Tree species are striking for their high within-population diversity and low among-population differentiation for nuclear genes. The usual explanation for this difference is that pollen flow, and therefore gene flow, is much higher for trees. However, this explanation relies on equilibrium hypotheses. Since trees have very recently recolonized temperate areas, they have experienced many foundation events. Only extremely high levels of gene flow could have counterbalanced these successive founder effects. We develop a model to study the impact of life-cycle of forest trees, in particular of the length of their juvenile phase, on genetic diversity and differentiation during the glacial period and the following colonization period. We show that both a reasonably high level of pollen flow and the life-cycle characteristics of trees are needed to explain the observed structure of genetic diversity.

We also show that the model that fits best the genetic data is a model of frequent events of long-distance migration of limited amplitude rather than a model of rare events of very long-distance migration. This seems to be confirmed by field observations. This study is an example of how genetic data can be used to infer which demographic model is more realistic for a given species or group of species.