Population Differentiation in the Range Expansion of a Native Maritime Plant, Solidago Sempervirens L

Solidago sempervirens, a halophytic goldenrod native to coastal dunes and marshes of eastern North America, has expanded its range inland along roads in parts of the northeastern United States during the past 20–30 yr. This expansion has been concurrent with the application of deicing salts on roadways in winter. During the same period, the species has appeared inland along the St. Lawrence waterway and the Great Lakes of Canada and the United States and near salt mines or seeps in western and west-central New York. In a greenhouse study of nine populations from coastal Massachusetts to west-central New York, we asked whether S. sempervirens has a physiological dependence on salt and whether populations differ genetically with respect to ecologically important traits, such as salt tolerance, phenology, and biomass allocation. Progeny from 20 maternal families per population were subjected to three watering treatments: control (no salt), medium salt (2% salinity), and high salt (4% salinity). Phenotypic traits differed significantly by treatment, population, and population response to treatment. Total plant mass decreased in response to salt, indicating no physiological dependence on salt; the species association with saline environments may therefore result from a tolerance for salt and to low competitive ability in nonsaline habitats. The reduction in total plant mass with increasing salt primarily resulted from a decrease in average shoot mass and less to a reduction in root mass. Even so, root allocation (i.e., the allometric exponent relating root mass to shoot mass) decreased from the control to the midand high-salt treatments. The propensity to flower during the course of the experiment also decreased with increasing salt. Flowering propensity increased from eastern to western populations in parallel with a gradient of increasing continentality. There was also some indication that coastal populations were less tolerant of high salt than inland populations. These geographic patterns of trait differentiation may indicate adaptive divergence among populations. There was significant variation attributable to family for biomass traits, but few trait heritabilities differed significantly from zero within populations.