The University of Vermont recently put out a press release called "Mellow in Europe, Crazy in America," about a paper by Sébastien Lavergne and Jane Molofsky. Their research, published in the Proceedings of the National Academy of Sciences, is an excellent comparison of the genetic diversity of reed canarygrass (Phalaris arundinacea) in its native vs. introduced range. We just discussed this paper in my class last week and I have to say it is a very nice study. The authors demonstrate that repeated introductions of the grass from different parts of Europe gave it multiple opportunities for genetic recombination in the USA - in fact alleles that are found only in France or only in the Czech Republic are found together in the USA. The result has been an increase of genetic diversity in these introduced populations, and new genotypes that are not known in Europe. Introduced populations also displayed increased phenotypic variation, expressed in the development of a significantly greater number of tillers (vegetative reproduction via new shoots that grow from the base of a clump of grass), more leaf production and more overall above-ground biomass. The authors attribute these new phenotypes to the rapid spread of reed canarygrass in the USA.
The situation is complicated by the presence of what the authors term "pre-settlement populations" of reed canarygrass in the USA - the "Is it native?" conundrum. However, this study indicates that at least 85% of the genetic diversity of reed canarygrass in the USA is of European origin and it appears that the invasive phenotypes are likely European, similar to what was found with Phragmites.