Urban wetlands are exposed to a variety of anthropogenic impacts that may hinder ecosystem function. In particular, salt from winter road de-icing and metals accumulate more in urban wetlands compared to wetlands surrounded by natural land use. These stressors can negatively impact native plant communities and contribute to invasive species establishment. Phalaris arundinacea is an invasive wetland grass found across North America that has been shown to be mildly salt tolerant and variably tolerant to metal contamination. The species is also highly variable, thus urban populations of P. arundinacea could become locally adapted in the presence of anthropogenic stressors. We quantified responses to salt, copper and zinc addition by P. arundinacea collected from four populations spanning an urbanization gradient (natural, rural, moderate urban and intense urban), based on surrounding land use. We grew plants from seed, divided them to create genetically identical clones and conducted a common garden experiment in Columbus, Ohio for one growing season. In the experiment, clones of five genotypes per population were treated with copper, salt, zinc, or a no-stress control. We measured twelve morphological and physiological above and belowground traits, testing for trait and trait plasticity differences based on source population, treatment and their interaction.
Results/Conclusions
We found no significant interactions between treatment and source population for plant traits. Aboveground biomass and height were negatively impacted by all stress treatments. Compared to control plants, only salt addition reduced specific leaf area, photosynthetic rate, belowground biomass and specific root length. Copper addition reduced belowground biomass and zinc addition increased root mass ratio compared to control. We saw significant differences between natural and intense urban populations in a suite of traits (aboveground biomass, leaf dry matter content, specific root length and root mass ratio), indicating that plants from the intense urban population . Finally, plasticity in fine root diameter in response to all stress treatments averaged together differed by source population; under stressful conditions, fine root diameter increased in plants from the intense urban population but decreased in plants from the natural population, compared to controls. Fine root diameter is positively associated with penetration force in the soil and nutrient uptake. Overall, these results are consistent with local adaptation to metal and salt contamination for P. arundinacea plants experiencing conditions of intense urbanization.