Genetic differentiation of native and introduced populations of Quercus rubra L

Native to North America, Northern red oak (Quercus rubra) was introduced in Europe in the XVIIth century for ornamental and forestry purposes. This species is now widespread in European forests due to plantations and natural regeneration. In invasive herbaceous plants, introduced populations are often genetically different from native populations. However, this has been poorly investigated in exotic tree species. Our objective was to explore the phenotypic variation between native and introduced populations of Q. rubra and to test for adaptation to the new environmental conditions since the introduction. We used three progeny test gardens, in South-Western, Central and North-Eastern France, composed of 106 American and European populations with 1-25 families per population. The gardens were settled from 1980s and trees were monitored regularly for growth (diameter, height) and leaf phenology (budburst, coloration). Since trees had reached sexual maturity, we have monitored acorn production for two years. Within each garden, data were analyzed using mixed analyses of variance; Qst indexes were calculated to evaluate genetic differentiation between populations. Overall, introduced populations presented higher trait values than native populations: growth rate was higher and spring phenology was advanced. Fruit set was higher in introduced trees, although depending of the year. Qst estimates clearly demonstrated the existence of a high genetic differentiation between native populations, for growth and phenology. Introduced populations presented a lower level of differentiation than native populations. These results suggest several hypotheses: (i) introduced populations only represent a part of the global diversity existing in the native range (ii) populations have evolved since introduction under new environmental selective pressures (ii) populations were selected by man since introduction. These hypotheses are being investigated, notably through a molecular approach