Aim: To determine the principal drivers of variation in the evolutionary diversity of forest tree communities, with a focus on the temperate forests of South America. Location: Forests across southern South America, extending from tropical forests in southern Brazil, to the temperate forests of southern Chile and Argentina. Methods: We compiled a database of 742 forest tree community inventories spread over six countries and 12 biomes, or major vegetation types. In total, these inventories covered 3075 species of shrubs and trees. We combined this dataset with a temporally calibrated phylogeny that included all species. For each community, we evaluated multiple measures of evolutionary diversity, including phylogenetic diversity sensu stricto (PD), which is the sum of the branch lengths of a phylogeny that includes all species in a community, and its equivalent standardized for variation in species richness (ses.PD), which we refer to as lineage diversity. Results: We found that biome affiliation is the most important determinant of the evolutionary diversity of woody plant communities, with climate also showing a significant influence. Communities in wet evergreen tropical forest have the highest species richness and the highest PD, but the lowest lineage diversity, while temperate forests in southern South America show the lowest species richness and PD, but the highest lineage diversity. Main conclusions: Our results contradict the idea that temperate floras represent recently derived, evolutionarily poor subsets of tropical floras. Rather, the high lineage diversity we find in the temperate forest communities supports the ‘austral conservatism hypothesis’, which states that the flora of southern South america has evolved independently from the Neotropical Domain over tens of millions of years. Our identification of the evolutionary distinctness and richness of this flora suggests that it deserves as much conservation attention as the more species-rich tropical forests of South America and that southern South American forests should not be lumped into the Neotropical Floristic Province.