracilis and Drymocallis glandulosa (formerly Potentilla), or non-congeners (native grass and native forb). To assess the distribution of P. recta in relation to the congeners, we compared the relative distances to the congeners from P. recta and three non-congener forbs (Geum triflorum, Geranium viscosissimum, or Hieracium scouleri). The relative distance for P. recta or the three non-congener forbs was calculated as the distance to the nearest-native congener minus the distance to the nearest non-congener, divided by the distance to the nearest non-congener.
In the greenhouse, we found biotic resistance to P. recta from the whole microbial community collected from both congeners and non-congeners. Additionally, the magnitude of this negative effect was correlated with the proportion of AMF colonization. In contrast, we found no evidence of general biotic resistance from the small biota fraction. In the field, the identity of neighbor plants did not appear to influence P. recta distributions, although P. recta was marginally (p = 0.07) more distant from the congener P. gracilis. Overall, our results contradict previous findings and suggest that the biotic resistance may be largely non-specific and therefore unlikely to influence field distributions of this problematic invader in our system. Molecular characterization allowing for comparisons of fungal communities in P. recta, congener and non-congener rhizosphere soil is underway and will be discussed.