Salt Tolerance of Invasive Phalaris arundinacea Exceeds That of Native Carex stricta (Wisconsin)

Invasive plants that are more salt tolerant than their native competitors would be favored in wetlands that receive inflows of road salt (the most common being NaCl). Although sodium and chlorine ions are essential in minute quantities for plant growth, high concentrations are stressful to non-salt-tolerant plants. While some inland wetlands are naturally saline, wetlands of the Upper Midwestern USA tend to experience saline runoff following winter application of deicing salt. And since more than 70% of U.S. roads are in snowy regions (FHWA 2009), considerable salt-enriched runoff makes its way into ditches and downstream wetlands.

Salt causes most plants to wilt, which triggers the production of abscisic acid, a hormone that mediates stomatal closure, thus reducing carbon dioxide intake and photosynthesis (Redondo-Gómez et al. 2007). In addition, sodium can replace potassium in leaf tissues, causing necrosis; chlorine and sodium ions can disrupt amino acid bonding in proteins; and affected plants divert energy from growth to export toxic ions from cells. The visible effects of salt exposure include leaf necrosis, decreased productivity, and mortality.

If invasive species are more salt tolerant than natives, road salts could contribute to their establishment and abundance in affected wetlands. Reed canarygrass (Phalaris arundinacea) has been called a model invader (Lavergne and Molofsky 2004). It has replaced the native tussock sedge (Carex stricta) over large areas of sedge meadow (Zedler 2009). Here, we tested the hypothesis that reed canarygrass is more salt tolerant than tussock sedge.

Figure 1.

Growth (net new biomass) of reed canarygrass and tussock sedge with increasing NaCl additions.

We tested salt tolerance of similar-size individuals by growing them in a University of Wisconsin–Madison greenhouse under seven treatments (watering with 0.0, 0.5, 1.0, 2.0, 4.0, 8.0, and 16.0 ppt NaCl) and assessing four response variables: new biomass production, necrosis severity, mortality, and ash weight (mineral content, which would be high where salt accumulated). We collected propagules of each species from a rural, groundwater-fed wetland with no record of salt influx in the Town of Dunn, Dane County. On October 1, 2008, we took 35 individuals of reed canarygrass from a shallow coldwater spring near the headwater. We had previously collected seeds of tussock sedge growing in the adjacent sedge meadow and germinated them in May 2008 in a greenhouse to produce 35 individuals.

On October 3, we pruned all 70 individuals to 10 cm height and weighed them wet, after washing and blotting, then planted each in its own 1 L container with moist vermiculite. One replicate per treatment was randomly assigned to each of the five blocks within a 1 × 1.25 m glass enclosure in the greenhouse. We watered the plants weekly; controls received 200 mL of deionized water, and NaCl treatments received 200 mL of solution with either 0.5 g, 1 g, 4 g, 8 g, or 16 g of NaCl per liter of deionized water. On October 3, October 21, and November 11, 2008, we mixed 14.7 g of Miracle-Gro water-soluble all-purpose plant food with 4 L of deionized water and watered all 70 containers equally to provide good growing conditions. We illuminated plants with a 430 W high-pressure sodium lamp on a 24-hour schedule. A HOBO data logger recorded temperatures during the treatment period (minimum 15.5°C, maximum 31.6°C). The experiment ended on December 15, 2008.

Intermittently (Oct. 27 and 29; Nov. 6, 11, and 18; Dec. 15), we visually rated necrosis using classes that ranged from 1 (= none to minimal) to 5 (= complete). We pooled classes 1–2 as low levels and 3–5 as high in comparing the two species to create a 2 × 2 contingency table with pooled NaCl treatments (0–4 vs. 8–16 ppt), which we analyzed with the chi-square test. We also recorded the number of plants that died. To determine net biomass production, we rinsed each individual (including dead mass), blotted it, obtained wet...