Environmental variation and its effect on phenotypic variation have long been of interest to evolutionary ecologists. Several hypotheses suggest how plants can live across steep environmental gradients. These include clonal integration, phenotypic plasticity and genetic differentiation. The main objective of this dissertation was to determine what strategies salt marsh plants use to live across the environmental gradients of the marsh. Field surveys of plant phenotypes, environmental parameters and allozyme patterns as well as a greenhouse study and a reciprocal transplant study were conducted to meet this objective. Field surveys of 12 species indicated that plant phenotypic variation is correlated to environmental variation. In contrast, allozyme patterns showed no association between alleles or genotypes with microhabitats. Similarly, levels of diversity did not differ across microhabitats along the gradient. Much of the variation in the distribution of genetic diversity, however, was predictable based on the gradient. In addition, genetic diversity was surprisingly high and clone size was limited. In both the greenhouse and field reciprocal transplant studies there was evidence of phenotypic plasticity for all traits measured. The greenhouse study on outcrossed seedlings revealed genetic variation for only final height and concentrations of leaf elements Na, P, and Mg. Alternatively, the field experiment on clones of field collected plants, found genetic variation in almost all salt tolerance traits. In high and low salt gardens, there was significant selection for increased total leaf area and water use efficiency (WUE). However, patterns of selection were significantly different in the two gardens only for stabilizing selection on WUE. These studies suggest that salt marsh plants are highly plastic. Although there is a lot of genetic variation for salt tolerance traits, and some evidence for differentiation between the two habitats, there is little evidence that these habitats select on traits differently. Differential selection in the two habitats on WUE was the one exception, however there was no evidence of differentiation for this trait. These studies therefore reveal the importance of phenotypic plasticity as the predominant strategy for living across the environmental gradients of the salt marsh.
