Historical, archaeological and paleoecological data provide a better understanding of the current distribution of species, which cannot be inferred directly from the sole knowledge of their fundamental ecological niche, dispersal ability and interactions with other species. The long time is an additional component, which is the object of historical ecology, the branch of ecology that is interested in legacies of the past in the current functioning of ecosystems. We will present some examples of the significance of these long-term effects. The first concerns the impact of past land-use changes on vegetation. Europe has known for the last two centuries a significant increase in forest area. We show the multiple impacts on the composition of plant communities of this forest expansion, in the Mediterranean area among others. This has led to the development of the important concept of ancient and recent forests, which is now classically included in assessments of the degree of naturalness of forests. We have tried to characterize the duration of these impacts: following the recolonization of a former agricultural soil, how long time does it take to erase the extinction debt of the species previously present but destined to disappear, and the colonization credit of the species that still have to arrive? The research that we have conducted on several Roman sites shows that these impacts have a multi-millennial span. We analyze the mechanisms of this strikingly long memory of ecosystems. In a second example, we will search for anthropogenic traces in the current spatial range of Fagus sylvatica in France. The resampling of dendrometric inventories from the XVIIIth century, the distribution of toponyms of beech at the national scale and soil charcoal analyses show how this tree species has been disadvantaged by forest management, perhaps early in the Neolithic period. Knowledge of such large-scale manipulations is needed to properly characterize the beech climatic niche, in order to be able to project its future distribution range into different climate change scenarios. The reaction times of plant communities to natural or anthropogenic disturbances can be very long, as shown by the examples presented. This has many consequences: climacic equilibria are rarely or never reached, the impacts of human actions should be appreciated over much longer time horizons than they are today, the understanding of our biotic environment cannot be done without an historical perspective. Time must be considered as a relevant attribute of the species niche.