The length of time land has been available for colonization by plants and other organisms could provide a partial explanation of the contemporary richness gradients of trees. According to this hypothesis, increasing times of land availability entail higher chances of recolonization, which eventually have positive effects on tree richness. To test this, we generated a dataset of the Holarctic trees and evaluated the influence of cell age, a measure of the time since an area became free of ice, on the observed tree richness gradients. We found that cell age is associated with richness in both Europe and North America, after controlling for contemporary climate patterns, suggesting that the historical pattern of glacial retreat in response to post-Pleistocene global warming has left a signal still detectable after at least 14 000 yr. The results were consistent using a range of modelling approaches or whether Europe and North America were analyzed separately or in concert. We conclude that, although secondary to contemporary climate, the post-glacial recolonization hypothesis is broadly supported at temperate latitudes.