Recently established, temperate tree populations combine a high level of differentiation for adaptive traits, suggesting rapid genetic evolution, with a high level of genetic diversity within population, suggesting a limited impact of genetic drift and purifying selection. To study experimentally the evolutionary forces in a recently established population, we assessed the spatial and temporal patterns of genetic diversity within a disjunct population of Cedrus atlantica established 140 years ago in south-eastern France from a North African source. The population is expanding through natural regeneration. Three generations were sampled, including founder trees. We analysed 12 isozyme loci, three of which were previously found in tight association with selected genes, and quantitative traits. No bottleneck effect was detected in the founder generation, but a simple test of allelic association revealed an initial disequilibrium which disappeared in the following generations. The impact of genetic drift during secondary evolution was limited, as suggested by the weak temporal differentiation. The genetic load was not reduced after 3 generations, and the quantitative variation for adaptive traits did not change either. Thus, initial genetic changes first proceed from a rapid re-organisation of the diversity through mating and recombination, whereas genetic erosion through drift and selection is delayed due to temporal and spatial stochasticity. Two life-history traits of trees contribute to slowing down the processes of genetic erosion: perenniality and large spatial scale. Thus, one would expect recently established tree populations to have a higher diversity than older ones, which seems in accordance with experimental surveys.