Vallisneria americana Michx. (Hydrocharitaceae) is an ecologically important submersed aquatic plant that once dominated freshwater to oligohaline environments in eastern North America. After dramatic declines it is the target of many restoration initiatives. To increase knowledge of the capacity of remaining populations to either adapt through natural selection or acclimate to emerging environmental conditions, I combined genetic data and common environment experiments to quantify V. americana genetic diversity and differentiation at local to regional scales, evaluate evidence of local adaptation to different climate conditions, and assess evidence of inbreeding or outbreeding depression.

I quantified the structure of genetic diversity in five sites from the tidal Potomac, Hudson, and Kennebec Rivers, and 33 sites across the species' distribution in the Potomac.  Genotypic (0.1-1.0) and allelic diversity (1.5-5.5), observed heterozygosity (0.34-0.72), and relatedness (-0.06-1.00) varied greatly along rivers and across latitude. Hudson V. americana had the lowest genetic diversity and Potomac had the highest. Differentiation and network analysis of relatedness revealed no common genetic diversity distribution patterns within rivers. Major differences in genetic structure were observed across the tidal and non-tidal Potomac.  

Common environment experiments evaluating growth and reproductive performance of Potomac, Hudson, and Kennebec V. americana grown in different temperature and photoperiod conditions only found evidence of local adaption in Potomac plants. Few overall differences in morphological and life history traits were observed between local and foreign plants. Plants grown under global warming conditions had reduced performance. Limited evidence of local adaptation and high acclimation to different conditions suggest that populations have high potential for resilience in the face of climate change, so long as temperatures do not exceed thermal tolerances. Climate change mitigation strategies that involve transplanting individuals may also be successful.

To investigate consequences of restoration strategies that translocate individuals, I evaluated seed production and germination success of controlled reproductive crosses between V. americana within and among genetically differentiated populations in the Chesapeake Bay. There were no consistent patterns of inbreeding or outbreeding depression in crosses. Effects of mixing sources were site-specific and not predicted by levels of relatedness among individuals, genetic diversity within, or differentiation among populations.