Mountain forests across Mexico and Central America host roughly 40 per cent of the world’s oak species, and a landmark study published in the Proceedings of the National Academy of Sciences (PNAS) has now traced the 25-million-year mountain expansion behind the planet’s greatest oak biodiversity hotspot. That is according to a research team led by University of Chicago PhD candidate Kieran Althaus, an affiliate of The Morton Arboretum’s Science and Conservation Department, working alongside senior author Andrew Hipp, the Arboretum’s Director of the Herbarium and Lead Scientist in Plant Systematics.
Drawing on data from 322 of the world’s approximately 450 oak species, the international research team reconstructed oak evolutionary history and found that two major groups, red and white oaks, independently migrated into the mountains of Mexico around 25 million years ago, then diversified along parallel paths into at least 160 distinct species.
Wood Central understands the result places Mexico and Central America at the centre of a global oak system that has produced the fastest expansion of oak diversity recorded anywhere on the planet, with the region now home to roughly four in every ten oak species worldwide.
According to the team, oaks proved unusually quick to adapt when they hit high-elevation terrain, with rugged habitats and climatic flexibility producing one of the most dynamic plant evolution events anywhere on the planet. The parallel diversification of red and white oak lineages now stands as a rare global case study for evolutionary biology at scale.
“Mountain ecosystems prove key to generating oak diversity,” Althaus said, with the paper, titled “Timing and origins of Mexican and Central American oak diversity,” concluding that the fastest expansion of oak diversity anywhere in the world has taken place across Mexico and Central America.
The findings carry weight for forest management as well as taxonomy, with more than 30 per cent of the world’s oak species now at risk of extinction even as oaks support entire ecosystems, hosting unique communities of fungi, insects, birds and mammals, and carrying deep cultural value for indigenous communities across the region. Co-author Socorro Gonzalez, a researcher at the Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional (CIIDIR) of the Instituto Politécnico Nacional in Mexico, said the ability to predict how plant communities respond to environmental change should be considered an important tool for future forest management.
Co-authors include the Arboretum’s Director of the Global Tree Conservation Program, Silvia Alvarez-Clare, and Plant Systematics Research Program Manager, Marlene Hahn, alongside researchers from Harvard University, Duke University, La Universidad Nacional Autónoma de México (UNAM), the Benemérita Universidad Autónoma de Puebla (BUAP) and the herbarium and botanic garden at CIIDIR.
“This work reflects years of shared effort and deep, sustained collaboration across borders,” Hipp said, with the publication marking the first chapter of Althaus’s PhD dissertation and a milestone in more than 15 years of fieldwork, botanical expeditions and data sharing between researchers in the US and Mexico.
For more information: K.N. Althaus, M. Hahn, S. Alvarez-Clare, J. Cavender-Bares, A.J. Coombes, M.D.S. González-Elizondo, A. González-Rodríguez, P.S. Manos, H. Rodríguez-Correa, S. Valencia-Ávalos, & A.L. Hipp, Timing and origins of Mexican and Central American oak diversity, Proc. Natl. Acad. Sci. U.S.A. 123 (19) e2537040123, https://doi.org/10.1073/pnas.2537040123 (2026).
