Freezing and water availability structure the evolutionary diversity of trees across the Americas

The historical course of evolutionary diversification shapes the current distribution of biodiversity, but the main forces constraining diversification are unclear. We unveil the evolutionary structure of tree species diversity across the Americas to assess whether an inability to move (dispersal limitation) or to evolve (niche conservatism) is the predominant constraint in plant diversification and biogeography. We find a fundamental divide in tree lineage composition between tropical and extratropical environments, defined by the absence versus presence of freezing temperatures. Within the Neotropics, we uncover a further evolutionary split between moist and dry forests. Our results demonstrate that American tree lineages, though broadly distributed geographically, tend to retain their ancestral environmental relationships and that phylogenetic niche conservatism is the primary force structuring the distribution of tree biodiversity.