Purple loosestrife (Lythrum salicaria) is a highly invasive species able to quickly take over entire wetlands and form persistent and prolific seed banks. Its seed banking ability is largely due to its bountiful seed production. Cattail species (Typha spp.), which are some of loosestrife’s most common competitors, have comparable seed production rates but less abundant seed banks. Thus, seed survival belowground may differ between purple loosestrife and cattails. Because of the importance of fungal pathogens to seed viability, we examined whether soil fungi differentially affect seed germination rates of purple loosestrife and cattail species. Following the predator escape hypothesis, it is possible that purple loosestrife experiences a reduced fungal pathogen load in North America, where as cattails in their native range are targeted by native pathogens. We buried packets of purple loosestrife and cattail seeds in field soil and treated half with fungicide during greenhouse incubation. To investigate whether fungi-seed interactions depend on environmental context, samples were kept dry, well-watered, or saturated. After 12 weeks, the remaining packets were exhumed and seeds were monitored for germination in a growth chamber. We characterized the microbial communities within seeds by culturing fungi and bacteria from additional seeds we buried and exhumed after four weeks.
Our results suggest that purple loosestrife seeds are present at significantly higher abundances in the seed bank than cattails (V = 795, p <0.001, paired Wilcoxon test). We found a significant correlation between purple loosestrife’s aboveground and belowground density (r = 0.523, p < 0.001), but not cattail. Purple loosestrife seeds were infected by soil fungi at a significantly higher rate than cattail seeds (c2 =9.645, p = 0.002). Although fungicide did not reduce fungal infection rates in purple loosestrife, fungal composition based on morphotype varied with fungicide treatment. Fungicide provided a protective effect for purple loosestrife seeds only in well-watered soils. By contrast fungicide application increased cattail germination in both well-watered and saturated soils, but negatively impacted cattail seeds in dry soils. Our results indicate that in saturated conditions, where both of these species commonly occur, fungi-mediated seed mortality may partially determine differential seed bank densities.