Systematic and population studies of the invasive plant Tamarix

In one of the United States' worst invasions, Eurasian Tamarix plant species (family Tamaricaceae) have spread rapidly to dominate over 600,000 riparian and wetland hectares. The effective implementation of biological or legislative control of Tamarix has been influenced by phylogenetic questions at the family, species, and population levels. Here, I use a combination of molecular systematic and phylogeographic analyses to characterize the Tamarix invasion.; Generic relationships provide an important indicator of potential biological control agent host switching. The exact relationships of the genera in the family Tamaricaceae and its sister family Frankeniaceae (which contains some taxa native to the U.S.) are controversial, and have not been tested with molecular data. Here, systematic analysis of nuclear and chloroplast DNA sequence data establishes relationships of all genera, and confirms that the two families are indeed monophyletic.; The Tamarix invasion consists of many species, some of which are morphologically very similar. The specific identities of these taxa involved in the invasion are controversial and hybridization may have played a role in the taxonomic confusion. The morphology and taxonomic history of 12 putative U.S. invasive Tamarix species are examined, and using chloroplast and nuclear sequence data, I conclude that there are four invasive entities in the U.S.: T. aphylla, T. parviflora, genetically indistinguishable T. ramosissima and T. chinensis, and genetically indistinguishable T. gallica and T. canariensis. A second analysis of Tamarix systematics using four chloroplast intron markers determines that current subgeneric taxa do not accurately describe natural groups in the genus.; Initial biological control tests show that imported insects have differential effectiveness on what appear to be a single species of Tamarix collected from different regions of the U.S., raising the issue of geographical variation. At the population level, DNA sequence markers are developed and a phylogeographic analysis of these indicates that the most common plant in this U.S. Tamarix invasion is a hybrid combination of two species-specific genotypes that are geographically isolated in their native Eurasian range. The presence of potentially novel hybrids in the U.S. illustrates how importation of exotics can alter population structure of species and contribute to invasions.