Adaptive Evolution of Freezing Tolerance in Oaks Is Key to Their Dominance in North America

Freezing tolerance plays a pivotal role in shaping the distribution and diversification of organisms. We investigated the dynamics of adaptation to climate and potential trade-offs between stem freezing tolerance and growth rate in 48 Quercus species. Species from colder regions exhibited higher freezing tolerance, lower growth rates and higher winter-acclimation potential than species from warmer climates. Despite an evolutionary lag, freezing tolerance in oaks is closely aligned with its optimal state. Deciduous species showed marked variability in freezing tolerance across their broad climatic range, while evergreen species, confined to warm climates, displayed low freezing tolerance. Annual growth rates were constrained in all deciduous species, but those that evolved in warm latitudes lost freezing tolerance, precluding a trade-off between freezing tolerance and growth. We provide evidence that the capacity to adapt to a wide range of thermal environments was critical to adaptive radiation and the current dominance of the North American oaks.