invasive tree; population diversity; quantitative genetics
Abstract :
[en] 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.