Cranberry (Vaccinium macrocarpon), a commercially grown evergreen dwarf shrub, is a dominant taxon in temperate bogs in North America. It spreads clonally by runners, and reproduces sexually predominantly by self-fertilization on upright stems. The objective of this project was to investigate genetic and clonal variation and phenotypic plasticity of V. macrocarpon. Specifically, I wanted to test whether there exists an inverse relationship between population genetic variation and the amount of overall phenotypic plasticity of vegetative characteristics.

As background information I assessed the vegetation and edaphic factors of marginal cranberry bogs found in the mid-to-southern Appalachians. A gradient of nutrient availabilities was found among bogs that was positively associated strongly with the dominance of the more generalist Rubus hispidus and negatively with ericaeous bog shrubs such as V. macrocarpon. Eutrophication may lead to the replacement of endemic bog species with generalist plastic species.

Theoretically, it would be plausible for environmental heterogeneity or stress to allow selection for more phenotypically plastic clones within a species. A single adaptively plastic clone for growth strategy could sweep a site, excluding intraspecific competitors. That is, selection could favor clones with high plasticity that could subsequently lead to a loss of genetic variation within a population. The environmental and genetic conditions favoring this would more likely exist in distributionally marginal sites because of spatial and temporal heterogeneity and island-like biogeography. Field and common garden experiments in which nutrients were manipulated were performed to test for an inverse relationship between phenotypic plasticity and genetic heterogeneity. Random amplified polymorphic DNA (RAPD) profiling was coupled with ecological measurements of plant growth on the experimental clones and other clones from the experimental populations to estimate genetic heterogeneity.

Genetic heterogeneity was found to be significantly lower in marginal populations than in central populations. Phenotypic plasticity was somewhat higher in a more marginal population in the field sites, but direct statistical comparisons could not be made. The common garden study was inconclusive, possibly due to transplant shock, but a trend among natural populations was towards higher plasticity among marginal clones. Additional research on other species is needed to clarify the possible inverse relationship between phenotypic plasticity and genetic variation within populations.