Latitudinal population differentiation in two species of Solidago (Asteraceae) introduced into Europe

Solidago altissima and S. gigantea were introduced from North America to Europe similar to 250 yr ago and are now considered aggressive weeds in abandoned fields and conservation areas. We studied patterns of genetic differentiation in these two species along their present latitudinal range in Europe (44 degrees-61 degrees N). Two generations of clonally propagated ramets from randomly selected genets of 24 populations of each species were grown in a common-garden experiment at latitude 47 degrees N from 1991 to 1992. Both species showed significant variation among populations in morphological and life-history characters: at this southern location, plants from northern populations were smaller and flowered earlier than plants from southern populations. The gradient of clinal variation was more pronounced in the second year of cultivation than in the first and was steeper in S. altissima than in S. gigantea. Within-population variation among genotypes was significant for most characters in the case of S. altissima. Phenological rate (reciprocal of days to flowering) and size at maturity showed a significant negative correlation among populations but not among genotypes within populations, indicating that genetic trade-offs may occur at one but not another infraspecific level. We suggest that the pattern of among-population variation reflects rapid adaptive population differentiation after introduction of the species to Europe.