Examining urban weed response to elevated salt levels

Stressors from the urban environment cause adverse effects on city-dwelling organisms. While there is evidence that some species are adapting to urban stressors, such as the urban heat island and impervious surfaces, other stressors such as road salt are less studied. We investigated whether plants are adapting to elevated salt concentrations with two common garden experiments using three common weed species: Veronica arvensis, Stellaria media, and Lamium purpureum. We collected plants from parks around Baltimore, MD, USA, collecting plant maternal lineages from roadside soils (i.e. higher soil salinity) and inner park soils (i.e. lower soil salinity). We hypothesized that roadside lineages would tolerate salt treatment better than park lineages. In our germination experiment on wild-collected seeds, we observed a non-significant trend where the germination rate of roadside L. purpureum lineages was reduced less by salt treatment than for park genetic lineages, while S. media had similar declines between groups. In our greenhouse experiment on the F1 generation, roadside V. arvensis lineages displayed a less adverse reaction to salt treatment than park V. arvensis lineages. L. purpureum did not display this difference, but roadside lineages of both species exhibited less tissue damage overall. The results of our experiments were likely influenced by the maternal environment, and work on F2 generations is required to confirm them. Our findings show some support for urban salt tolerance adaptation, and point to the heterogeneity of the urban environment and the complexity of plant reactions to salinity.