Latitudinal trends in growth and phenology of the invasive alien plant Impatiens glandulifera (Balsaminaceae)

Geographical differentiation of populations can be interpreted as a result of adaptive processes in response to environmental gradients and biotic interactions. Such adaptations are particularly interesting in invasive alien species which have been present in an area for a relatively short time. There are few observations of latitudinal trends in alien plants, and no account exists for Impatiens glandulifera, a problematic annual weed in most countries of central and north-western Europe. Here we describe variation in growth and phenology in 26 populations of this species from nine European regions in a common garden in Denmark. Above-ground biomass (an estimate of fecundity), height and basal diameter were lower in the northern populations which were first to produce flowers. Some differences were also observed in biomass allocation to leaves, flowers and fruits, albeit without a latitudinal pattern. The latitudinal trends in growth and survival disappeared in a field experiment, probably due to suboptimal site conditions. Most variation in plant traits was explained by differences among regions with some minor effects of populations within regions. Besides latitude, no other geographical, climatic or population trait correlated with the observed differences in growth and phenology. Differences in latitude may mainly represent variation in length of the growing season. The causes and potential consequences of such latitudinal trends for population dynamics and dispersal of alien plants are discussed.