Background/Question/Methods Comparative studies of native and non-native populations of invasive species can help to determine the relative importance of environment vs genetic factors in affecting the growth and establishment of a particular invasive plant species. We present results from the first year of a three year study of Eurasian and North American populations of Lythrum salicaria L. (purple loosestrife). The aim of the study was to determine whether invasive North American and native Eurasian populations, taken from across the latitudinal gradient in both primary and secondary ranges, differed in terms of growth, morphology, and biomass allocation patterns. Using plants from four Eurasian (Finland, Czech Republic, Spain, Turkey) and four North American (Alberta, Canada; Wisconsin, Illinois, Tennessee, USA) populations, common gardens were established in spring 2006 in Louisiana, USA and Třeboń, Czech Republic. In each country, one-third of the plants were harvested when they reached the same phenological stage. Differences in growth and morphological traits between and within the common gardens were tested using reaction norms and ANOVAs, after accounting for covariance factors (time grown) and data transformations to achieve homogeneity of variance.

Results/Conclusions Within each common garden, plant growth traits (e.g., dry weight, stem height, etc) differed significantly among the populations. Within-trait differences were greater between European populations originating from the most northern and southern locations than for the similar invasive North American ones, indicating lower genetic variability in the invasive plants, as expected. Reaction norms indicated that both genetic and environmental factors are important in explaining the differences among and between the invasive and native populations. Interestingly, the native southern populations have similar growth traits as the invasive populations. It is possible that invasive populations of L. salicaria originated from more competitive populations in the southern portion of the native range and then, after hybridization with native North American congeners (e.g., L. alatum), adapted to local conditions in their new area. Genetic variation among the native and non-native populations will be further tested using DNA fingerprinting methods, such as AFLPs, to examine these hypotheses.

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