The diversity, growth, and fitness of the invasive plants Fallopia japonica (Japanese knotweed) and Fallopia xbohemica in the United States (polygonaceae)

Fallopia japonica (Japanese knotweed) is an infamous perennial that is established throughout the U.S., Canada and Europe, where its strong rhizomatous growth is a great obstacle to remediation. Another congener, F. sachalinensis (Giant knotweed), and their hybrid Fallopia xbohemica, have also naturalized, with hybrids often comprising a large component of invading populations. Evidence suggests the invasion history and dynamics of these knotweeds differ between invaded continents, and in particular, that in North America seedlings are an important component of populations, and genetic diversity is greater. I addressed this with several experiments. First, I characterized hybridization in New England using morphological and molecular tools, and found introgression and morphological variation not previously detected in Europe (Gammon et al. 2007). Next, I used a chloroplast marker to compare North American and Japanese accessions, revealing at least three separate introductions and several singletons, in contrast to the one F. japonica var. japonica genet known in Europe (Gammon and Kesseli 2009). Third, in a common garden experiment with seedlings from controlled crosses and field-collected seed, I found many first and second-generation hybrids had equal or greater vegetative and reproductive capabilities than did the parent species F. japonica. These data suggest that the observed success of hybrids in North America may not solely be due to the advanced asexual spread of a rare genet(s), but that hybrid seedlings may regularly out-establish parentals. Finally, an analysis of the ploidy levels of plants in the common garden and at local field sites showed greater intra and inter-specific cytotype variation than has been reported in Europe, which is likely the result of multiple introductions of F. japonica and/or hybrids into North America, and subsequent reproduction and introgression. When examined together, these experiments show definitive similarities between invasions in Europe and the U.S., such as the dominance of the same F. japonica genotype on both continents, and the prevalence of F1 hybrids in nature. However, more genotype introductions and greater intra- and inter-specific reproduction in North America appear to have created a more complex swarm, which could translate into a greater potential for post-invasion adaptation.